Apache MPM Introduction

What is MPM?

MPM (Multi-processing modules, multiplexing module) is the core feature that affects performance in apache2.x.

 

It is a pluggable concurrency model supported by Apache 2.x. during compilation, we can select only one concurrency model.

Configuration File:/usr/local/apache2/CONF/extra/httpd-mpm.conf

If Apache is installed by default, it may be configured in the httpd. conf file. Find the configuration according to the actual situation.

Format:
Go to the Apache directory and compile Apache as follows:

Linux code

1. ./Configure -- help | grep mpm

The displayed content is as follows:

Linux code

1. -- With-MPM = mpm
2. Choose the process model for Apache to use.
3. MPM = {BEOs | worker | prefork | mpmt_os2 | perchild | leader | threadpool}

1. BEOs and mpmt_os2 are the default MPM on BEOs and OS/2 respectively.

2. perchild is designed to run different sub-processes with different identities of users and groups, which is particularly useful when running Multiple CGI virtual hosts, it will do better than the suexec mechanism in version 1.3.

3. The leader and threadpool are both worker-based variants and are still in the experimental stage. In some cases, they will not work as expected, so Apache is not officially recommended.

4. If prefork explicitly specifies an MPM without "-with-MPM", prefork is the default MPM on Linux/UNIX platforms. the pre-Dispatch process method adopted by Apache 1.3 is also used. prefork itself does not use threads. Version 2.0 uses it to maintain compatibility with version 1.3. On the other hand, prefork uses a separate sub-process to process different requests, processes are independent from each other, which makes them one of the most stable MPM.How prefork worksYes. After the control process creates a "startservers" sub-process, it creates a process to meet the needs of the minspareservers settings. Wait for one second to create two more processes. Wait for another second to create four more processes ...... In this way, the number of Created processes is increased exponentially, up to 32 processes per second until the value set by minspareservers is met. This is the origin of prefork. This mode does not have to generate new processes when requests arrive, thus reducing system overhead and increasing performance.
5. Compared with prefork, worker is a brand new MPM supporting multi-thread and multi-process hybrid model in 2.x. Because threads are used for processing, a relatively large number of requests can be processed, and the overhead of system resources is smaller than that of process-based servers. however, worker also uses multiple processes, and each process generates multiple threads to obtain the stability of the process server. this MPM will work in Apache 2. X development trend.Working principle of workerYes. A "startservers" sub-process is generated by the main control process. Each sub-process contains a fixed number of threadsperchild threads, and each thread processes Requests independently. Similarly, in order not to generate a thread when the request arrives, minsparethreads and maxsparethreads set the minimum and maximum number of Idle threads, while maxclients sets the total number of threads in all sub-processes. if the total number of threads in the existing sub-process cannot meet the load, the control process will derive a new sub-process.

How can I determine which MPM module is used by the current server?If prefork is used, after make compilation and make install, "httpd-L" is used to determine the currently used mpm, as shown below: [Aaron @ webslave1 extra] $/usr/local/apache2/bin/httpd-lcompiled in modules: core. C ...... prefork. C ...... the prefork. C (if you see worker. C indicates that worker MPM is used, and so on ). Check the default httpd. conf configuration file, which contains the following configuration sections:
Linux code

1. <Ifmodule prefork. c>
2. Startservers 5
3. Minspareservers 5
4. Maxspareservers 10 maximum number of idle Processes
5. Maxclients 150 the number of concurrent connections that can be processed simultaneously by Apache (the most important !!
6. Maxrequestsperchild 0 Number of requests that each sub-process can process
7. </Ifmodule>

Maxspareservers sets the maximum number of idle processes. If the number of idle processes is greater than this value, Apache automatically kill unnecessary processes. This value should not be too large, but if it is smaller than minspareservers, Apache will automatically adjust it to minspareservers + 1. If the site load is large, consider increasing minspareservers and maxspareservers at the same time.

Maxrequestsperchild sets the number of requests that each sub-process can process. Each sub-process is automatically destroyed after processing "maxrequestsperchild" requests. 0 means that the sub-process will never be destroyed. Although the default value is 0, each sub-process can process more requests, setting it to a non-zero value also has two important advantages:

This prevents accidental Memory leakage;
When the server load drops, the number of sub-processes is automatically reduced.

Therefore, you can adjust this value based on the server load. I think about 10000 is suitable.

Maxclients is the most important of these commands. It sets the request that Apache can process at the same time and is the parameter that has the greatest impact on Apache performance. The default value of 150 is far from enough. If the total number of requests has reached this value (you can confirm it through PS-Ef | grep HTTP | WC-l), the subsequent requests will be queued, until a processed request is completed. This is the main reason why there are still a lot of system resources and HTTP access is slow. The system administrator can dynamically adjust the value based on hardware configuration and load conditions. In theory, the larger the value, the more requests can be processed, but the default limit of Apache cannot be greater than 256. If you set this value to greater than 256, Apache cannot start. In fact, 256 is not enough for sites with a slightly higher load. In Apache 1.3, this is a hard limitation. To increase this value, you must search for 256 in src/include/httpd. h under the source code tree manually modified before "Configure", and you will find the line "# define hard_server_limit 256. Change 256 to the value to be increased (such as 4000), and then re-compile Apache. Added the serverlimit command in Apache 2.0 to increase maxclients without re-compiling Apache. The following is the prefork configuration section of the author:
Linux code 

1. <Ifmodule prefork. c>
2. Startservers 10
3. Minspareservers 10
4. Maxspareservers 15
5. Serverlimit 2000
6. Maxclients 1000
7. Maxrequestsperchild 10000
8. </Ifmodule>

In the preceding configuration, the maximum serverlimit value is 20000, which is sufficient for most sites. If you must increase the value
The following two lines in/httpd-2.2.15/Server/MPM/prefork. C can be modified accordingly: Linux code 

1. # Define default_server_limit 256
2. # Define max_server_limit 20000

The working principle of worker is that the main control process generates a "startservers" subprocess. Each subprocess contains a fixed number of threadsperchild threads, and each thread processes Requests independently. Similarly, in order not to generate a thread when the request arrives, minsparethreads and maxsparethreads set the minimum and maximum number of Idle threads, while maxclients sets the total number of threads in all sub-processes. If the total number of threads in the existing sub-process cannot meet the load, the control process will derive a new sub-process.

The maximum default values of minsparethreads and maxsparethreads are 75 and 250, respectively. These two parameters have little impact on Apache performance and can be adjusted as needed.
Threadsperchild is the most performance-related command in worker MPM. The maximum default value of threadsperchild is 64. If the load is large, 64 is not enough. The threadlimit command must be explicitly used. The maximum default value is 20000. The preceding two values are located in the following two lines in the source code tree server/MPM/worker. C:

Linux code  

1. # Define default_thread_limit 64
2. # Define max_thread_limit 20000

These two rows correspond to the number of restrictions of threadsperchild and threadlimit. It is best to change 64 to the desired value before configure. Note: Do not set these two values too high to exceed the processing capability of the system, so that the system is unstable because Apache does not start.
In worker mode, the total number of requests that can be processed simultaneously is determined by the total number of sub-processes multiplied by the value of threadsperchild. The value must be greater than or equal to maxclients. If the load is large and the number of existing sub-processes cannot meet the requirements, the control process will derive a new sub-process. By default, the maximum number of sub-processes is 16. When you increase the number, you must explicitly declare serverlimit (the maximum value is 20000 ). These two values are located in the following two lines in the source code tree server/MPM/worker. C:

Linux code  

1. # Define default_server_limit 16
2. # Define max_server_limit 20000

Note that if serverlimit is explicitly declared, the value multiplied by threadsperchild must be greater than or equal to maxclients, and maxclients must be an integer multiple of threadsperchild, otherwise, Apache automatically adjusts to a value (which may be an unexpected value ). The following is the author's worker configuration section:
Linux code 

1. <Ifmodule worker. c>
2. Startservers 3
3. Maxclients 2000
4. Serverlimit 25
5. Minsparethreads 50
6. Maxsparethread S 200
7. Threadlimit 200
8. Threadsperchild 100
9. Maxrequestsperchild 0
10. </Ifmodule>

Through the above description, we can understand the working principles of the two important mpm, prefork and worker in Apache 2.0, and can configure APACHE-related core parameters according to the actual situation, to achieve maximum performance and stability.