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    SubjectRe: 2.6.35-rc3 deadlocks on semaphore operations
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    On Fri, Jun 25, 2010 at 11:09 PM, Manfred Spraul
    <manfred@colorfullife.com> wrote:
    > On 06/24/2010 09:22 PM, Luca Tettamanti wrote:
    >>
    >> Argh, "seems" was indeed appropriate. Manfred your patch does
    >> alleviate the problem but something is still wrong. I noticed (I'm
    >> developing an ajax heavy web app) that sometimes an apache worker
    >> hangs; I can reproduce the problem with ab (apache benchmark) and a
    >> high concurrency level (I'm testing with 100 and 10k requests, and I
    >> get only 2-5 dropped requests). This does not happen with 2.4.34.
    >> Any idea on how I can debug this further?
    >>
    >>
    >
    > Are there still hung httpd processes?

    They don't seem really hung as before, I see two different behaviours:
    * Near the end of the run ab is frozen for a few seconds, but in the
    end all requests are processed; however I see a few "length" errors,
    meaning that the received page does not match the expected content
    (I'm testing a static page):

    Concurrency Level: 100
    Time taken for tests: 14.682 seconds
    Complete requests: 10000
    Failed requests: 10
    (Connect: 0, Receive: 0, Length: 10, Exceptions: 0)
    Write errors: 0
    Total transferred: 79987946 bytes
    HTML transferred: 77190466 bytes
    Requests per second: 681.11 [#/sec] (mean)
    Time per request: 146.819 [ms] (mean)
    Time per request: 1.468 [ms] (mean, across all concurrent requests)
    Transfer rate: 5320.36 [Kbytes/sec] received

    Connection Times (ms)
    min mean[+/-sd] median max
    Connect: 0 1 0.4 0 3
    Processing: 3 29 428.9 15 14678
    Waiting: 0 15 1.3 15 19
    Total: 5 30 429.0 16 14682
    ERROR: The median and mean for the initial connection time are more
    than twice the standard
    deviation apart. These results are NOT reliable.
    Percentage of the requests served within a certain time (ms)
    50% 16
    66% 17
    75% 17
    80% 17
    90% 17
    95% 17
    98% 18
    99% 18
    100% 14682 (longest request)

    strace on apache shows:
    [pid 3787] restart_syscall(<... resuming interrupted call ...> <unfinished ...>
    [pid 3789] restart_syscall(<... resuming interrupted call ...> <unfinished ...>
    [pid 3788] restart_syscall(<... resuming interrupted call ...> <unfinished ...>
    [pid 3784] restart_syscall(<... resuming interrupted call ...> <unfinished ...>
    [pid 3783] restart_syscall(<... resuming interrupted call ...> <unfinished ...>
    [pid 3782] restart_syscall(<... resuming interrupted call ...> <unfinished ...>
    [pid 3239] restart_syscall(<... resuming interrupted call ...> <unfinished ...>
    [pid 3233] restart_syscall(<... resuming interrupted call ...> <unfinished ...>
    [pid 3238] restart_syscall(<... resuming interrupted call ...> <unfinished ...>
    [pid 3237] restart_syscall(<... resuming interrupted call ...>


    * Sometimes ab pauses at the end but not all requests are successful; ab says:

    apr_socket_recv: Connection timed out (110)
    Total of 9932 requests completed

    The problem seems more acute if there's other application running on
    the system (e.g. right now I have kde, firefox, thunderbird active and
    I see more errors; note that nothing is using much CPU time). I've
    taken a sysrq-t log during one of ab pauses, it's attached.

    Might be related: KDE (4.4) takes an usual amount of time to start up:
    the splash screen is fronzen for a while on the screen before the
    desktop is available (.xsession-error does not show anything
    interesting).

    Luca
    PS: I'll be offline until Monday 5th...
    [unhandled content-type:application/x-gzip]
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