Re: Optimizing Read-Only Scalability

On Thu, May 14, 2009 at 1:28 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndQuadrant.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