Tom Lane wrote:
> NikhilS <[EMAIL PROTECTED]> writes:
>> What is the opinion of the list as to the best way of measuring if the
>> following implementation is ok?
>> As mentioned in earlier mails, this will reduce the per-backend usage of
>> memory by an amount which will be a fraction (single digit percentage)
>> of (NBuffers
>> * int) size. I have done pgbench/dbt2 runs and I do not see any negative
>> impact because of this.
> I find it extremely telling that you don't claim to have seen any
> positive impact either.
> I think that the original argument
> is basically bogus.  At 500000 buffers (4GB in shared memory) the
> per-backend space for PrivateRefCount is still only 2MB, which is
> simply not as significant as Simon claims; a backend needs at least
> that much for catalog caches etc.  There is, furthermore, no evidence
> that running shared_buffers that high is a good idea in the first
> place, or that there aren't other performance bottlenecks that will
> manifest before this one becomes interesting.

hmm - we are continuily running into people with dedicated servers that
have 16GB RAM or even more available and most tuning docs recommend some
 20-30% of system RAM to get dedicated to shared_buffers. So having some
500k buffers allocated does not sound so unrealistic in practise and
combined with the fact that people often have a few hundred backends
that could add up to some noticable overhead.
If that is actually a problem given that those people tend to have heaps
of memory is another story but if we can preserve some memory ...


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