Tom Lane wrote:
"Luke Lonergan" <[EMAIL PROTECTED]> writes:
The issue is summarized like this: the buffer cache in PGSQL is not "scan
resistant" as advertised.
Sure it is. As near as I can tell, your real complaint is that the
bufmgr doesn't attempt to limit its usage footprint to fit in L2 cache;
which is hardly surprising considering it doesn't know the size of L2
cache. That's not a consideration that we've ever taken into account.
To add a little to this - forgetting the scan resistant point for the
moment... cranking down shared_buffers to be smaller than the L2 cache
seems to help *any* sequential scan immensely, even on quite modest HW:
e.g: PIII 1.26Ghz 512Kb L2 cache, 2G ram,
SELECT count(*) FROM lineitem (which is about 11GB) performance:
Shared_buffers Elapsed
-------------- -------
400MB 101 s
128KB 74 s
When I've profiled this activity, I've seen a lot of time spent
searching for/allocating a new buffer for each page being fetched.
Obviously having less of them to search through will help, but having
less than the L2 cache-size worth of 'em seems to help a whole lot!
Cheers
Mark
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