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The following version of Apache is used. (MPM : event)

httpd -V
Server version: Apache/2.4.37 (Red Hat Enterprise Linux)
Server built:   Jun 15 2022 08:27:14
Server's Module Magic Number: xxxxxxxx:xx
Server loaded:  APR 1.6.3, APR-UTIL 1.6.1
Compiled using: APR 1.6.3, APR-UTIL 1.6.1
Architecture:   64-bit
Server MPM:     event
  threaded:     yes (fixed thread count)
    forked:     yes (variable process count)

The settings are as follows.

<IfModule mpm_event_module>
    StartServers 3
    ServerLimit 3
    ThreadsPerChild 25
    MaxRequestWorkers 75
</IfModule>

I understand the relationship between ServerLimit, ThreadsPerChild and MaxRequestWorkers. (ThreadsPerChild * ServerLimit = MaxRequestWorkers)

In the above configuration, I expect to run 1 parent process + 3 child processes for a total of 4 processes.
However, the following occurs For some reason, parent process 1 + child process 5 is launched.

ps  -ylC httpd
S   UID     PID    PPID  C PRI  NI   RSS    SZ WCHAN  TTY          TIME CMD
S     0 1234082       1  1  80   0 62164 85197 -      ?        00:00:00 httpd # Parent
S    48 1234083 1234082  0  80   0 55936 88443 -      ?        00:00:00 httpd # Child1
S    48 1234084 1234082  0  80   0 55860 88787 -      ?        00:00:00 httpd # Child2
S    48 1234085 1234082  0  80   0 63784 697650 -     ?        00:00:00 httpd # Child3
S    48 1234086 1234082  0  80   0 61732 648482 -     ?        00:00:00 httpd # Child4
S    48 1234087 1234082  0  80   0 65812 648482 -     ?        00:00:00 httpd # Child5

When loaded by sending a large number of Http requests, RSS increases only for Child 3, 4, and 5, with little change for Child 1 and 2. From this, I assume that Child3, 4, and 5 are the normal processes that we expect to see, and Child1 and 2 are processes that are being created for some special reason.

I have another question. Regarding the number of threads in each child process, I would expect it to be 25thread. However, it looks like this

ps aux -L | grep httpd | grep -v grep | perl -awln -F'\s+' -e 'print "$F[0] $F[1]"' | sort | uniq -c
      1 root   1234082 # Parent
      1 apache 1234083 # Child1
      1 apache 1234084 # Child2
     81 apache 1234085 # Child3
     65 apache 1234086 # Child4
     65 apache 1234087 # Child5

For some reason, Child3, 4, and 5 exceed the ThreadsPerChild value of 25 and reach numbers such as 65 and 81. (The anomaly of Child1 and Child2 is also apparent here.)

Question 1 : What are the Child1 and Child2 processes doing? Why do they start extra?

Question 2 : Why do Child3, 4, and 5 exceed ThreadsPerChild and have large thread counts?

As an additional note, similar results are obtained using the following settings.

<IfModule mpm_event_module>
    StartServers 4
    ServerLimit 4
    ThreadsPerChild 25
    MaxRequestWorkers 100
</IfModule>

Four child processes that are considered normal and two child processes that are considered abnormal are launched.

ps  -ylC httpd
S   UID     PID    PPID  C PRI  NI   RSS    SZ WCHAN  TTY          TIME CMD
S     0 1235661       1  2  80   0 62540 85203 -      ?        00:00:00 httpd
S    48 1235663 1235661  0  80   0 56108 88443 -      ?        00:00:00 httpd
S    48 1235664 1235661  0  80   0 56176 88786 -      ?        00:00:00 httpd
S    48 1235665 1235661  0  80   0 66036 697656 -     ?        00:00:00 httpd
S    48 1235666 1235661  0  80   0 63992 648488 -     ?        00:00:00 httpd
S    48 1235667 1235661  0  80   0 63992 664872 -     ?        00:00:00 httpd
S    48 1235668 1235661  0  80   0 63968 664872 -     ?        00:00:00 httpd
ps aux -L | grep httpd | grep -v grep | perl -awln -F'\s+' -e 'print "$F[0] $F[1]"' | sort | uniq -c
      1 root 1235661
      1 apache 1235663
      1 apache 1235664
     81 apache 1235665
     65 apache 1235666
     65 apache 1235667
     65 apache 1235668

Thanks in advance.

1 Answer 1

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Child1 and Child2 are created by Apache multiprocessing module at startup. These are commonly referred to as "spare" or "extra" process. In the config you specified "StartServers 3", which means apache starts with three spare processes in addition to the parent process.

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  • Thx for ur comment. I believe the processes defined by StartServer3 are Child3,4,5. the role and rationale for Child1,2 is still unclear. If I set StartServer=1, ServerLimit=1, 1parent+1child is expected, but 1parent+3child is launched. If StartServer=10, ServerLimit=10, 1parent+10child is expected, but 1parent+12child is launched. There are always two extra children. And these two extra children do not increase RSS even if HTTP requests increase. If these two extra children are "Spare" processes, is there a reference I can refer to? I would appreciate a little more detail.
    – fsk5304
    Jun 15, 2023 at 4:22

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