On Tue, Apr 12, 2016 at 3:48 PM, Alexander Korotkov < a.korot...@postgrespro.ru> wrote:
> On Tue, Apr 12, 2016 at 12:40 AM, Andres Freund <and...@anarazel.de> > wrote: > >> I did get access to the machine (thanks!). My testing shows that >> performance is sensitive to various parameters influencing memory >> allocation. E.g. twiddling with max_connections changes >> performance. With max_connections=400 and the previous patches applied I >> get ~1220000 tps, with 402 ~1620000 tps. This sorta confirms that we're >> dealing with an alignment/sharing related issue. >> >> Padding PGXACT to a full cache-line seems to take care of the largest >> part of the performance irregularity. I looked at perf profiles and saw >> that most cache misses stem from there, and that the percentage (not >> absolute amount!) changes between fast/slow settings. >> >> To me it makes intuitive sense why you'd want PGXACTs to be on separate >> cachelines - they're constantly dirtied via SnapshotResetXmin(). Indeed >> making it immediately return propels performance up to 1720000, without >> other changes. Additionally cacheline-padding PGXACT speeds things up to >> 1750000 tps. >> > > It seems like padding PGXACT to a full cache-line is a great improvement. > We have not so many PGXACTs to care about bytes wasted to padding. > Yes, it seems generally it is a good idea, but not sure if it is a complete fix for variation in performance we are seeing when we change shared memory structures. Andres suggested me on IM to take performance data on x86 m/c by padding PGXACT and the data for the same is as below: median of 3, 5-min runs Client_Count/Patch_ver 8 64 128 HEAD 59708 329560 173655 PATCH 61480 379798 157580 Here, at 128 client-count the performance with patch still seems to have variation. The highest tps with patch (170363) is close to HEAD (175718). This could be run-to-run variation, but I think it indicates that there are more places where we might need such padding or may be optimize them, so that they are aligned. I can do some more experiments on similar lines, but I am out on vacation and might not be able to access the m/c for 3-4 days. With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com
pad_pgxact_v1.patch
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