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

On Tue, Apr 19, 2016 at 10:14 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Tue, Apr 19, 2016 at 11:11 AM, Kevin Grittner <kgrittn(at)gmail(dot)com> wrote:
>> On Tue, Apr 19, 2016 at 9:57 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>> On Sun, Apr 17, 2016 at 2:26 AM, Andres Freund <andres(at)anarazel(dot)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.

That would be great!

--
Kevin Grittner
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