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Subject DO NOT REPLY [Bug 47180] New: DBM ssl_scache setting can cause extreme performance degradation
Date Mon, 11 May 2009 08:50:31 GMT

           Summary: DBM ssl_scache setting can cause extreme performance
           Product: Apache httpd-2
           Version: 2.2.11
          Platform: Sun
        OS/Version: Solaris
            Status: NEW
          Severity: normal
          Priority: P2
         Component: mod_ssl

We are running Apache 2.2 on Solaris 10 with the following SSL cache settings:-

SSLSessionCache         dbm:/usr/local/apache2/logs/ssl_scache
SSLSessionCacheTimeout  300

We have connections from clients using client certificates for authentication.
These connections are programs rather than interactive. As such each connection
exists for a single discrete request.

The session cache is checked each SSLSessionCacheTimeout seconds for expired
SSL session information. This is performed by all httpd processes
independently. This check consists of at least one sequential search of the DBM
file. During this all other SSL operations are blocked.

We have seen that under load the size of the DBM file grow to between 1 and 2
GB. As the file grows in size it takes longer to traverse it and the server
suffers intermittent performance issues. Once the time to traverse the file
exceeds the SSLSessionCacheTimeout the server responds to requests at the rate
of one per SSLSessionCacheTimeout.

It is possible to workaround this by disabling the session cache or changing to
use shared memory.

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