Apache Performance Tuning

DeveloperSide.NET Articles

Forewarning:

"Premature optimization is the root of all evil." -- Donald Knuth.

...in other words, don't implement in extra complexity if you don't
need it. A site handling a few thousand requests per day will do fine on
a default configuration and just about any hardware. This article is
geared towards a site that needs to handle multiple concurrent requests
[ten to several hundred per second].

General [in order of importance]

RAM

The single biggest issue affecting webserver performance is RAM. Have
as much RAM as your hardware, OS, and funds allow [within reason].

The more RAM your system has, the more processes [and threads] Apache
can allocate and use; which directly translates into the amount of
concurrent requests/clients Apache can serve.

Generally speaking, disk I/O is usually a close 2nd, followed by CPU
speed and network link. Note that a single PII 400 Mhz with 128-256 Megs
of RAM can saturate a T3 (45 Mbps) line.

Select MPM

Chose the right MPM for the right job:

prefork [default MPM for Apache 2.0 and 1.3]:

• Apache 1.3-based.
• Multiple processes, 1 thread per process, processes handle requests.
• Used for security and stability.
• Has higher memory consumption and lower performance over the newer Apache 2.0-based threaded MPMs.

worker:

• Apache 2.0-based.
• Multiple processes, many threads per process, threads handle requests.
• Used for lower memory consumption and higher performance.
• Does not provide the same level of isolation request-to-request, as a process-based MPM does.

winnt:

• The only MPM choice under Windows.
• 1 parent process, exactly 1 child process with many threads, threads handle requests.
• Best solution under Windows, as on this platform, threads are always "cheaper" to use over processes.

Configure MPM

Core Features and Multi-Processing Modules

Default Configuration

<IfModule prefork.c>
 StartServers   8
 MinSpareServers   5
 MaxSpareServers  20
 MaxClients   150
 MaxRequestsPerChild 1000
</IfModule>

<IfModule worker.c>
 StartServers   2
 MaxClients   150
 MinSpareThreads  25
 MaxSpareThreads  75 
 ThreadsPerChild  25
 MaxRequestsPerChild  0
</IfModule>

<IfModule mpm_winnt.c>
 ThreadsPerChild  250
 MaxRequestsPerChild  0
</IfModule>

Directives

MaxClients, for prefork MPM

MaxClients sets a limit on the number of simultaneous connections/requests that will be served.

I consider this directive to be the critical factor to a well
functioning server. Set this number too low and resources will go to
waste. Set this number too high and an influx of connections will bring
the server to a stand still. Set this number just right and your server
will fully utilize the available resources.

An approximation of this number should be derived by
dividing the amount of system memory (physical RAM) available by the
maximum size of an apache/httpd process; with a generous amount spared
for all other processes.

MaxClients ≈ (RAM - size_all_other_processes)/(size_apache_process)

Use 'ps -ylC httpd --sort:rss' to find process size. Divide number by 1024 to get megabytes. Also try 'top'.

Use 'free -m' for a general overview. The key figure to look at is the buffers/cache used value.

Use 'vmstat 2 5' to display the number of runnable, blocked, and waiting processes; and swap in and swap out.

Example:

• System: VPS (Virtual Private Server), CentOS 4.4, with 128MB RAM
• Apache: v2.0, mpm_prefork, mod_php, mod_rewrite, mod_ssl, and other modules
• Other Services: MySQL, Bind, SendMail
• Reported System Memory: 120MB
• Reported httpd process size: 7-13MB
• Assumed memory available to Apache: 90MB

Optimal settings:

• StartServers 5
• MinSpareServers 5
• MaxSpareServers 10
• ServerLimit 15
• MaxClients 15
• MaxRequestsPerChild 2000

With the above configuration, we start with 5-10 processes and set a
top limit of 15. Anything above this number will cause serious swapping
and thrashing under a load; due to the low amount of RAM available to
the [virtual] Server. With a dedicated Server, the default values
[ServerLimit 256] will work with 1-2GB of RAM.

When calculating MaxClients, take into consideration that the
reported size of a process and the effective size are two different
values. In this setup, it might be safe to use 20 or more workers...
Play with different values and check your system stats.

Note that when more connections are attempted than there are workers,
the connections are placed into a queue. The default queue size value
is 511 and can be adjusted with the ListenBackLog directive.

ThreadsPerChild, for winnt MPM

On the Windows side, the only useful directive is ThreadsPerChild,
which is usually set to a value of 250 [defaults to 64 without a value].
If you expect more, or less, concurrent connections/requests, set this
directive appropriately. Check process size with Task Manager, under
different values and server load.

MaxRequestsPerChild

Directive MaxRequestsPerChild is used to recycle processes. When this
directive is set to 0, an unlimited amount of requests are allowed per
process.

While some might argue that this increases server performance by not
burdening Apache with having to destroy and create new processes, there
is the other side to the argument...

Setting this value to the amount of requests that a website generates
per day, divided by the number of processes, will have the benefit of
keeping memory leaks and process bloat to a minimum [both of which are
a common problem]. The goal here is to recycle each process once per
day, as apache threads gradually increase their memory allocation as
they run.

Note that under the winnt MPM model, recycling the only request
serving process that Apache contains, can present a problem for some
sites with constant and heavy traffic.

Requests vs. Client Connections

On any given connection, to load a page, a client may request many
URLs: page, site css files, javascript files, image files, etc.

Multiple requests from one client in rapid succession can have the
same effect on a Server as "concurrent" connections [threaded MPMs and
directive KeepAlive taken into consideration]. If a particular website
requires 10 requests per page, 10 concurrent clients will require MPM
settings that are geared more towards 20-70 clients. This issue
manifests itself most under a process-based MPM [prefork].

Separate Static and Dynamic Content

Use separate servers for static and dynamic content. Apache processes
serving dynamic content will carry overhead and swell to the size of
the content being served, never decreasing in size. Each process will
incur the size of any loaded PHP or Perl libraries. A 6MB-30MB process
size [or 10% of server's memory] is not unusual, and becomes a waist of
resources for serving static content.

For a more efficient use of system memory, either use mod_proxy to
pass specific requests onto another Apache Server, or use a lightweight
server to handle static requests:

• lighttpd [has experimental win32 builds]
• tux [patched into RedHat, runs inside the Linux kernel and is at the top of the charts in performance]

The Server handling the static content goes up front.

Note that configuration settings will be quite different between a dynamic content Server and a static content Server.

mod_deflate

Reduce bandwidth by 75% and improve response time by using mod_deflate.


LoadModule deflate_module modules/mod_deflate.so
<Location />
AddOutputFilterByType DEFLATE text/html text/plain text/css text/xml application/x-javascript
</Location>


Loaded Modules

Reduce memory footprint by loading only the required modules.

Some also advise to statically compile in the needed modules, over building DSOs (Dynamic Shared Objects). Very bad advice.
You will need to manually rebuild Apache every time a new version or
security advisory for a module is put out, creating more work, more
build related headaches, and more downtime.

mod_expires

Include mod_expires for the ability to set expiration dates for
specific content; utilizing the 'If-Modified-Since' header cache control
sent by the user's browser/proxy. Will save bandwidth and drastically
speed up your site for [repeat] visitors.

Note that this can also be implemented with mod_headers.

KeepAlive

Enable HTTP persistent connections to improve latency times and
reduce server load significantly [25% of original load is not uncommon].

prefork MPM:


KeepAlive On
KeepAliveTimeout 2
MaxKeepAliveRequests 80


worker and winnt MPMs:


KeepAlive On
KeepAliveTimeout 15
MaxKeepAliveRequests 80


With the prefork MPM, it is recommended to set 'KeepAlive' to 'Off'.
Otherwise, a client will tie up an entire process for that span of time.
Though in my experience, it is more useful to simply set the
'KeepAliveTimeout' value to something very low [2 seconds seems to be
the ideal value]. This is not a problem with the worker MPM
[thread-based], or under Windows [which only has the thread-based winnt
MPM].

With the worker and winnt MPMs, the default 15 second timeout is
setup to keep the connection open for the next page request; to better
handle a client going from link to link. Check logs to see how long a
client remains on each page before moving on to another link. Set value
appropriately [do not set higher than 60 seconds].

SymLinks

Make sure 'Options +FollowSymLinks -SymLinksIfOwnerMatch' is set for
all directories. Otherwise, Apache will issue an extra system call per
filename component to substantiate that the filename is NOT a symlink;
and more system calls to match an owner.


<Directory />
Options FollowSymLinks
</Directory>


AllowOverride

Set a default 'AllowOverride None' for your filesystem. Otherwise,
for a given URL to path translation, Apache will attempt to detect an
.htaccess file under every directory level of the given path.


<Directory />
AllowOverride None
</Directory>


ExtendedStatus

If mod_status is included, make sure that directive 'ExtendedStatus'
is set to 'Off'. Otherwise, Apache will issue several extra time-related
system calls on every request made.


ExtendedStatus Off


Timeout

Lower the amount of time the server will wait before failing a request.

Timeout 45

Other/Specific

Cache all PHP pages, using Squid, and/or a PHP Accelerator and
Encoder application, such as APC. Also take a look at mod_cache under
Apache 2.2.

Convert/pre-render all PHP pages that do not change
request-to-request, to static HTML pages. Use 'wget' or 'HTTrack' to
crawl your site and perform this task automatically.

Pre-compress content and pre-generate headers for static pages;
send-as-is using mod_asis. Can use 'wget' or 'HTTrack' for this task.
Make sure to set zlib Compression Level to a high value (6-9). This will
take a considerable amount of load off the server.

Use output buffering under PHP to generate output and serve requests without pauses.

Avoid content negotiation for faster response times.

Make sure log files are being rotated. Apache will not handle large (2gb+) files very well.

Gain a significant performance improvement by using SSL session cache.

Outsource your images to Amazon's Simple Storage Service (S3).

Measuring Web Server Performance

Load Testing

Apache HTTP server benchmarking tool
httperf
The Grinder, a Java Load Testing Framework

Benchmarks

I have searched extensively for Apache, lighttpd, tux, and other
webserver benchmarks. Sadly, just about every single benchmark I could
locate appeared to have been performed completely without thought, or
with great bias.

Do not trust any posted benchmarks, especially ones done with the 'ab' tool.

The only way to get a valid report is to perform the benchmark yourself.

For valid results, note to test under a system with limited
resources, and maximum resources. But most importantly, configure each
httpd server application for the specific situation.

Information

System Tuning Info for Linux Servers