>Simon Riggs wrote:
>
>> 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?
>>
>+1 on the idea (I can speak to the technical side). What I can say is
>that it is pretty much known that after 8 cores we slow down. Although
>8.2 is better than any other release in this regard.
>
>Joshua D. Drake
>
Here's a quick update. We're working on moving the patches we made
against Postgres 8.2.4 to 8.3 to see what is still valid. So far, the
base 8.3 shows ~7% improvement at 8 cores over 8.2.4.
The NUM_BUFFER_PARTITIONS patch is fairly simple. We've noticed gains
with NUM_BUFFER_PARTITIONS set between 256 and 2048, but little to no
gain after 2048, although this might depend on the benchmark and
platform being used. We've measured ~3% gain from the 8.3 base with
NUM_BUFFER_PARTITIONS set to 2048. This might be the way this patch
behaves with 8.3 or we might find that the NUM_BUFFER_PARTITIONS patch
complements patch "X" as the 7.7% number reported for
NUM_BUFFER_PARTITIONS in our presentation had a number of other patches
enabled. This was also running at a 20 cores.
We plan to start releasing patches this week for your consideration,
along with their current gains.
David
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