Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On Tue, Apr 19, 2016 at 11:11 AM, Kevin Grittner <kgrittn@gmail.com> wrote:
> On Tue, Apr 19, 2016 at 9:57 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> On Sun, Apr 17, 2016 at 2:26 AM, Andres Freund <andres@anarazel.de> wrote:
>>>
>>> On 2016-04-16 16:44:52 -0400, Noah Misch wrote:
>>> > That is more controversial than the potential ~2% regression for
>>> > old_snapshot_threshold=-1.  Alvaro[2] and Robert[3] are okay releasing
>>> > that way, and Andres[4] is not.
>>>
>>> FWIW, I could be kinda convinced that it's temporarily ok, if there'd be
>>> a clear proposal on the table how to solve the scalability issue around
>>> MaintainOldSnapshotTimeMapping().
>>
>> It seems that for read-only workloads, MaintainOldSnapshotTimeMapping()
>> takes EXCLUSIVE LWLock which seems to be a probable reason for a performance
>> regression.  Now, here the question is do we need to acquire that lock if
>> xmin is not changed since the last time value of
>> oldSnapshotControl->latest_xmin is updated or xmin is lesser than equal to
>> oldSnapshotControl->latest_xmin?
>> If we don't need it for above cases, I think it can address the performance
>> regression to a good degree for read-only workloads when the feature is
>> enabled.
>
> Thanks, Amit -- I think something along those lines is the right
> solution to the scaling issues when the feature is enabled.  For
> now I'm focusing on the back-patching issues and the performance
> regression when the feature is disabled, but I'll shift focus to
> this once the "killer" issues are in hand.

Maybe Amit could try his idea in parallel.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company