Re: Wierd context-switching issue on Xeon

Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Did we ever come to a conclusion about excessive SMP context switching
> > under load?
>
> Yeah: it's bad.
>
> Oh, you wanted a fix?  That seems harder :-(.  AFAICS we need a redesign
> that causes less load on the BufMgrLock.  However, the traditional
> solution to too-much-contention-for-a-lock is to break up the locked
> data structure into finer-grained units, which means *more* lock
> operations in total.  Normally you expect that the finer-grained lock
> units will mean less contention.  But given that the issue here seems to
> be trading physical ownership of the lock's cache line back and forth,
> I'm afraid that the traditional approach would actually make things
> worse.  The SMP issue seems to be not with whether there is
> instantaneous contention for the locked datastructure, but with the cost
> of making it possible for processor B to acquire a lock recently held by
> processor A.

I see.  I don't even see a TODO in there.  :-(

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