8.2 + apache == a LOT of sigprocmask

Doug Barton dougb at FreeBSD.org
Thu Nov 17 06:46:28 UTC 2011

On 11/15/2011 02:09, Jeremy Chadwick wrote:
> On Tue, Nov 15, 2011 at 11:07:45AM +0200, Kostik Belousov wrote:
>> On Mon, Nov 14, 2011 at 12:51:35PM -0800, Doug Barton wrote:
>>> On 11/14/2011 12:31, Doug Barton wrote:
>>>> Trying to track down a load problem we're seeing on 8.2-RELEASE-p4 i386
>>>> in a busy web hosting environment I came across the following post:
>>>> http://lists.freebsd.org/pipermail/freebsd-questions/2011-October/234520.html
>>>> That basically describes what we're seeing as well, including the
>>>> "doesn't happen on Linux" part.
>>>> Does anyone have any ideas about this?
>>>> With incredibly similar stuff running on 7.x we didn't see this problem,
>>>> so it seems to be something new in 8.
>>> Just took a closer look at our ktrace, and actually our pattern is
>>> slightly different than the one in that post. In ours the second option
>>> is null, but the third is set:
>>> 74195 httpd    0.000017 RET   sigprocmask 0
>>> 74195 httpd    0.000013 CALL  sigprocmask(SIG_BLOCK,0,0xbfbf89d4)
>>> 74195 httpd    0.000009 RET   sigprocmask 0
>>> 74195 httpd    0.000013 CALL  sigprocmask(SIG_BLOCK,0,0xbfbf89d4)
>>> 74195 httpd    0.000009 RET   sigprocmask 0
>>> 74195 httpd    0.000012 CALL  sigprocmask(SIG_BLOCK,0,0xbfbf89d4)
>>> But repeated hundreds of times in a row.
>> The calls cannot come from rtld, they are generated by some setjmp()
>> invocation. If signal-safety is not needed, sigsetjmp() should be used
>> instead.
>> Quick grep of the apache httpd source shows a single setjmp() in their
>> copy of pcre. No idea is it to safe to change setjmp() into sigsetjmp(?, 0).
> I hate cross-posting, but: adding freebsd-apache@ to the list.  Some of
> the Apache folks (not just port committers) may have some insight to
> Kostik's findings.

Thanks to everyone for the responses. We tried Kostik's suggestion and
unfortunately it didn't reduce the number of sigprocmask() calls to a
statistically significant degree.

Does anyone have any other ideas on ways to debug this? We're sort of
running out of things to test. :-/

Given how important (and prevalent) the Apache + FreeBSD combination is,
I'm kind of disturbed that we're seeing this performance problem, and if
it's something in 8.x that's also in 9.x, it would be better to fix it
prior to 9.0-RELEASE.



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