Re: Straightforward changes for increased SMP scalability

Add to TODO:

* SMP scalability improvements
 http://archives.postgresql.org/pgsql-hackers/2007-07/msg00439.php


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Simon Riggs wrote:
> David Strong presented some excellent results of his SMP scalability
> testing at Ottawa in May.
> http://www.pgcon.org/2007/schedule/events/16.en.html
> 
> There are some easy things we can do to take advantage of those results,
> especially the ones that were hardware independent.
> 
> The hardware independent results were these two:
> - Avoid contention on WALInsertLock (+28% gain)
> - Increase NUM_BUFFER_PARTITIONS (+7.7% gain)
> 
> Scalability begins to slow down at 8 CPUs on 8.2.4 and David was able to
> show good gains even at 8 CPUs with these changes.
> 
> Proposals
> 
> 1. For the first result, I suggest that we introduce some padding into
> the shmem structure XLogCtlData to alleviate false sharing that may
> exist between holders of WALInsertLock, WALWriteLock and info_lck. The
> cost of this will be at most about 200 bytes of shmem, with a low risk
> change. The benefits are hard to quantify, but we know this is an area
> of high contention and we should do all we can to reduce that.
> This hasn't been discussed previously, though we have seen good benefit
> from avoiding false sharing in other cases, e.g. LWLOCK padding.
> 
> 2. Increase NUM_BUFFER_PARTITIONS from 16 to 256 (or higher).
> This has been discussed previously:
> http://archives.postgresql.org/pgsql-hackers/2006-09/msg00967.php
> 
> Both of these changes are simple enough to consider for 8.3
> 
> Comments?
> 
> -- 
>   Simon Riggs
>   EnterpriseDB  http://www.enterprisedb.com
> 
> 
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--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://postgres.enterprisedb.com
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