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

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.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company