Re: Optimizing Read-Only Scalability

On Thu, May 14, 2009 at 1:55 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Thu, May 14, 2009 at 1:28 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> 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.
>
> I think my point was that there's no evidence that GetSnapshotData
> is where the scalability issue is.  Without some evidence there's no
> point in kluging it up.

Sure. I don't think anyone was proposing to commit something without
first testing it.

Supposing that the patch can be shown to improve performance for
all-read-only workloads, and supposing further that the patch can be
shown to have no material negative impact on write-heavy workloads, it
would also be interesting to throw in a bit of scattered write traffic
and see whether that completely negates the benefit or not.

...Robert