Re: [HACKERS] Proposal for CSN based snapshots

On Mon, Sep 25, 2017 at 10:17 AM, Alexander Kuzmenkov
<a.kuzmenkov@postgrespro.ru> wrote:
> Here is some news about the CSN patch.
>
> * I merged it with master (58bd60995f), which now has the clog group update.
> With this optimization, CSN is now faster than the master by about 15% on
> 100 to 400 clients (72 cores, pgbench tpcb-like, scale 500). It does not
> degrade faster than master as it did before. The numbers of clients greater
> than 400 were not measured.

Hmm, that's gratifying.

> * Querying for CSN of subtransactions was not implemented in the previous
> version of the patch, so I added it. I tested the performance on the
> tpcb-like pgbench script with some savepoints added, and it was
> significantly worse than on the master. The main culprit seems to be the
> ProcArrayLock taken in GetSnapshotData, GetRecentGlobalXmin,
> ProcArrayEndTransaction. Although it is only taken in shared mode, just
> reading the current lock mode and writing the same value back takes about
> 10% CPU. Maybe we could do away with some of these locks, but there is some
> interplay with imported snapshots and replication slots which I don't
> understand well. I plan to investigate this next.

That's not so good, though.

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


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