Re: Linux/PostgreSQL scalability issue - problem with 8 cores

Added to TODO:

* Improve performance of shared invalidation queue for multiple CPUs

http://archives.postgresql.org/pgsql-performance/2008-01/msg00023.php

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Tom Lane wrote:
> Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> > Perhaps it would make sense to try to take the "fast path" in
> > SIDelExpiredDataEntries with only a shared lock rather than exclusive.
>
> I think the real problem here is that sinval catchup processing is well
> designed to create contention :-(. Once we've decided that the message
> queue is getting too full, we SIGUSR1 all the backends at once (or as
> fast as the postmaster can do it anyway), then they all go off and try
> to touch the sinval queue. Backends that haven't awoken even once
> since the last time will have to process the entire queue contents,
> and they're all trying to do that at the same time. What's worse, they
> take and release the SInvalLock once for each message they take off the
> queue. This isn't so horrid for one-core machines (since each process
> will monopolize the CPU for probably less than one timeslice while it's
> catching up) but it's pretty obvious where all the contention is coming
> from on an 8-core.
>
> Some ideas for improving matters:
>
> 1. Try to avoid having all the backends hit the queue at once. Instead
> of SIGUSR1'ing everybody at the same time, maybe hit only the process
> with the oldest message pointer, and have him hit the next oldest after
> he's done reading the queue.
>
> 2. Try to take more than one message off the queue per SInvalLock cycle.
> (There is a tuning tradeoff here, since it would mean holding the lock
> for longer at a time.)
>
> 3. Try to avoid having every backend run SIDelExpiredDataEntries every
> time through ReceiveSharedInvalidMessages. It's not critical to delete
> entries until the queue starts getting full --- maybe we could rejigger
> the logic so it only happens once when somebody notices the queue is
> getting full, or so that only the guy(s) who had nextMsgNum == minMsgNum
> do it, or something like that?
>
> regards, tom lane
>
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