Re: larger shared buffers slows down cluster

On 08/22/2012 05:19 PM, Jeff Janes wrote:


>>
>>      Shared Buffers   Time
>>        48Gb           2058ms
>>         8Gb            372ms
>>         1gb             67ms
>>
>>
>> Is this expected behaviour?
> Yeah.  Clustering the table means that all the indexes and the old
> version of the table all get dropped, and each time something is
> dropped the entire buffer pool is scoured to remove the old buffers.
>
> In my hands, this is about 10 times better in 9.2 than 9.1.4, at 8GB.
> Because now the scouring is done once per object, not once per fork.
> Also, the check is done without an initial spinlock.
>
> It perhaps could be improved further by only scouring the pool once,
> at the end of the transaction, with a hash of all objects to be
> dropped.
>
>


FYI, I have rerun the tests on amazon with 9.2 BETA - the improvement I 
saw ranged from a factor of roughly 2 (with 1Gb of shared memory) to 6 
(with 48Gb).

cheers

andrew