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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 Thu, 01 Apr 2010 13:14:44 GMT
Hi,

Just to keep people informed (and search engines). It seems to be a
httpd bug on usage of LDAP cache shared memory and graceful restart:
https://issues.apache.org/bugzilla/show_bug.cgi?id=48958

Manuel

On Fri, Mar 19, 2010 at 9:34 AM, Manuel Vacelet
<manuel.vacelet@gmail.com> wrote:
> 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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