Re: Bug: Buffer cache is not scan resistant

One more thing: the L2 is invalidated when re-written from the kernel IO cache, but the pages addressed in L2 retain their values when 'writeen thru' which allows the new data to be re-used up the executor chain.

- Luke

Msg is shrt cuz m on ma treo

-----Original Message-----
From: Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us]
Sent: Sunday, March 04, 2007 08:36 PM Eastern Standard Time
To: Luke Lonergan
Cc: PGSQL Hackers; Doug Rady; Sherry Moore
Subject: Re: [HACKERS] Bug: Buffer cache is not scan resistant

"Luke Lonergan" <llonergan(at)greenplum(dot)com> 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.

I'm also less than convinced that it'd be helpful for a big seqscan:
won't reading a new disk page into memory via DMA cause that memory to
get flushed from the processor cache anyway? I wonder whether your
numbers are explained by some other consideration than you think.

regards, tom lane