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From bugzi...@apache.org
Subject DO NOT REPLY [Bug 39275] New: - slow child_init causes MaxClients warning
Date Tue, 11 Apr 2006 20:09:20 GMT
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http://issues.apache.org/bugzilla/show_bug.cgi?id=39275

           Summary: slow child_init causes MaxClients warning
           Product: Apache httpd-2
           Version: 2.3-HEAD
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: PatchAvailable
          Severity: normal
          Priority: P2
         Component: worker
        AssignedTo: bugs@httpd.apache.org
        ReportedBy: chrisd@pearsoncmg.com
                CC: chrisd@pearsoncmg.com


Per this thread on the httpd-dev mailing list:

http://marc.theaimsgroup.com/?t=114453426100001&r=1&w=2

I have been seeing something similar with 2.2.0 using the worker
MPM, where with the following settings, I get over 10 child processes
initializing immediately (e.g., up to 15), and then they drop back to
10.  I see the "server reached MaxClients" message as well right
after httpd startup, although nothing is connecting yet.

<IfModule mpm_worker_module>
    StartServers         10
    MaxClients          150
    MinSpareThreads      25
    MaxSpareThreads     100
    ThreadsPerChild      10
</IfModule>

In my case, the problem relates to how long the child_init phase
takes to execute.  I can "tune" this by raising DBDMin (and DBDKeep)
so that mod_dbd attempts to open increasingly large numbers of
DB connections during child_init.  With DBDMin set to 0 or 1,
all is well; no funny behaviour.  Up at DBDMin and DBDKeep at 3,
that's when (for me) things go pear-shaped.

In server/mpm/worker/worker.c, after make_child() creates a
child process it immediately sets the scoreboard parent slot's pid
value.  The main process goes into server_main_loop() and begins
executing perform_idle_server_maintenance() every second; this
looks at any process with a non-zero pid in the scoreboard and
assumes that any of its worker threads marked SERVER_DEAD are,
in fact, dead.

However, if the child processes are starting "slowly" because
ap_run_child_init() in child_main() is taking its time, then
start_threads() hasn't even been run yet, so the threads aren't
marked SERVER_STARTING -- they're just set to 0 as the default
value.  But 0 == SERVER_DEAD, so the main process sees a lot
of dead worker threads and begins spawning new child processes,
up to MaxClients/ThreadsPerChild in the worst case.  In this case,
when no worker threads have started yet, but all possible child
processes have been spawned (and are working through their
child_init phases), then the following is true and the
"server reached MaxClients" message is printed, even though
the server hasn't started accepting connections yet:

    else if (idle_thread_count < min_spare_threads) {
        /* terminate the free list */
        if (free_length == 0) {

I considered wedging another thread status into the
scoreboard, between SERVER_DEAD (the initial value) and
SERVER_STARTING.  The make_child() would set all the thread
slots to this value and start_threads() would later flip them
to SERVER_STARTING after actually creating the worker threads.

That would have various ripple effects on other bits of
httpd, though, like mod_status and other MPMs, etc.  So instead
I tried adding a status field to the process_score scoreboard
structure, and making the following changes to worker.c such that
this field is set by make_child to SERVER_STARTING and then
changed to SERVER_READY once the start thread that runs
start_threads() has done its initial work.

During this period, while the new child process is running
ap_run_child_init() and friends, perform_idle_server_maintenance()
just counts that child process's worker threads as all being
effectively in SERVER_STARTING mode.  Once the process_score.status
field changes to SERVER_READY, perform_idle_server_maintenance()
begins to look at the individual thread status values.

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