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    SubjectRe: Costly Context Switches
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    On Monday 13 December 2010 14:51:04 Arnaldo Carvalho de Melo wrote:
    > Em Sun, Dec 12, 2010 at 08:07:07PM +0100, Andres Freund escreveu:
    > > On Sunday 12 December 2010 16:11:12 Ralf Hildebrandt wrote:
    > > > I recently made a parallel installation of dovecot-2.0 on my mailbox
    > > > server, which is running dovecot-1.2 without any problems whatsoever.
    > > >
    > > > Using dovecot-2.0 on the same hardware, same kernel, with the same
    > > > users and same mailboxes and usage behaviour results in an immense
    > > > increase in the load numbers.
    > > >
    > > > Switching back to 1.2 results in a immediate decrease of the load back
    > > > to "normal" numbers.
    > > >
    > > > This is mainly due to a 10-20 fold increase of the number of context
    > > > switches. The same problem has been reported independently by Cor
    > > > Bosman of XS4All, on different hardware (64bit instead of 32bit,
    > > > real hardware instead of virtual hardware).
    > > >
    > > > So, now the kernel related question: How can I find out WHY the
    > > > context switches are happening? Are there any "in kernel" statistics
    > > > I could look at?
    > >
    > > "strace" or "perf trace syscall-counts" would be a good start.
    >
    > Better to record just "cs" (Context Switches) events and also to collect
    > callchains when those events take place:
    Hm. Its also a good starting point but it may be harder to see the differences
    between dovecot-2.0 and dovecot-1.2 that way because its harder to see if its
    the usage being different causing the problem (i.e. calling in a different
    order, trashing caches) or if its the amount of syscalls that changed. But I
    agree that both are very usefull analyze problems like that.

    Andres


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