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From Manuel Vacelet <manuel.vace...@gmail.com>
Subject [users@httpd] Re: mod_ldap cache and strace doesn't tell the same story
Date Fri, 19 Mar 2010 08:34:05 GMT
I continue my investigations...

The mod_ldap cache failure seems related to regular graceful restart
(~30mn) the application send to apache to take into account conf
update.

Does it make sense ?
How graceful restart could impact caching of mod_ldap?

Manuel

On Wed, Mar 17, 2010 at 4:16 PM, Manuel Vacelet
<manuel.vacelet@gmail.com> wrote:
> On Tue, Mar 16, 2010 at 2:18 PM, Manuel Vacelet
> <manuel.vacelet@gmail.com> wrote:
>> On Tue, Mar 16, 2010 at 11:06 AM, Manuel Vacelet
>> <manuel.vacelet@gmail.com> wrote:
>>> Hi all,
>>>
>>> I'm facing a strange issue.
>>> I have a major performance penalty on some apache related operations
>>> (svn checkout that used to take 5mn but that suddenly takes more than
>>> 7 hours).
>>>
>>> I'm running a standard RHEL5.3 with apache 2.2.3 (fyi the server runs
>>> mod_php, mod_auth_mysql and some other modules too).
>>>
>>> Thanks to strace, I identified that the httpd process that is "very
>>> long" (the httpd process that discuss with svn client that takes hour
>>> to complete).
>>> The process makes a lot of operation on a file descriptor attached to
>>> my ldap server and is regulary blocking with following syscall:
>>> poll([{fd=3, events=POLLIN|POLLPRI|POLLERR|POLLHUP}], 1, -1 <unfinished ...>
>>>
>>> fd3 is a socket (/proc/<pid>/fd/3) and the inode of this socket is
>>> used to communicate with the ldap server (shown by netstat)
>>>
>>> This is where I no longer understand what happens:
>>> - I'm using mod_ldap so there should be a cache of all ldap info.
>>> - ldap-status confirm that the credentials used for the svn operation
>>> are in cache.
>>> - The cache is far from full (2%)
>>> - The hit ratio is close to 100%
>>>
>>> => Why there is an activity between my httpd process and the ldap server ?
>>
>> I think I have an answer to this question:
>> In some cases, the apache process (I'm running prefork) doesn't use
>> mod_ldap cache.
>>
>> I made a test:
>> In parallel of a very slow checkout, I ran another one. The second
>> checkout was running as normal rate.
>> A couple of strace on the server later, it appears that, the apache
>> process that server the second checkout never talk to the ldap server
>>
>> So the new question is:
>> Why 2 apache processes forked from the same root could behave so differently ?
>
> Does anybody have an explanation to this whole thing ?
>
> Why "sometimes" mod_ldap never cache my credentials ?
> Or in other words: "what would make mod_ldap not to cache users credentials" ?
>
> Manuel
>

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