Apache workers

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title

Apache workers

author

Sam Bashton (NobleProg Ltd)

</slideshow>

Apache worker model ⌘

• Apache can function using different models
• Most common - prefork

Apache prefork ⌘

• One central Apache process to accept incoming connections, read configuration etc
• Multiple child processes to serve requests
• Each child can serve only one request at a time

Apache prefork settings ⌘

```
StartServers       8
MinSpareServers    5
MaxSpareServers   20
ServerLimit      256
MaxClients       256
MaxRequestsPerChild  4000
```

Tuning Apache prefork ⌘

• StartServers

• Number of Apache processes to create on first start

• MinSpareServers

• Minimum number of spare Apache processes that should be available at any time. If less spare workers than this are available, Apache will start more

• MaxSpareServers

• Maximum number of spare workers.If more than this are available, Apache will kill them off at a rate of 1/second to bring it below this.

Tuning Apache prefork ⌘

• Default EL5 config has a maximum of 256 child processes
• This means maximum 256 concurrent requests
• In reality, 256 may well be too high
• When using PHP, Apache processes are often 30MB+
• 30 * 256 = 7680MB of RAM required, just for workers
• Under these conditions, 256 MaxClients suitable only for machines with at 8GB RAM

Sizing MaxClients ⌘

• Tool ps_mem.py can be used

• http://www.pixelbeat.org/scripts/ps_mem.py
• Shows actual RAM in use by each process
• Should be used on a server with a representative workload

• Set MaxClients too high, and you'll start swapping

What are the workers doing? ⌘

• Apache module `mod_status` shows this data
• Module is loaded by default but not enabled
• Uncomment the section in /etc/httpd/conf/httpd.conf to enable

• Change `.example.com` to `127.0.0.1`

• Once enabled, run `service httpd fullstatus` to see data

Exercise ⌘

• Enable mod_status and find the status of each worker
• How much memory is each httpd process using on your VM?
• What is a suitable value for MaxClients on this VM based on the above?