<?xml version="1.0" encoding="utf-8"?>

<!DOCTYPE manualpage SYSTEM "/style/manualpage.dtd">

<?xml-stylesheet type="text/xsl" href="/style/manual.en.xsl"?>

<manualpage metafile="perf-scaling.xml.meta">

<parentdocument href="./">Miscellaneous Documentation</parentdocument>

<title>Performance Scaling</title>

<summary>

<p>The Performance Tuning page in the Apache 1.3 documentation says:

</p>

<blockquote><p>

"Apache is a general webserver, which is designed to be

correct first, and fast

second. Even so, its performance is quite satisfactory. Most

sites have less than 10Mbits of outgoing bandwidth, which

Apache can fill using only a low end Pentium-based

webserver."</p>

</blockquote>

<p>However, this sentence was written a few years ago, and in the

meantime several things have happened. On one hand, web server

hardware has become much faster. On the other hand, many sites now

are allowed much more than ten megabits per second of outgoing

bandwidth. In addition, web applications have become more complex.

The classic brochureware site is alive and well, but the web has

grown up substantially as a computing application platform and

webmasters may find themselves running dynamic content in Perl, PHP

or Java, all of which take a toll on performance.

</p>

<p>Therefore, in spite of strides forward in machine speed and

bandwidth allowances, web server performance and web application

performance remain areas of concern. In this documentation several

aspects of web server performance will be discussed.

</p>

</summary>

<section id="what-will-and-will-not-be-discussed">

<title>What Will and Will Not Be Discussed

</title>

<p>The session will focus on easily accessible configuration and tuning

options for Apache httpd 2.2 and 2.4 as well as monitoring tools.

Monitoring tools will allow you to observe your web server to

gather information about its performance, or lack thereof.

We'll assume that you don't have an unlimited budget for

server hardware, so the existing infrastructure will have to do the

job. You have no desire to compile your own Apache, or to recompile

the operating system kernel. We do assume, though, that you have

some familiarity with the Apache httpd configuration file.

</p>

</section>

<section id="monitoring-your-server">

<title>Monitoring Your Server

</title>

<p>The first task when sizing or performance-tuning your server is to

find out how your system is currently performing. By monitoring

your server under real-world load, or artificially generated load,

you can extrapolate its behavior under stress, such as when your

site is mentioned on Slashdot.

</p>

<section id="monitoring-tools">

<title>Monitoring Tools

</title>

<section id="top">

<title>top

</title>

<p>The top tool ships with Linux and FreeBSD. Solaris offers

<code>prstat(1)</code>. It collects a number of statistics for the

system and for each running process, then displays them

interactively on your terminal. The data displayed is

refreshed every second and varies by platform, but

typically includes system load average, number of processes

and their current states, the percent CPU(s) time spent

executing user and system code, and the state of the

virtual memory system. The data displayed for each process

is typically configurable and includes its process name and

ID, priority and nice values, memory footprint, and

percentage CPU usage. The following example shows multiple

httpd processes (with MPM worker and event) running on an

Linux (Xen) system:

</p>

<example><pre>

top - 23:10:58 up 71 days, 6:14, 4 users, load average: 0.25, 0.53, 0.47

Tasks: 163 total, 1 running, 162 sleeping, 0 stopped, 0 zombie

Cpu(s): 11.6%us, 0.7%sy, 0.0%ni, 87.3%id, 0.4%wa, 0.0%hi, 0.0%si, 0.0%st

Mem: 2621656k total, 2178684k used, 442972k free, 100500k buffers

Swap: 4194296k total, 860584k used, 3333712k free, 1157552k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND

16687 example_ 20 0 1200m 547m 179m S 45 21.4 1:09.59 httpd-worker

15195 www 20 0 441m 33m 2468 S 0 1.3 0:41.41 httpd-worker

1 root 20 0 10312 328 308 S 0 0.0 0:33.17 init

2 root 15 -5 0 0 0 S 0 0.0 0:00.00 kthreadd

3 root RT -5 0 0 0 S 0 0.0 0:00.14 migration/0

4 root 15 -5 0 0 0 S 0 0.0 0:04.58 ksoftirqd/0

5 root RT -5 0 0 0 S 0 0.0 4:45.89 watchdog/0

6 root 15 -5 0 0 0 S 0 0.0 1:42.52 events/0

7 root 15 -5 0 0 0 S 0 0.0 0:00.00 khelper

19 root 15 -5 0 0 0 S 0 0.0 0:00.00 xenwatch

20 root 15 -5 0 0 0 S 0 0.0 0:00.00 xenbus

28 root RT -5 0 0 0 S 0 0.0 0:00.14 migration/1

29 root 15 -5 0 0 0 S 0 0.0 0:00.20 ksoftirqd/1

30 root RT -5 0 0 0 S 0 0.0 0:05.96 watchdog/1

31 root 15 -5 0 0 0 S 0 0.0 1:18.35 events/1

32 root RT -5 0 0 0 S 0 0.0 0:00.08 migration/2

33 root 15 -5 0 0 0 S 0 0.0 0:00.18 ksoftirqd/2

34 root RT -5 0 0 0 S 0 0.0 0:06.00 watchdog/2

35 root 15 -5 0 0 0 S 0 0.0 1:08.39 events/2

36 root RT -5 0 0 0 S 0 0.0 0:00.10 migration/3

37 root 15 -5 0 0 0 S 0 0.0 0:00.16 ksoftirqd/3

38 root RT -5 0 0 0 S 0 0.0 0:06.08 watchdog/3

39 root 15 -5 0 0 0 S 0 0.0 1:22.81 events/3

68 root 15 -5 0 0 0 S 0 0.0 0:06.28 kblockd/0

69 root 15 -5 0 0 0 S 0 0.0 0:00.04 kblockd/1

70 root 15 -5 0 0 0 S 0 0.0 0:00.04 kblockd/2</pre></example>

<p>Top is a wonderful tool even though it's slightly resource

intensive (when running, its own process is usually in the

top ten CPU gluttons). It is indispensable in determining

the size of a running process, which comes in handy when

determining how many server processes you can run on your

machine. How to do this is described in <a href="#sizing-maxClients">sizing MaxClients</a>.

Top is, however, an interactive tool and running it

continuously has few if any advantages.

</p>

</section>

<section id="free">

<title>free

</title>

<p>This command is only available on Linux. It shows how much

memory and swap space is in use. Linux allocates unused

memory as file system cache. The free command shows usage

both with and without this cache. The free command can be

used to find out how much memory the operating system is

using, as described in the paragraph <a href="#sizing-maxClients">sizing MaxClients</a>.

The output of free looks like this:

</p>

<example><pre>

sctemme@brutus:~$ free

total used free shared buffers cached

Mem: 4026028 3901892 124136 0 253144 841044

-/+ buffers/cache: 2807704 1218324

Swap: 3903784 12540 3891244

</pre></example>

</section>

<section id="vmstat">

<title>vmstat

</title>

<p>This command is available on many unix platforms. It

displays a large number of operating system metrics. Run

without argument, it displays a status line for that

moment. When a numeric argument is added, the status is

redisplayed at designated intervals. For example,

<code>vmstat 5</code>

causes the information to reappear every five seconds.

Vmstat displays the amount of virtual memory in use, how

much memory is swapped in and out each second, the number

of processes currently running and sleeping, the number of

interrupts and context switches per second and the usage

percentages of the CPU.

</p>

<p>

The following is <code>vmstat</code> output of an idle server:

</p>

<example><pre>

[sctemme@GayDeceiver sctemme]$ vmstat 5 3

procs memory swap io system cpu

r b w swpd free buff cache si so bi bo in cs us sy id

0 0 0 0 186252 6688 37516 0 0 12 5 47 311 0 1 99

0 0 0 0 186244 6696 37516 0 0 0 16 41 314 0 0 100

0 0 0 0 186236 6704 37516 0 0 0 9 44 314 0 0 100

</pre></example>

<p>And this is output of a server that is under a load of one

hundred simultaneous connections fetching static content:

</p>

<example><pre>

[sctemme@GayDeceiver sctemme]$ vmstat 5 3

procs memory swap io system cpu

r b w swpd free buff cache si so bi bo in cs us sy id

1 0 1 0 162580 6848 40056 0 0 11 5 150 324 1 1 98

6 0 1 0 163280 6856 40248 0 0 0 66 6384 1117 42 25 32

11 0 0 0 162780 6864 40436 0 0 0 61 6309 1165 33 28 40

</pre></example>

<p>The first line gives averages since the last reboot. The

subsequent lines give information for five second

intervals. The second argument tells vmstat to generate

three reports and then exit.

</p>

</section>

<section id="se-toolkit">

<title>SE Toolkit

</title>

<p>The SE Toolkit is a system monitoring toolkit for Solaris.

Its programming language is based on the C preprocessor and

comes with a number of sample scripts. It can use both the

command line and the GUI to display information. It can

also be programmed to apply rules to the system data. The

example script shown in Figure 2, Zoom.se, shows green,

orange or red indicators when utilization of various parts

of the system rises above certain thresholds. Another

included script, Virtual Adrian, applies performance tuning

metrics according to.

</p>

<p>The SE Toolkit has drifted around for a while and has had

several owners since its inception. It seems that it has

now found a final home at Sunfreeware.com, where it can be

downloaded at no charge. There is a single package for

Solaris 8, 9 and 10 on SPARC and x86, and includes source

code. SE Toolkit author Richard Pettit has started a new

company, Captive Metrics4 that plans to bring to market a

multiplatform monitoring tool built on the same principles

as SE Toolkit, written in Java.

</p>

</section>

<section id="dtrace">

<title>DTrace

</title>

<p>Given that DTrace is available for Solaris, FreeBSD and OS

X, it might be worth exploring it. There's also

mod_dtrace available for httpd.

</p>

</section>

<section id="mod_status">

<title>mod_status

</title>

<p>The mod_status module gives an overview of the server

performance at a given moment. It generates an HTML page

with, among others, the number of Apache processes running

and how many bytes each has served, and the CPU load caused

by httpd and the rest of the system. The Apache Software

Foundation uses <module>mod_status</module> on its own

<a href="http://apache.org/server-status">web site</a>.

If you put the <code>ExtendedStatus On</code>

directive in your <code>httpd.conf</code>,

the <module>mod_status</module>

page will give you more information at the cost of a little

extra work per request.

</p>

</section>

</section>

<section id="web-server-log-files">

<title>Web Server Log Files

</title>

<p>Monitoring and analyzing the log files httpd writes is one of

the most effective ways to keep track of your server health and

performance. Monitoring the error log allows you to detect

error conditions, discover attacks and find performance issues.

Analyzing the access logs tells you how busy your server is,

which resources are the most popular and where your users come

from. Historical log file data can give you invaluable insight

into trends in access to your server, which allows you to

predict when your performance needs will overtake your server

capacity.

</p>

<section id="ErrorLog">

<title>Error Log

</title>

<p>The error log will contain messages if the server has

reached the maximum number of active processes or the

maximum number of concurrently open files. The error log

also reflects when processes are being spawned at a

higher-than-usual rate in response to a sudden increase in

load. When the server starts, the stderr file descriptor is

redirected to the error logfile, so any error encountered

by httpd after it opens its logfiles will appear in this

log. This makes it good practice to review the error log

frequently.

</p>

<p>Before Apache httpd opens its logfiles, any errors will be

written to the stderr stream. If you start httpd manually,

this error information will appear on your terminal and you

can use it directly to troubleshoot your server. If your

httpd is started by a startup script, the destination of

early error messages depends on their design. The

<code>/var/log/messages</code>

file is usually a good bet. On Windows, early error

messages are written to the Applications Event Log, which

can be viewed through the Event Viewer in Administrative

Tools.

</p>

<p>

The Error Log is configured through the <directive module="core">ErrorLog</directive>

and <directive module="core">LogLevel</directive>

configuration directives. The error log of httpd's main

server configuration receives the log messages that pertain

to the entire server: startup, shutdown, crashes, excessive

process spawns, etc. The <directive module="core">ErrorLog</directive>

directive can also be used in virtual host containers. The

error log of a virtual host receives only log messages

specific to that virtual host, such as authentication

failures and 'File not Found' errors.

</p>

<p>On a server that is visible to the Internet, expect to see a

lot of exploit attempt and worm attacks in the error log. A

lot of these will be targeted at other server platforms

instead of Apache, but the current state of affairs is that

attack scripts just throw everything they have at any open

port, regardless of which server is actually running or

what applications might be installed. You could block these

attempts using a firewall or <a href="http://www.modsecurity.org/">mod_security</a>,

but this falls outside the scope of this discussion.

</p>

<p>

The <directive module="core">LogLevel</directive>

directive determines the level of detail included in the

logs. There are eight log levels as described here:

</p>

<table>

<tr>

<td>

<p><strong>Level</strong></p>

</td>

<td>

<p><strong>Description</strong></p>

</td>

</tr>

<tr>

<td>

<p>emerg</p>

</td>

<td>

<p>Emergencies - system is unusable.</p>

</td>

</tr>

<tr>

<td>

<p>alert</p>

</td>

<td>

<p>Action must be taken immediately.</p>

</td>

</tr>

<tr>

<td>

<p>crit</p>

</td>

<td>

<p>Critical Conditions.</p>

</td>

</tr>

<tr>

<td>

<p>error</p>

</td>

<td>

<p>Error conditions.</p>

</td>

</tr>

<tr>

<td>

<p>warn</p>

</td>

<td>

<p>Warning conditions.</p>

</td>

</tr>

<tr>

<td>

<p>notice</p>

</td>

<td>

<p>Normal but significant condition.</p>

</td>

</tr>

<tr>

<td>

<p>info</p>

</td>

<td>

<p>Informational.</p>

</td>

</tr>

<tr>

<td>

<p>debug</p>

</td>

<td>

<p>Debug-level messages</p>

</td>

</tr>

</table>

<p>The default log level is warn. A production server should

not be run on debug, but increasing the level of detail in

the error log can be useful during troubleshooting.

Starting with 2.3.8 <directive module="core">LogLevel</directive>

can be specified on a per module basis:

</p>

<highlight language="config">

LogLevel debug mod_ssl:warn

</highlight>

<p>

This puts all of the server in debug mode, except for

<module>mod_ssl</module>, which tends to be very noisy.

</p>

</section>

<section id="AccessLog">

<title>Access Log

</title>

<p>Apache httpd keeps track of every request it services in its

access log file. In addition to the time and nature of a

request, httpd can log the client IP address, date and time

of the request, the result and a host of other information.

The various logging format features are documented in the

manual. This file exists by default for the main server and can be

configured per virtual host by using the <directive module="mod_log_config">TransferLog</directive>

or <directive module="mod_log_config">CustomLog</directive>

configuration directive.

</p>

<p>The access logs can be analyzed with any of several free and

commercially available programs. Popular free analysis

packages include Analog and Webalizer. Log analysis should

be done offline so the web server machine is not burdened

by processing the log files. Most log analysis packages

understand the Common Log Format. The fields in the log

lines are explained in in the following:

</p>

<example><pre>

195.54.228.42 - - [24/Mar/2007:23:05:11 -0400] "GET /sander/feed/ HTTP/1.1" 200 9747

64.34.165.214 - - [24/Mar/2007:23:10:11 -0400] "GET /sander/feed/atom HTTP/1.1" 200 9068

60.28.164.72 - - [24/Mar/2007:23:11:41 -0400] "GET / HTTP/1.0" 200 618

85.140.155.56 - - [24/Mar/2007:23:14:12 -0400] "GET /sander/2006/09/27/44/ HTTP/1.1" 200 14172

85.140.155.56 - - [24/Mar/2007:23:14:15 -0400] "GET /sander/2006/09/21/gore-tax-pollution/ HTTP/1.1" 200 15147

74.6.72.187 - - [24/Mar/2007:23:18:11 -0400] "GET /sander/2006/09/27/44/ HTTP/1.0" 200 14172

74.6.72.229 - - [24/Mar/2007:23:24:22 -0400] "GET /sander/2006/11/21/os-java/ HTTP/1.0" 200 13457

</pre></example>

<table>

<tr>

<td>

<p><strong>Field</strong></p>

</td>

<td>

<p><strong>Content</strong></p>

</td>

<td>

<p><strong>Explanation</strong></p>

</td>

</tr>

<tr>

<td>

<p>Client IP</p>

</td>

<td>

<p>195.54.228.42</p>

</td>

<td>

<p>IP address where the request originated</p>

</td>

</tr>

<tr>

<td>

<p>RFC 1413 ident</p>

</td>

<td>

<p>-</p>

</td>

<td>

<p>Remote user identity as reported by their identd</p>

</td>

</tr>

<tr>

<td>

<p>username</p>

</td>

<td>

<p>-</p>

</td>

<td>

<p>Remote username as authenticated by Apache</p>

</td>

</tr>

<tr>

<td>

<p>timestamp</p>

</td>

<td>

<p>[24/Mar/2007:23:05:11 -0400]</p>

</td>

<td>

<p>Date and time of request</p>

</td>

</tr>

<tr>

<td>

<p>Request</p>

</td>

<td>

<p>&quot;GET /sander/feed/ HTTP/1.1&quot;</p>

</td>

<td>

<p>Request line</p>

</td>

</tr>

<tr>

<td>

<p>Status Code</p>

</td>

<td>

<p>200</p>

</td>

<td>

<p>Response code</p>

</td>

</tr>

<tr>

<td>

<p>Content Bytes</p>

</td>

<td>

<p>9747</p>

</td>

<td>

<p>Bytes transferred w/o headers</p>

</td>

</tr>

</table>

</section>

<section id="rotating-log-files">

<title>Rotating Log Files

</title>

<p>There are several reasons to rotate logfiles. Even though

almost no operating systems out there have a hard file size

limit of two Gigabytes anymore, log files simply become too

large to handle over time. Additionally, any periodic log

file analysis should not be performed on files to which the

server is actively writing. Periodic logfile rotation helps

keep the analysis job manageable, and allows you to keep a

closer eye on usage trends.

</p>

<p>On unix systems, you can simply rotate logfiles by giving

the old file a new name using mv. The server will keep

writing to the open file even though it has a new name.

When you send a graceful restart signal to the server, it

will open a new logfile with the configured name. For

example, you could run a script from cron like this:

</p>

<example>

APACHE=/usr/local/apache2<br />

HTTPD=$APACHE/bin/httpd<br />

mv $APACHE/logs/access_log

$APACHE/logarchive/access_log-`date +%F`<br />

$HTTPD -k graceful

</example>

<p>This approach also works on Windows, just not as smoothly.

While the httpd process on your Windows server will keep

writing to the log file after it has been renamed, the

Windows Service that runs Apache can not do a graceful

restart. Restarting a Service on Windows means stopping it

and then starting it again. The advantage of a graceful

restart is that the httpd child processes get to complete

responding to their current requests before they exit.

Meanwhile, the httpd server becomes immediately available

again to serve new requests. The stop-start that the

Windows Service has to perform will interrupt any requests

currently in progress, and the server is unavailable until

it is started again. Plan for this when you decide the

timing of your restarts.

</p>

<p>

A second approach is to use piped logs. From the

<directive module="mod_log_config">CustomLog</directive>,

<directive module="mod_log_config">TransferLog</directive>

or <directive module="core">ErrorLog

</directive>

directives you can send the log data into any program using

a pipe character (<code>|</code>). For instance:

</p>

<example>CustomLog "|/usr/local/apache2/bin/rotatelogs

/var/log/access_log 86400" common

</example>

<p>The program on the other end of the pipe will receive the

Apache log data on its stdin stream, and can do with this

data whatever it wants. The rotatelogs program that comes

with Apache seamlessly turns over the log file based on

time elapsed or the amount of data written, and leaves the

old log files with a timestamp suffix to its name. This

method for rotating logfiles works well on unix platforms,

but is currently broken on Windows.

</p>

</section>

<section id="logging-and-performance">

<title>Logging and Performance

</title>

<p>Writing entries to the Apache log files obviously takes some

effort, but the information gathered from the logs is so

valuable that under normal circumstances logging should not

be turned off. For optimal performance, you should put your

disk-based site content on a different physical disk than

the server log files: the access patterns are very

different. Retrieving content from disk is a read operation

in a fairly random pattern, and log files are written to

disk sequentially.

</p>

<p>

Do not run a production server with your error <directive module="core">LogLevel</directive>

set to debug. This log level causes a vast amount of

information to be written to the error log, including, in

the case of SSL access, complete dumps of BIO read and

write operations. The performance implications are

significant: use the default warn level instead.

</p>

<p>If your server has more than one virtual host, you may give

each virtual host a separate access logfile. This makes it

easier to analyze the logfile later. However, if your

server has many virtual hosts, all the open logfiles put a

resource burden on your system, and it may be preferable to

log to a single file. Use the <code>%v</code>

format character at the start of your <directive module="mod_log_config">LogFormat</directive>

and starting 2.3.8 of your <directive module="core">ErrorLog</directive>

to make httpd print the hostname of the virtual host that

received the request or the error at the beginning of each

log line. A simple Perl script can split out the log file

after it rotates: one is included with the Apache source

under <code>support/split-logfile</code>.

</p>

<p>

You can use the <directive module="mod_log_config">BufferedLogs</directive>

directive to have Apache collect several log lines in

memory before writing them to disk. This might yield better

performance, but could affect the order in which the

server's log is written.

</p>

</section>

</section>

<section id="generating-a-test-load">

<title>Generating A Test Load

</title>

<p>It is useful to generate a test load to monitor system

performance under realistic operating circumstances. Besides

commercial packages such as <a href="http://learnloadrunner.com/">LoadRunner</a>

,there are a number of freely available tools to generate a

test load against your web server.

</p>

<ul>

<li>Apache ships with a test program called ab, short for

Apache Bench. It can generate a web server load by

repeatedly asking for the same file in rapid succession.

You can specify a number of concurrent connections and have

the program run for either a given amount of time or a

specified number of requests.

</li>

<li>Another freely available load generator is http load11 .

This program works with a URL file and can be compiled with

SSL support.

</li>

<li>The Apache Software Foundation offers a tool named flood12

. Flood is a fairly sophisticated program that is

configured through an XML file.

</li>

<li>Finally, JMeter13 , a Jakarta subproject, is an all-Java

load-testing tool. While early versions of this application

were slow and difficult to use, the current version 2.1.1

seems to be versatile and useful.

</li>

<li>

<p>ASF external projects, that have proven to be quite

good: grinder, httperf, tsung, <a href="http://funkload.nuxeo.org/">FunkLoad</a>

</p>

</li>

</ul>

<p>When you load-test your web server, please keep in mind that if

that server is in production, the test load may negatively

affect the server's response. Also, any data traffic you

generate may be charged against your monthly traffic allowance.

</p>

</section>

</section>

<section id="configuring-for-performance">

<title>Configuring for Performance

</title>

<section id="apache-configuration">

<title>Httpd Configuration

</title>

<p>The Apache 2.2 httpd is by default a pre-forking web server.

When the server starts, the parent process spawns a number of

child processes that do the actual work of servicing requests.

But Apache httpd 2.0 introduced the concept of the

Multi-Processing Module (MPM). Developers can write MPMs to

suit the process- or threadingarchitecture of their specific

operating system. Apache 2 comes with special MPMs for Windows,

OS/2, Netware and BeOS. On unix-like platforms, the two most

popular MPMs are Prefork and Worker. The Prefork MPM offers the

same pre-forking process model that Apache 1.3 uses. The Worker

MPM runs a smaller number of child processes, and spawns

multiple request handling threads within each child process. In

2.4 MPMs are no longer hard-wired. They too can be exchanged

via <directive module="mod_so">LoadModule</directive>.

The default MPM in 2.4 is the event MPM.

</p>

<p>The maximum number of workers, be they pre-forked child

processes or threads within a process, is an indication of how

many requests your server can manage concurrently. It is merely

a rough estimate because the kernel can queue connection

attempts for your web server. When your site becomes busy and

the maximum number of workers is running, the machine

doesn't hit a hard limit beyond which clients will be

denied access. However, once requests start backing up, system

performance is likely to degrade.

</p>

<p>Finally, if the httpd server in question is not executing any third-party

code, via <code>mod_php</code>, <code>mod_perl</code> or similar,

we recommend the use of <module outdated="true">mpm_event</module>. This MPM is ideal

for situations where httpd serves as a thin layer between clients and

backend servers doing the real job, such as a proxy or cache.

</p>

<section id="MaxClients">

<title>MaxClients

</title>

<p>

The <code>MaxClients</code>

directive in your Apache httpd configuration file specifies

the maximum number of workers your server can create. It

has two related directives, <code>MinSpareServers

</code>

and <code>MaxSpareServers

</code>

,which specify the number of workers Apache keeps waiting

in the wings ready to serve requests. The absolute maximum

number of processes is configurable through the <code>

ServerLimit

</code>

directive.

</p>

</section>

<section id="spinning-threads">

<title>Spinning Threads

</title>

<p>For the prefork MPM of the above directives are all there is

to determining the process limit. However, if you are

running a threaded MPM the situation is a little more

complicated. Threaded MPMs support the <code>

ThreadsPerChild

</code>

directive1 . Apache requires that <code>MaxClients</code>

is evenly divisible by <code>ThreadsPerChild

</code>

.If you set either directive to a number that doesn't

meet this requirement, Apache will send a message of

complaint to the error log and adjust the <code>

ThreadsPerChild

</code>

value downwards until it is an even factor of

<code>MaxClients</code>.

</p>

</section>

<section id="sizing-maxClients">

<title>Sizing MaxClients

</title>

<p>Optimally, the maximum number of processes should be set so

that all the memory on your system is used, but no more. If

your system gets so overloaded that it needs to heavily

swap core memory out to disk, performance will degrade

quickly. The formula for determining <directive module="mpm_common" name="MaxRequestWorkers">MaxClients</directive>

is fairly simple:

</p>

<example>

total RAM - RAM for OS - RAM for external programs<br />

MaxClients =

-------------------------------------------------------<br />

RAM per httpd process

</example>

<p>The various amounts of memory allocated for the OS, external

programs and the httpd processes is best determined by

observation: use the top and free commands described above

to determine the memory footprint of the OS without the web

server running. You can also determine the footprint of a

typical web server process from top: most top

implementations have a Resident Size (RSS) column and a

Shared Memory column.

</p>

<p>The difference between these two is the amount of memory

per-process. The shared segment really exists only once and

is used for the code and libraries loaded and the dynamic

inter-process tally, or 'scoreboard,' that Apache

keeps. How much memory each process takes for itself

depends heavily on the number and kind of modules you use.

The best approach to use in determining this need is to

generate a typical test load against your web site and see

how large the httpd processes become.

</p>

<p>The RAM for external programs parameter is intended mostly

for CGI programs and scripts that run outside the web

server process. However, if you have a Java virtual machine

running Tomcat on the same box it will need a significant

amount of memory as well. The above assessment should give

you an idea how far you can push <code>MaxClients

</code>

,but it is not an exact science. When in doubt, be

conservative and use a low <code>MaxClients

</code>

value. The Linux kernel will put extra memory to good use

for caching disk access. On Solaris you need enough

available real RAM memory to create any process. If no real

memory is available, httpd will start writing 'No space

left on device' messages to the error log and be unable

to fork additional child processes, so a higher <code>

MaxClients

</code>

value may actually be a disadvantage.

</p>

</section>

<section id="selecting-your-mpm">

<title>Selecting your MPM

</title>

<p>The prime reason for selecting a threaded MPM is that

threads consume fewer system resources than processes, and

it takes less effort for the system to switch between

threads. This is more true for some operating systems than

for others. On systems like Solaris and AIX, manipulating

processes is relatively expensive in terms of system

resources. On these systems, running a threaded MPM makes

sense. On Linux, the threading implementation actually uses

one process for each thread. Linux processes are relatively

lightweight, but it means that a threaded MPM offers less

of a performance advantage than in other environments.

</p>

<p>Running a threaded MPM can cause stability problems in some

situations For instance, should a child process of a

preforked MPM crash, at most one client connection is

affected. However, if a threaded child crashes, all the

threads in that process disappear, which means all the

clients currently being served by that process will see

their connection aborted. Additionally, there may be

so-called &quot;thread-safety&quot; issues, especially with

third-party libraries. In threaded applications, threads

may access the same variables indiscriminately, not knowing

whether a variable may have been changed by another thread.

</p>

<p>This has been a sore point within the PHP community. The PHP

processor heavily relies on third-party libraries and

cannot guarantee that all of these are thread-safe. The

good news is that if you are running Apache on Linux, you

can run PHP in the preforked MPM without fear of losing too

much performance relative to the threaded option.

</p>

</section>

<section id="spinning-locks">

<title>Spinning Locks

</title>

<p>Apache httpd maintains an inter-process lock around its

network listener. For all practical purposes, this means

that only one httpd child process can receive a request at

any given time. The other processes are either servicing

requests already received or are 'camping out' on

the lock, waiting for the network listener to become

available. This process is best visualized as a revolving

door, with only one process allowed in the door at any

time. On a heavily loaded web server with requests arriving

constantly, the door spins quickly and requests are

accepted at a steady rate. On a lightly loaded web server,

the process that currently &quot;holds&quot; the lock may

have to stay in the door for a while, during which all the

other processes sit idle, waiting to acquire the lock. At

this time, the parent process may decide to terminate some

children based on its <code>MaxSpareServers

</code>

directive.

</p>

</section>

<section id="the-thundering-herd">

<title>The Thundering Herd

</title>

<p>The function of the 'accept mutex' (as this

inter-process lock is called) is to keep request reception

moving along in an orderly fashion. If the lock is absent,

the server may exhibit the Thundering Herd syndrome.

</p>

<p>Consider an American Football team poised on the line of

scrimmage. If the football players were Apache processes

all team members would go for the ball simultaneously at

the snap. One process would get it, and all the others

would have to lumber back to the line for the next snap. In

this metaphor, the accept mutex acts as the quarterback,

delivering the connection &quot;ball&quot; to the

appropriate player process.

</p>

<p>Moving this much information around is obviously a lot of

work, and, like a smart person, a smart web server tries to

avoid it whenever possible. Hence the revolving door

construction. In recent years, many operating systems,

including Linux and Solaris, have put code in place to

prevent the Thundering Herd syndrome. Apache recognizes

this and if you run with just one network listener, meaning

one virtual host or just the main server, Apache will

refrain from using an accept mutex. If you run with

multiple listeners (for instance because you have a virtual

host serving SSL requests), it will activate the accept

mutex to avoid internal conflicts.

</p>

<p>

You can manipulate the accept mutex with the <code>

AcceptMutex

</code>

directive. Besides turning the accept mutex off, you can

select the locking mechanism. Common locking mechanisms

include fcntl, System V Semaphores and pthread locking. Not

all are available on every platform, and their availability

also depends on compile-time settings. The various locking

mechanisms may place specific demands on system resources:

manipulate them with care.

</p>

<p>There is no compelling reason to disable the accept mutex.

Apache automatically recognizes the single listener

situation described above and knows if it is safe to run

without mutex on your platform.

</p>

</section>

</section>

<section id="tuning-the-operating-system">

<title>Tuning the Operating System

</title>

<p>People often look for the 'magic tune-up' that will

make their system perform four times as fast by tweaking just

one little setting. The truth is, present-day UNIX derivatives

are pretty well adjusted straight out of the box and there is

not a lot that needs to be done to make them perform optimally.

However, there are a few things that an administrator can do to

improve performance.

</p>

<section id="ram-and-swap-space">

<title>RAM and Swap Space

</title>

<p>The usual mantra regarding RAM is &quot;more is

better&quot;. As discussed above, unused RAM is put to good

use as file system cache. The Apache processes get bigger

if you load more modules, especially if you use modules

that generate dynamic page content within the processes,

like PHP and mod_perl. A large configuration file-with many

virtual hosts-also tends to inflate the process footprint.

Having ample RAM allows you to run Apache with more child

processes, which allows the server to process more

concurrent requests.

</p>

<p>While the various platforms treat their virtual memory in

different ways, it is never a good idea to run with less

disk-based swap space than RAM. The virtual memory system

is designed to provide a fallback for RAM, but when you

don't have disk space available and run out of

swappable memory, your machine grinds to a halt. This can

crash your box, requiring a physical reboot for which your

hosting facility may charge you.

</p>

<p>Also, such an outage naturally occurs when you least want

it: when the world has found your website and is beating a

path to your door. If you have enough disk-based swap space

available and the machine gets overloaded, it may get very,

very slow as the system needs to swap memory pages to disk

and back, but when the load decreases the system should

recover. Remember, you still have <code>MaxClients

</code>

to keep things in hand.

</p>

<p>Most unix-like operating systems use designated disk

partitions for swap space. When a system starts up it finds

all swap partitions on the disk(s), by partition type or

because they are listed in the file <code>/etc/fstab

</code>

,and automatically enables them. When adding a disk or

installing the operating system, be sure to allocate enough

swap space to accommodate eventual RAM upgrades.

Reassigning disk space on a running system is a cumbersome

process.

</p>

<p>Plan for available hard drive swap space of at least twice

your amount of RAM, perhaps up to four times in situations

with frequent peaking loads. Remember to adjust this

configuration whenever you upgrade RAM on your system. In a

pinch, you can use a regular file as swap space. For

instructions on how to do this, see the manual pages for

the <code>mkswap

</code>

and <code>swapon

</code>

or <code>swap

</code>

programs.

</p>

</section>

<section id="ulimit-files-and-processes">

<title>ulimit: Files and Processes

</title>

<p>Given a machine with plenty of RAM and processor capacity,

you can run hundreds of Apache processes if necessary. . .

and if your kernel allows it.

</p>

<p>Consider a situation in which several hundred web servers

are running; if some of these need to spawn CGI processes,

the maximum number of processes would occur quickly.

</p>

<p>However, you can change this limit with the command

</p>

<example>

ulimit [-H|-S] -u [newvalue]

</example>

<p>This must be changed before starting the server, since the

new value will only be available to the current shell and

programs started from it. In newer Linux kernels the

default has been raised to 2048. On FreeBSD, the number

seems to be the rather unusual 513. In the default user

shell on this system, <code>csh

</code>

the equivalent is <code>limit

</code>

and works analogous the the Bourne-like <code>ulimit

</code>

:

</p>

<example>

limit [-h] maxproc [newvalue]

</example>

<p>Similarly, the kernel may limit the number of open files per

process. This is generally not a problem for pre-forked

servers, which just handle one request at a time per

process. Threaded servers, however, serve many requests per

process and much more easily run out of available file

descriptors. You can increase the maximum number of open

files per process by running the

</p>

<example>ulimit -n [newvalue]

</example>

<p>command. Once again, this must be done prior to starting

Apache.

</p>

</section>

<section id="setting-user-limits-on-system-startup">

<title>Setting User Limits on System Startup

</title>

<p>Under Linux, you can set the ulimit parameters on bootup by

editing the <code>/etc/security/limits.conf

</code>

file. This file allows you to set soft and hard limits on a

per-user or per-group basis; the file contains commentary

explaining the options. To enable this, make sure that the

file <code>/etc/pam.d/login

</code>

contains the line

</p>

<example>session required /lib/security/pam_limits.so

</example>

<p>All items can have a 'soft' and a 'hard'

limit: the first is the default setting and the second the

maximum value for that item.

</p>

<p>

In FreeBSD's <code>/etc/login.conf

</code>

these resources can be limited or extended system wide,

analogously to <code>limits.conf</code>.

'Soft' limits can be specified with <code>-cur</code>

and 'hard' limits with <code>-max</code>.

</p>

<p>Solaris has a similar mechanism for manipulating limit

values at boot time: In <code>/etc/system</code>

you can set kernel tunables valid for the entire system at

boot time. These are the same tunables that can be set with

the <code>mdb</code>

kernel debugger during run time. The soft and hard limit

corresponding to ulimit -u can be set via:

</p>

<example>

set rlim_fd_max=65536<br />

set rlim_fd_cur=2048

</example>

<p>Solaris calculates the maximum number of allowed processes

per user (<code>maxuprc</code>) based on the total amount

available memory on the system (<code>maxusers</code>).

You can review the numbers with

</p>

<example>sysdef -i | grep maximum

</example>

<p>but it is not recommended to change them.

</p>

</section>

<section id="turn-off-unused-services-and-modules">

<title>Turn Off Unused Services and Modules

</title>

<p>Many UNIX and Linux distributions come with a slew of

services turned on by default. You probably need few of

them. For example, your web server does not need to be

running sendmail, nor is it likely to be an NFS server,

etc. Turn them off.

</p>

<p>On Red Hat Linux, the chkconfig tool will help you do this

from the command line. On Solaris systems <code>svcs</code>

and <code>svcadm</code>

will show which services are enabled and disable them

respectively.

</p>

<p>In a similar fashion, cast a critical eye on the Apache

modules you load. Most binary distributions of Apache

httpd, and pre-installed versions that come with Linux

distributions, have their modules enabled through the

<directive>LoadModule</directive> directive.

</p>

<p>Unused modules may be culled: if you don't rely on

their functionality and configuration directives, you can

turn them off by commenting out the corresponding

<directive>LoadModule</directive>

lines. Read the documentation on each module's

functionality before deciding whether to keep it enabled.

While the performance overhead of an unused module is

small, it's also unnecessary.

</p>

</section>

</section>

</section>

<section id="caching-content">

<title>Caching Content

</title>

<p>Requests for dynamically generated content usually take

significantly more resources than requests for static content.

Static content consists of simple filespages, images, etc.-on disk

that are very efficiently served. Many operating systems also

automatically cache the contents of frequently accessed files in

memory.

</p>

<p>Processing dynamic requests, on the contrary, can be much more

involved. Running CGI scripts, handing off requests to an external

application server and accessing database content can introduce

significant latency and processing load to a busy web server. Under

many circumstances, performance can be improved by turning popular

dynamic requests into static requests. In this section, two

approaches to this will be discussed.

</p>

<section id="making-popular-pages-static">

<title>Making Popular Pages Static

</title>

<p>By pre-rendering the response pages for the most popular queries

in your application, you can gain a significant performance

improvement without giving up the flexibility of dynamically

generated content. For instance, if your application is a

flower delivery service, you would probably want to pre-render

your catalog pages for red roses during the weeks leading up to

Valentine's Day. When the user searches for red roses,

they are served the pre-rendered page. Queries for, say, yellow

roses will be generated directly from the database. The

mod_rewrite module included with Apache is a great tool to

implement these substitutions.

</p>

<section id="example-a-statically-rendered-blog">

<title>Example: A Statically Rendered Blog

</title>

<!--we should provide a more useful example here.

<p>Blosxom is a lightweight web log package that runs as a CGI.

It is written in Perl and uses plain text files for entry

input. Besides running as CGI, Blosxom can be run from the

command line to pre-render blog pages. Pre-rendering pages

to static HTML can yield a significant performance boost in

the event that large numbers of people actually start

reading your blog.

</p>

<p>To run blosxom for static page generation, edit the CGI

script according to the documentation. Set the $static dir

variable to the <directive>DocumentRoot</directive>

of the web server, and run the script from the command line

as follows:

</p>

<example>$ perl blosxom.cgi -password='whateveryourpassword'

</example>

<p>This can be run periodically from Cron, after you upload

content, etc. To make Apache substitute the statically

rendered pages for the dynamic content, we'll use

mod_rewrite. This module is included with the Apache source

code, but is not compiled by default. It can be built with

the server by passing the option <code>--enable-rewrite[=shared]</code>

to the configure command. Many binary distributions of

Apache come with <module>mod_rewrite </module> included. The following is an

example of an Apache virtual host that takes advantage of

pre-rendered blog pages:

</p>

<highlight language="config">

Listen *:8001

&lt;VirtualHost *:8001&gt;

ServerName blog.sandla.org:8001

ServerAdmin sander@temme.net

DocumentRoot "/home/sctemme/inst/blog/httpd/htdocs"

&lt;Directory "/home/sctemme/inst/blog/httpd/htdocs"&gt;

Options +Indexes

Require all granted

RewriteEngine on

RewriteCond %{REQUEST_FILENAME} !-f

RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule ^(.*)$ /cgi-bin/blosxom.cgi/$1 [L,QSA]

&lt;/Directory&gt;

RewriteLog /home/sctemme/inst/blog/httpd/logs/rewrite_log

RewriteLogLevel 9

ErrorLog /home/sctemme/inst/blog/httpd/logs/error_log

LogLevel debug

CustomLog /home/sctemme/inst/blog/httpd/logs/access_log common

ScriptAlias /cgi-bin/ /home/sctemme/inst/blog/bin/

&lt;Directory "/home/sctemme/inst/blog/bin"&gt;

Options +ExecCGI

Require all granted

&lt;/Directory&gt;

&lt;/VirtualHost&gt;

</highlight>

<p>

The <directive>RewriteCond</directive>

and <directive>RewriteRule</directive>

directives say that, if the requested resource does not

exist as a file or a directory, its path is passed to the

Blosxom CGI for rendering. Blosxom uses Path Info to

specify blog entries and index pages, so this means that if

a particular path under Blosxom exists as a static file in

the file system, the file is served instead. Any request

that isn't pre- rendered is served by the CGI. This

means that individual entries, which show the comments, are

always served by the CGI which in turn means that your

comment spam is always visible. This configuration also

hides the Blosxom CGI from the user-visible URL in their

Location bar. mod_rewrite is a fantastically powerful and

versatile module: investigate it to arrive at a

configuration that is best for your situation.

</p>

</section>

</section>

<section id="caching-content-with-mod_cache">

<title>Caching Content With mod_cache

</title>

<p>The mod_cache module provides intelligent caching of HTTP

responses: it is aware of the expiration timing and content

requirements that are part of the HTTP specification. The

mod_cache module caches URL response content. If content sent

to the client is considered cacheable, it is saved to disk.

Subsequent requests for that URL will be served directly from

the cache. The provider module for mod_cache, mod_disk_cache,

determines how the cached content is stored on disk. Most

server systems will have more disk available than memory, and

it's good to note that some operating system kernels cache

frequently accessed disk content transparently in memory, so

replicating this in the server is not very useful.

</p>

<p>To enable efficient content caching and avoid presenting the

user with stale or invalid content, the application that

generates the actual content has to send the correct response

headers. Without headers like <code>Etag:</code>,

<code>Last-Modified:</code> or <code>Expires:</code>,

<module>mod_cache</module> can not make the right decision on whether to cache

the content, serve it from cache or leave it alone. When

testing content caching, you may find that you need to modify

your application or, if this is impossible, selectively disable

caching for URLs that cause problems. The mod_cache modules are

not compiled by default, but can be enabled by passing the

option <code>--enable-cache[=shared]</code>

to the configure script. If you use a binary distribution of

Apache httpd, or it came with your port or package collection,

it may have <module>mod_cache</module> already included.

</p>

<section id="example-wiki">

<title>Example: wiki.apache.org

</title>

<!-- Is this still the case? Maybe we should give

<p>

The Apache Software Foundation Wiki is served by

MoinMoin. MoinMoin is written in Python and runs as

a CGI. To date, any attempts to run it under

mod_python has been unsuccessful. The CGI proved to

place an untenably high load on the server machine,

especially when the Wiki was being indexed by search

engines like Google. To lighten the load on the

server machine, the Apache Infrastructure team

turned to mod_cache. It turned out MoinMoin needed a

small patch to ensure proper behavior behind the

caching server: certain requests can never be cached

and the corresponding Python modules were patched to

send the proper HTTP response headers. After this

modification, the cache in front of the Wiki was

enabled with the following configuration snippet in

<code>httpd.conf</code>:

</p>

<highlight language="config">

CacheRoot /raid1/cacheroot

CacheEnable disk /

# A page modified 100 minutes ago will expire in 10 minutes

CacheLastModifiedFactor .1

# Always check again after 6 hours

CacheMaxExpire 21600

</highlight>

<p>This configuration will try to cache any and all content

within its virtual host. It will never cache content for

more than six hours (the <directive module="mod_cache">CacheMaxExpire</directive>

directive). If no <code>Expires:</code>

header is present in the response, <module>mod_cache</module> will compute

an expiration period from the <code>Last-Modified:</code>

header. The computation using <directive module="mod_cache">CacheLastModifiedFactor</directive>

is based on the assumption that if a page was recently

modified, it is likely to change again in the near future

and will have to be re-cached.

</p>

<p>

Do note that it can pay off to <em>disable</em>

the <code>ETag:</code>

header: For files smaller than 1k the server has to

calculate the checksum (usually MD5) and then send out a

<code>304 Not Modified</code>

response, which will use up some CPU and still saturate

the same amount of network resources for the transfer (one

TCP packet). For resources larger than 1k it might prove

CPU expensive to calculate the header for each request.

Unfortunately there does currently not exist a way to cache

these headers.

</p>

<highlight language="config">

&lt;FilesMatch "\.(jpe?g|png|gif|js|css|x?html|xml)"&gt;

FileETag None

&lt;/FilesMatch&gt;

</highlight>

<p>

This will disable the generation of the <code>ETag:</code>

header for most static resources. The server does not

calculate these headers for dynamic resources.

</p>

</section>

</section>

</section>

<section id="further-considerations">

<title>Further Considerations

</title>

<p>Armed with the knowledge of how to tune a sytem to deliver the

desired the performance, we will soon discover that <em>one</em>

system might prove a bottleneck. How to make a system fit for

growth, or how to put a number of systems into tune will be

discussed in <a href="http://wiki.apache.org/httpd/PerformanceScalingOut">PerformanceScalingOut</a>.

</p>

</section>

</manualpage>