Re: POC: Cache data in GetSnapshotData()

On Thu, Mar 10, 2016 at 1:04 PM, Mithun Cy <mithun.cy@enterprisedb.com> wrote:

On Thu, Mar 3, 2016 at 11:50 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>What if you apply both this and Amit's clog control log patch(es)?  Maybe the combination of the two helps substantially more than either >one alone.

I did the above tests along with Amit's clog patch. Machine :8 socket, 64 core. 128 hyperthread.

clients	BASE	ONLY CLOG CHANGES	% Increase	ONLY SAVE SNAPSHOT	% Increase	CLOG CHANGES + SAVE SNAPSHOT	% Increase
64	29247.658034	30855.728835	5.4981181711	29254.532186	0.0235032562	32691.832776	11.7758992463
88	31214.305391	33313.393095	6.7247618606	32109.248609	2.8670931702	35433.655203	13.5173592978
128	30896.673949	34015.362152	10.0939285832	***	***	34947.296355	13.110221549
256	27183.780921	31192.895437	14.7481857938	***	***	32873.782735	20.9316056164

With clog changes, perf of caching the snapshot patch improves.

This data looks promising to me and indicates that saving the snapshot has benefits and we can see noticeable performance improvement especially once the CLog contention gets reduced.  I wonder if we should try these tests with unlogged tables, because I suspect most of the contention after CLogControlLock and ProcArrayLock is for WAL related locks, so you might be able to see better gain with these patches.  Do you think it makes sense to check the performance by increasing CLOG buffers (patch for same is posted in Speed up Clog thread [1]) as that also relieves contention on CLOG as per the tests I have done?

[1] - http://www.postgresql.org/message-id/CAA4eK1LMMGNQ439BUm0LcS3p0sb8S9kc-cUGU_ThNqMwA8_Tug@mail.gmail.com

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com