How directives behave and which directives are mainly available hinges on the loaded MPM. MPM is short for MultiProcess Modules, and they determine the basis for how Apache addresses multiprocessing. We will cover the following subsections:
- General Optimization
- Event/Worker Optimization
- Prefork Optimization
Each part will focus on how the directives affect performance for their respective MPM and some common considerations that should be assessed when optimizing Apache with those specific MPMs.
1. General Optimization
IfModule
An important directive to learn when working with Apache servers is the IfModule conditional statement. There are two parts to the IfModule statement. A beginning, which also accepts a module name or module source file name, as well as a closing statement. When the provided module is loaded into Apache, then all directives between the beginning IfModule statement and the closing IfModule statement are also read into the Apache running configuration. Please review the provided example below for further clarification:
<ifModule mpm_prefork_module>
MaxSpareServers 16
</ifModule>
Timeout 60
The above example defines the MaxSpareServers directive only when loaded by mpm_prefork_module. The Timeout directive is applied every time due to it being outside of the IfModule closing statement.
IfModule statements are used to maintain compatibility within Apache configuration between module changes. Maintaining compatibility is done by grouping directives into IfModule statements, so they are only used when the required module is loaded. Ensuring a syntactically correct configuration file even when swapping modules.
Appropriately wrapping everything in an IfModule statement is a best practice standard with Apache and should be adhered to for superior compatibility in config files.
Timeout
The numerical value of seconds Apache waits for all common I/O events. Apache will abandon requests that fail to complete before the provided Timeout value.
Determining the right Timeout depends on both traffic habits and hosted applications. Ideally, the Timeout should be as low as possible while still allowing the vast majority of regular traffic to operate without issue. Large timeouts, those above 1 minute, open the server to SlowLoris style DOS attacks and foster a long wait in the browser when it encounters a problem. Lower timeouts allow Apache to recover from errant stuck connections quickly. It becomes necessary to strike a balance between the two extremes.
Avoid increasing the global Timeout when addressing issues with a single script, or user, that requires a long Timeout. Problems can usually be resolved by a .htaccess file or include a file to increase the Timeout directive for that specific script.
KeepAlive
A simple on|off toggle enables the KeepAlive protocols with supported browsers. The KeepAlive feature can provide as much as a 50% reduction in latency, significantly boosting the performance of Apache. KeepAlive accomplishes this by reusing the same initial connections a browser creates when connecting to Apache for all follow-up requests which occur within a short period.
KeepAlive is a powerful feature and in general, should be enabled in most situations. It works great for reducing some of the CPU and Network overhead with modern element-heavy websites. For example, an easy way to visualize KeepAlive is with the “hold the door” phrase. Imagine a queue of people entering a building through a single doorway. Each person is required to open the door, walk through it, then close the door before the next person does the same process. Mostly, that’s how Apache works without KeepAlive. When enabled, the door stays open until all the people in line are through the door before it closes again.
Two additional related directives also govern KeepAlive. MaxKeepAliveRequests and KeepAliveTimeout. As discussed in the next section, each one plays a vital role in fine-tuning the KeepAlive directive.
MaxKeepAliveRequests
Sets a limit on the number of requests an individual KeepAlive connection is permitted to handle. Once reached, Apache forces the connection to terminate, and creates a new one for any additional requests.
Determining an ideal setting here is open to interpretation. Generally, you want this value to be at least as high as the largest count of elements (HTML, Text, CSS, Images, Etc..) served by the most heavily trafficked pages on the server.
Set MaxKeepAliveRequests to double that of the largest count of elements on common pages. (Services like webpagetest.org or gtmetrix.com can count elements on a page).
KeepAliveTimeout
This directive is measured in seconds and will remain idle waiting for additional requests from its initiator. Since these types of connections are only accessible to their initiator, we want to keep KeepAliveTimeout very low. A low value prevents too many KeepAlive connections from locking out new visitors due to connection priority.
A large MaxKeepAliveRequests directive with a very low KeepAliveTimeout allows active visitors to reuse connections while also quickly recovering threads from idle visitors. Configuration: Set MaxKeepAliveRequests to 500+, Set KeepAliveTimeout to 2 Requirements: Works best on MPM Event.
2. MPM Event/Worker Optimization
This section details the use and performance considerations that are essential when running Worker based MPMs, including both MPM Event and MPM Worker. These MPMs are considered multi-threaded solutions and some directives behave differently based on the loaded MPM. The information provided in this section is only a portion about Worker based MPMs.
In Worker-based MPMs: ServerLimit, ThreadsPerChild, and MaxRequestWorkers are intrinsically linked with each other. It is essential to understand the role of each one and how changing one affects the others. The following directives govern the fine-tuning of the thread handling capabilities of Apache web servers.
MPM Worker and MPM Event
The two modules, MPM Event, and MPM Worker for most intents and purposes operate identically. The difference is apparent in the way each handles KeepAlive requests. The MPM Worker locks threads for the duration of the KeepAlive process and directly affects the number of available threads able to handle new requests. The MPM Event uses a Listener thread for each child. These Listener threads handle standard requests, and KeepAlive requests alike meaning thread locking will not reduce the capacity of the server. Without thread locking, MPM Event is the superior choice but only in Apache 2.4. Before Apache 2.4 the MPM Event was unstable and prone to problems.
ServerLimit
ServerLimit represents the upper limit of children Apache is allowed. The practical usage for ServerLimit is creating a hard ceiling in Apache to protect against input errors with MaxRequestWorkers. The cap prevents spawning vastly more children than a system can handle, resulting in downtime, revenue loss, reputation loss, or even data loss.
ServerLimit ties in directly with the thrashing point discussed earlier in this article. The thrashing point is the maximum number of children Apache can run before memory usage tips the scale into perpetual swap. Match the ServerLimit to the calculated thrashing point to safeguard the server.
ThreadsPerChild
Used to define the limit of threads that each Apache child can manage. Every thread running can handle a single request. The default of 25 works well for most cases and is a fair balance between children and threads.
There is an upper limit on this directive as well, and the limit is controlled by the ThreadLimit directive, which defaults to 64 threads. The adjustments to increase ThreadsPerChild past 64 threads also need to be made to ThreadLimit.
Increasing this value allows each child to handle more requests keeping memory consumption down while allowing a larger MaxRequestWorkers directive. A key benefit of running more threads within each child is shared memory cache access. Threads from one child cannot access caches from another child. Boosting the number of threads per child squeezes out more performance due to this sharing of cache data. The major downside for increased threads per child occurs during child recycling. The capacity of the server is diminished by the number of threads configured for each child when that child process is eventually recycled (graceful restart).
Inversely the opposite reaction is achieved by lowering ThreadsPerChild. Fewer threads per child require more children to run an equal amount of MaxRequestWorkers. Since children are full copies of Apache, this increases Apache’s overall memory footprint but reduces the impact when recycling children. Fewer threads mean fewer potential “stuck” threads during the recycle procedure, keeping the higher capacity of requests available overall children. Having fewer threads per child provides increased shared memory isolation. For instance, dropping ThreadsPerChild to 1 gives the same request isolation of MPM Prefork but inherits its massive performance tax as well, requiring one child per one request.
When setting ThreadsPerChild always consider the server environment and hardware.
- A memory-heavy shared server hosting numerous independent accounts might opt for a lower ThreadsPerChild, reducing the potential impact of one user affecting another.
- A dedicated Apache server in a high capacity load-balanced configuration can choose to increase ThreadsPerChild significantly for a better overall performance of each thread.
ThreadLimit
Used to set the maximum value of ThreadsPerChild. This directive is a hard ceiling for ThreadsPerChild. It helps protect against typographical errors with the ThreadsPerChild directive which could quickly spin a server out of control if too many threads are allowed due to an input error. This setting needs to be adjusted in some high-end servers when the system needs more than the default of 64 threads per child.
MaxRequestWorkers / MaxClients
The directive sets the limit for active worker threads across all running children and acts as a soft ceiling with ServerLimit taking control as the hard limit. When the number of total running threads has reached or exceeded MaxRequestWorkers, Apache no longer spawns new children.
Determining the MaxRequestWorkers is a critical part of server optimization. An optimal setting is based on several changing variables. This means its configuration needs to be reevaluated and tailored periodically over time, changed by watching traffic habits and system resource usage. The Apache status Scoreboard is an effective tool for the analysis of Apache performance.
It is typical of Worker based MPM systems to run an isolated third-party PHP handler like Mod_fcgid, PHP-FPM, and mod_lsapi. These modules are responsible for processing PHP code outside of Apache and frees up Apache to handle all other non-PHP requests such as HTML, TEXT, CSS, Images, etc… These requests are far less taxing on server resources which allows Apache to handle larger volumes of requests, such as those beyond 400 MaxRequestWorkers.
MinSpareThreads
The least number of Threads that should remain open, waiting for new requests. MinSpareThreads is a multiple of ThreadsPerChild and cannot exceed MaxSpareThreads, though it can match it.
Set MinSpareThreads to equal 50% of MaxRequestWorkers.
Spare threads are idle workers' threads. These threads are merely waiting for new incoming requests and are governed by the Apache child process that spawned them. If there are fewer available threads than MinSpareThreads, The Apache parent will generate a new child with another ThreadsPerChild worth of threads.
MaxSpareThreads
This directive governs the total number of idle threads allowed on the server across all children. Any threads above this limit direct their parent to shut down to reduce memory consumption during off-peak hours.
Having a limit to the number of idle open threads is excellent for smaller servers with hardware constraints. However, it mostly unneeded on today’s modernizing hardware.
Configuring Apache as an open throttle is a high-performance configuration for servers with significant RAM and multiple CPU cores. When running the open throttle configuration, all available threads become available at all times. Apache’s memory usage will stay near its peak at all times, a side effect due to running all the configured children into memory preemptively. This configuration will produce the best possible response times from Apache by maintaining persistent open connections ready to do work and removing the overhead of spawning new processes in response to traffic surges. Match both MinSpareThreads and MaxSpareThreads to MaxRequestWorkers and make sure there is enough server RAM to run all MaxRequestWorkers at once.
StartServers
This directive governs the initial amount of children the Apache Parent process spawns when the Apache service is started or restarted. This is commonly left unchanged since Apache continuously checks the current running children in conjunction with ThreadsPerChild and compares it to MinSpareThreads to determine if more children get forked. This process is repeated perpetually, with a doubling of new children on each iteration, until MinSpareThreads is satisfied.
Manually calculating StartServers is done by dividing MaxRequestWorkers by ThreadsPerChild, rounding down to the nearest whole number. This process forces all children to be created without delay at startup and begins handling requests immediately. This aspect is especially useful in modern Apache servers which require periodic restarts to load in directive changes.
MaxConnectionsPerChild / MaxRequestsPerChild
The number of requests a single Apache child process can handle equals a cumulative total on the child server across all threads it controls. Each request handled by a thread counts toward this limit to its parent. Once the child server has reached its limit, the child is then recycled.
This directive is a stop-gap for accidental memory leaks. Some code executed through Apache threads may contain memory leaks. Leaked memory is portions of memory that subprocess failed to release properly, so they are inaccessible to any outside processes. The longer a leaking program is left running, the more memory it will leak. Setting a MaxConnectionsPerChild limit is a specific method for assuring Apache is periodically recycling programs to reduce the impact of leaked memory on the system. When using external code handlers like Mod_fcgid, PHP-FPM, or mod_lsapi, it becomes necessary to set MaxConnectionsPerChild to 0 (unlimited), doing so prevents periodic error pages caused by Apache terminating threads prematurely.
If the server encounters a memory leak never set the MaxConnectionsPerChild / MaxRequestsPerChild too low, instead start with 10,000 and reduce it incrementally.
3. MPM Prefork Optimization
This MPM Prefork section details the use and performance considerations for various directives when running this module. This MPM is a non-threaded multi-processor designed for compatibility. It consists of a single Apache parent process, which is used to govern all-new Apache processes also known as children. The following directives show how Apache is capable of performance tuning when using MPM Prefork. Unlike Worker based MPMs, optimizing MPM Prefork is generally simple and straightforward. There is a 1:1 ratio of Apache processes to incoming requests. However, MPM Prefork does not scale well with hardware, and the more traffic it encounters, the more hardware it will need to keep up with the pace. It should be noted that some directives behave differently based on which MPM is loaded. The information provided in this section is only the portion about MPM Prefork.
MaxRequestWorkers / MaxClients
Used to control the upper limit of children that the Apache parent server is allowed to have in memory at one time. These children (also called workers) handle requests on a 1:1 ratio. This translates into the maximum number of simultaneous requests the server can handle.
If this directive is too low, Apache under-utilizes the available hardware which translates to wasted money and long delays in page load times during peak hours. Alternatively, if this directive is too high, Apache outpaces the underlying hardware sending the system into a thrashing scenario which can lead to server crashes and potential data loss.
MinSpareServers
This directive defines a minimum number of spare children the Apache parent process can maintain in its memory. An additional server is a preforked idle Apache child that is ready to respond to a new incoming request. Having idle children waiting for new requests is essential for providing the fastest server response times. When the total idle children on the server drop below this value, a new child is preforked at the rate of one per second until this directive is satisfied. The “one per second” rule is in place to prevent surges of the creation process that overload the server, however, this failsafe comes at a cost. The one-per-second spawn rate is particularly slow when it comes to handling page requests. So it’s highly beneficial to make sure enough children are preforked and ready to handle incoming requests.
Never set this to zero. Setting this to 25% of MaxRequestWorkers ensures plenty of resources are ready and waiting for requests.
MaxSpareServers
MasSpareServers controls the maximum number of idle Apache child servers running at one time. An idle child is one who is not currently handling a request but waiting for a new request. When there are more than MaxSpareServers idle children, Apache kills off the excess.
If the MaxSpareServers value is less than MinSpareServers, Apache will automatically adjust MaxSpareServers to equal MinSpareServers plus one.
Like with MinSpareServers, this value should always be altered with available server resources in mind.
The rule of thumb is to set this to double the value of MinSpareServers.
Configuring Apache as an open throttle is a high-performance configuration for servers with significant RAM and multiple CPU cores. When running the open throttle configuration, all available Apache children become available at all times. As a side effect of running open throttle, the Apache memory usage will stay near its peak at all times, due to running all the configured children into memory preemptively. This configuration will produce the best possible response times by maintaining persistent open connections. Furthermore, in response to traffic surges, it removes the overhead that comes from spawning new processes. Match both MinSpareServers and MaxSpareServers to MaxRequestWorkers and make sure there is enough server RAM to run all MaxRequestWorkers at once.
StartServers
Created at startup, are the initial amount of Apache child servers.
This seldom changed directive only impacts Apache startup and restart processes. Generally not altered because Apache uses internal logic to work out how many child servers should be running.
Many modern servers periodically restart Apache to address configuration changes, rotate log files or other internal processes. When this occurs during a high load traffic surge, every bit of downtime matters. You can manually set the StartServers directive to mirror that of your MinSpareServers to shave off time from the Apache startup.
The StartServers directive should mirror that of MinSpareServers.
ServerLimit
The ServerLimit directive represents the upper limit of MaxRequestWorkers. This setting is generally used as a safeguard or ceiling against input errors when modifying MaxRequestWorkers.
It becomes necessary to adjusted ServerLimit when the server is expected to handle more than the default of 256 requests simultaneously.
ServerLimit ties in directly with the thrashing point. The thrashing point is the maximum number of children Apache can run before memory usage tips the scale into perpetual swap. Match the ServerLimit to the calculated thrashing point to safeguard the server.
Increasing ServerLimit is not recommended with MPM Prefork. Running more than 256 simultaneous requests is hardware intensive when using the MPM Prefork module.
MaxConnectionsPerChild / MaxRequestsPerChild
This directive equals the number of requests a single Apache child server can handle.
This directive is a stop-gap for accidental memory leaks. Code executed through Apache may contain faults that leak memory. These leaks add up over time making less and less of the shared memory pool of the child usable. The way to recover from leaked memory is to recycle the affected Apache child process. Setting a MaxConnectionsPerChild limit will protect from this type of memory leakage.
The rule of thumb is never to set this too low. If the server encounters memory leak issues start with 10,000 and reduce incrementally.