Performance Scaling +

Available Languages: en

+ + +

The Performance Tuning page in the Apache 1.3 documentation says: +

“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.” +

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. +

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. +

+ +

What Will and Will Not Be Discussed +
Monitoring Your Server +
Configuring for Performance +
Caching Content +
Further Considerations +

What Will and Will Not Be Discussed +

+ +

The session will focus on easily accessible configuration and tuning + options for Apache httpd 2.2 and 2.3 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. +

+ +

Monitoring Your Server +

+ +

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. +

+ + +

Monitoring Tools +

+ + + +

top +

+ +

The top tool ships with Linux and FreeBSD. Solaris offers + `prstat'. 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: +

+ +

+ 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 +

+ +

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 ' + sizing MaxClients + + '. Top is, however, an interactive tool and running it + continuously has few if any advantages. +

+ +

free +

+ +

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 ' + Sizing MaxClients + + '. The output of free looks like this: +

+ +

+ sctemme@brutus:~$ free + total used free shared buffers cached + Mem: 4026028 3901892 124136 0 253144 + 841044 + -/+ buffers/cache: 2807704 1218324 + Swap: 3903784 12540 3891244 +

+ + +

vmstat +

+ +

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, + vmstat 5 + + 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. +

+ The following is vmstat + + output of an idle server: +

+ + +

+ [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 i + 0 0 0 0 186252 6688 37516 0 0 12 5 47 + 311 0 1 9 + 0 0 0 0 186244 6696 37516 0 0 0 16 41 + 314 0 0 10 + 0 0 0 0 186236 6704 37516 0 0 0 9 44 + 314 0 0 100 +

+ +

And this is output of a server that is under a load of one + hundred simultaneous connections fetching static content: +

+ +

+ 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 +

+ +

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. +

+ + + +

SE Toolkit +

+ +

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. +

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. +

+ + + +

DTrace +

+ +

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. +

+ + + +

mod_status +

+ +

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 mod_status on its own + web site + + .If you put the ExtendedStatus On + + directive in your httpd.conf + + ,the mod_status + + page will give you more information at the cost of a little + extra work per request. +

+ + + + +

Web Server Log Files +

+ +

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. +

+ + +

Error Log +

+ +

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. +

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 + /var/log/messages + + 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. +

+ The Error Log is configured through the ErrorLog + + and LogLevel + + 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 ErrorLog + + 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. +

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 + mod_security + + ,but this falls outside the scope of this discussion. +

+ The LogLevel + + directive determines the level of detail included in the + logs. There are eight log levels as described here: +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

+ + Level + + +	+ + Description + + +
+ emerg + +	+ Emergencies - system is unusable. + +
+ alert + +	+ Action must be taken immediately. + +
+ crit + +	+ Critical Conditions. + +
+ error + +	+ Error conditions. + +
+ warn + +	+ Warning conditions. + +
+ notice + +	+ Normal but significant condition. + +
+ info + +	+ Informational. + +
+ debug + +	+ Debug-level messages + +

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 LogLevel + + can be specified on a per module basis: +

+ +

+ LogLevel debug mod_ssl:warn +

+ +

+ This puts all of the server in debug mode, except for + mod_ssl + + ,which tends to be very noisy. +

+ + + +

Access Log +

+ +

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 TransferLog + + or CustomLog + + configuration directive. +

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: +

+ + +

+ 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 +

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

+ + Field + + +	+ + Content + + +	+ + Explanation + + +
+ Client IP + +	+ 195.54.228.42 + +	+ IP address where the request originated + +
+ RFC 1413 ident + +	+ - + +	+ Remote user identity as reported by their + identd + +
+ username + +	+ - + +	+ Remote username as authenticated by Apache + +
+ timestamp + +	+ [24/Mar/2007:23:05:11 -0400] + +	+ Date and time of request + +
+ Request + +	+ "GET /sander/feed/ HTTP/1.1" + +	+ Request line + +
+ Status Code + +	+ 200 + +	+ Response code + +
+ Content Bytes + +	+ 9747 + +	+ Bytes transferred w/o headers + +

+ + +

Rotating Log Files +

+ +

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. +

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: +

+ + +

+ APACHE=/usr/local/apache2 + HTTPD=$APACHE/bin/httpd + mv $APACHE/logs/access_log + $APACHE/logarchive/access_log-‘date +%F‘ + $HTTPD -k graceful +

+ +

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. +

+ A second approach is to use piped logs. From the + CustomLog + + ,TransferLog + + or ErrorLog + + directives you can send the log data into any program using + a pipe character (| + + ). For instance: +

+ +

CustomLog "|/usr/local/apache2/bin/rotatelogs + /var/log/access_log 86400" common +

+ +

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. +

+ + + +

Logging and Performance +

+ +

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. +

+ Do not run a production server with your error + LogLevel + + 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. +

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 %v + + format character at the start of your + LogFormat + + and starting 2.3.8 of your ErrorLogFormat + + 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 support/split-logfile + + . +

+ You can use the BufferedLogs + + 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. +

+ + + + +

Generating A Test Load +

+ +

It is useful to generate a test load to monitor system + performance under realistic operating circumstances. Besides + commercial packages such as + LoadRunner + + ,there are a number of freely available tools to generate a + test load against your web server. +

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. +
Another freely available load generator is http load11 . + This program works with a URL file and can be compiled with + SSL support. +
The Apache Software Foundation offers a tool named flood12 + . Flood is a fairly sophisticated program that is + configured through an XML file. +
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. +
+
ASF external projects, that have proven to be quite + good: grinder, httperf, tsung, + FunkLoad + +
+

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. +

+ + + +

Configuring for Performance +

+ + + +

Apache Configuration +

+ +

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 + 2.3+ MPMs are no longer hard-wired. They too can be exchanged + via LoadModule + + .The default MPM in 2.3 is the event MPM. +

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. +

+ + +

MaxClients +

+ +

+ The MaxClients + + directive in your Apache httpd configuration file specifies + the maximum number of workers your server can create. It + has two related directives, MinSpareServers + + and MaxSpareServers + + ,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 + ServerLimit + + directive. +

+ + + +

Spinning Threads +

+ +

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 + ThreadsPerChild + + directive1 . Apache requires that MaxClients + + is evenly divisible by ThreadsPerChild + + .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 + ThreadsPerChild + + value downwards until it is an even factor of + MaxClients + + . +

+ + + +

Sizing MaxClients +

+ +

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 MaxClients + + is fairly simple: +

+ +

+ total RAM − RAM for OS − RAM for external programs + MaxClients = + ------------------------------------------------------- + RAM per httpd process +

+ +

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. +

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. +

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 MaxClients + + ,but it is not an exact science. When in doubt, be + conservative and use a low MaxClients + + 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 + MaxClients + + value may actually be a disadvantage. +

+ + + +

Selecting your MPM +

+ +

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. +

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 "thread-safety" 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. +

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. +

+ + + +

Spinning Locks +

+ +

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 "holds" 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 MaxSpareServers + + directive. +

+ + + +

The Thundering Herd +

+ +

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. +

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 "ball" to the + appropriate player process. +

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. +

+ You can manipulate the accept mutex with the + AcceptMutex + + 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. +

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. +

+ + + + +

Tuning the Operating System +

+ +

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. +

+ + +

RAM and Swap Space +

+ +

The usual mantra regarding RAM is "more is + better". 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. +

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. +

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 MaxClients + + to keep things in hand. +

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 /etc/fstab + + ,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. +

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 mkswap + + and swapon + + or swap + + programs. +

+ + + +

ulimit: Files and Processes +

+ +

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. +

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. +

However, you can change this limit with the command +

+ +

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

+ +

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, csh + + the equivalent is limit + + and works analogous the the Bourne-like ulimit + + : +

+ +

+ limit [-h] maxproc [newvalue] +

+ +

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 +

+ +

ulimit -n [newvalue] +

+ +

command. Once again, this must be done prior to starting + Apache. +

+ + + +

Setting User Limits on System Startup +

+ +

Under Linux, you can set the ulimit parameters on bootup by + editing the /etc/security/limits.conf + + 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 /etc/pam.d/login + + contains the line +

+ +

session required /lib/security/pam_limits.so +

+ +

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. +

+ In FreeBSD's /etc/login.conf + + these resources can be limited or extended system wide, + analogously to limits.conf + + .'Soft' limits can be specified with -cur + + and 'hard' limits with -max + + . +

Solaris has a similar mechanism for manipulating limit + values at boot time: In /etc/system + + you can set kernel tunables valid for the entire system at + boot time. These are the same tunables that can be set with + the mdb + + kernel debugger during run time. The soft and hard limit + corresponding to ulimit -u can be set via: +

+ +

+ set rlim_fd_max=65536 + set rlim_fd_cur=2048 +

+ +

Solaris calculates the maximum number of allowed processes + per user (maxuprc + + )based on the total amount available memory on the system (+ maxusers + + ). You can review the numbers with +

+ +

sysdef -i | grep maximum +

+ +

but it is not recommended to change them. +

+ + + +

Turn Off Unused Services and Modules +

+ +

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. +

On Red Hat Linux, the chkconfig tool will help you do this + from the command line. On Solaris systems svcs + + and svcadm + + will show which services are enabled and disable them + respectively. +

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 + LoadModule + + directive. +

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 + LoadModule + + 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. +

+ + + + +

Caching Content +

+ +

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. +

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. +

+ + +

Making Popular Pages Static +

+ +

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. +

+ + +

Example: A Statically Rendered Blog +

+ +

+ 'we should provide a more useful example here. + One showing how to make Wordpress or Drupal suck less. + + ' +

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. +

To run blosxom for static page generation, edit the CGI + script according to the documentation. Set the $static dir + variable to the DocumentRoot + + of the web server, and run the script from the command line + as follows: +

+ +

$ perl blosxom.cgi -password='whateveryourpassword' +

+ +

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 + --enable-rewrite[=shared] + + to the configure command. Many binary distributions of + Apache come with mod_rewrite included. The following is an + example of an Apache virtual host that takes advantage of + pre-rendered blog pages: +

+ +

Listen *:8001 + <VirtualHost *:8001> + + ServerName blog.sandla.org:8001 + ServerAdmin sander@temme.net + DocumentRoot "/home/sctemme/inst/blog/httpd/htdocs" + <Directory + "/home/sctemme/inst/blog/httpd/htdocs"> + + Options +Indexes + Order allow,deny + Allow from all + RewriteEngine on + RewriteCond %{REQUEST_FILENAME} !-f + RewriteCond %{REQUEST_FILENAME} !-d + RewriteRule ^(.*)$ /cgi-bin/blosxom.cgi/$1 [L,QSA] + + </Directory> + 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/ + <Directory "/home/sctemme/inst/blog/bin"> + + Options +ExecCGI + Order allow,deny + Allow from all + + </Directory> + + </VirtualHost> +

+ +

+ The RewriteCond + + and RewriteRule + + 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. +

+ + + + +

Caching Content With mod_cache +

+ +

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. +

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 Etag: + + ,Last-Modified: + + or Expires: + + ,mod_cache 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 --enable-cache[=shared] + + 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 mod_cache already included. +

+ + +

Example: wiki.apache.org +

+ +

+ 'Is this still the case? Maybe we should give + a better example here too. + +

+ 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 httpd.conf + + : +

+ +

+ 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 +

+ +

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 CacheMaxExpire + + directive). If no Expires: + + header is present in the response, mod_cache will compute + an expiration period from the Last-Modified: + + header. The computation using CacheLastModifiedFactor + + 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. +

+ Do note that it can pay off to disable + + the ETag: + + header: For files smaller than 1k the server has to + calculate the checksum (usually MD5) and then send out a + 304 Not Modified + + response, which will take waste 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. +

+ <FilesMatch \.(jpe?g|png|gif|js|css|x?html|xml)> + + FilesETag None + + </FilesMatch> +

+ +

+ This will disable the generation of the ETag: + + header for most static resources. The server does not + calculate these headers for dynamic resources. +

+ + + + +

Further Considerations +

+ +

Armed with the knowledge of how to tune a sytem to deliver the + desired the performance, we will soon discover that one + + 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 + PerformanceScalingOut + + . +