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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...
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