--On Thursday, October 18, 2007 9:58 AM +0200 Pascal Gienger <Pascal.Gienger@xxxxxxxxxxxxxxx> wrote:
Scott Adkins <adkinss@xxxxxxxx> wrote:Meanwhile, we hacked around this in a very cool way. We copied the imapd process 60 times (assuming average of 12,000 processes, shooting for 200 processes per executable, that is 60 individual executables). These were named /usr/cyrus/bin/imapd_001 through /usr/cyrus/bin/imapd_060. We then symlinked the "imapd" binary to imapd_001. We then wrote a cron job that ran once a minute and relinked the imapd symlink to the next numbered executable,And how did you solve the problem of the "deadlock" resulting in deleteing the symbolic link, setting the new one? Between these events an exec of imapd would break resulting in an error to the customer. Funny hack though.
Actually, I don't see a deadlock situation at all... I am guessing that
theorettically, it is possible... but the "ln -sf" option makes the
overwriting of the symlink an atomic action (as much as it can), which
is why it seems to work. I stress tested this as much as I could prior
to putting it into production, simulating tons of users logging in and
out of the server while rotating the symlink as fast as I could. There
was not one case of a connection failure at any point during the test.
There are other factors that are involved too... Process re-use keeps a
lot of problems to a minimum... We don't use preforking, but I imagine
that preforking would further reduce any possibilities of deadlocking.
Users coming into the system would be funneled to a socket associated
with an IMAP process that has already been running for a bit, so it is
clean from their perspective.
The only issue is actually between master and imapd, and I don't know what
master does if it tries to spawn an imapd and it fails... it may just try
again... I dounbt it just out-right refused the incoming connection. I
need to look at the source code to understand it better.
Scott
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Scott W. Adkins Work (740)593-9478 Fax (740)593-1944
UNIX Systems Engineer <mailto:adkinss@xxxxxxxx>
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