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Re: First set of OSDL Shared Mem scalability results, some

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Curt Sampson <cjs(at)cynic(dot)net>
Cc: Kevin Brown <kevin(at)sysexperts(dot)com>,pgsql-performance(at)postgresql(dot)org
Subject: Re: First set of OSDL Shared Mem scalability results, some
Date: 2004-10-24 14:39:35
Message-ID: 8438.1098628775@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
Curt Sampson <cjs(at)cynic(dot)net> writes:
> On Sat, 23 Oct 2004, Tom Lane wrote:
>> Seems to me the overhead of any such scheme would swamp the savings from
>> avoiding kernel/userspace copies ...

> Well, one really can't know without testing, but memory copies are
> extremely expensive if they go outside of the cache.

Sure, but what about all the copying from write queue to page?

>> the locking issues alone would be painful.

> I don't see why they would be any more painful than the current locking
> issues.

Because there are more locks --- the write queue data structure will
need to be locked separately from the page.  (Even with a separate write
queue per page, there will need to be a shared data structure that
allows you to allocate and find write queues, and that thing will be a
subject of contention.  See BufMgrLock, which is not held while actively
twiddling the contents of pages, but is a serious cause of contention
anyway.)

			regards, tom lane

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