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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 Tue, 16 Mar 2010 13:18:20 GMT
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 ?

Manuel

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