Re: Optimizing Read-Only Scalability

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Optimizing Read-Only Scalability
Date: 2009-05-14 17:48:52
Message-ID: 603c8f070905141048w72f89405hbfc258c68c619847@mail.gmail.com
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On Thu, May 14, 2009 at 1:28 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
>> In a thread on -perform it has been observed that our Read-Only
>> scalability is not as good as it could be. One problem being that we
>> need to scan the whole of the ProcArray to derive a snapshot, which
>> becomes the dominant task with many users.
>
> GetSnapshotData doesn't take an exclusive lock.  Neither does start or
> end of a read-only transaction.  AFAIK there is no reason, and certainly
> no shred of experimental evidence, to think that ProcArrayLock
> contention is the bottleneck for read-only scenarios.

I think Simon's point was that it is O(n) rather than O(1), not that
it took an exclusive lock.

...Robert

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