Incidentally, we tried triggering NTA (L2 cache bypass) unconditionally and in various patterns and did not see the substantial gain as with reducing the working set size.
My conclusion: Fixing the OS is not sufficient to alleviate the issue. We see a 2x penalty (1700MB/s versus 3500MB/s) at the higher data rates due to this effect.
Msg is shrt cuz m on ma treo
From: Sherry Moore [mailto:sherry(dot)moore(at)sun(dot)com]
Sent: Tuesday, March 06, 2007 10:05 PM Eastern Standard Time
To: Simon Riggs
Cc: Sherry Moore; Tom Lane; Luke Lonergan; Mark Kirkwood; Pavan Deolasee; Gavin Sherry; PGSQL Hackers; Doug Rady
Subject: Re: [HACKERS] Bug: Buffer cache is not scan resistant
> and what you haven't said
> - all of this is orthogonal to the issue of buffer cache spoiling in
> PostgreSQL itself. That issue does still exist as a non-OS issue, but
> we've been discussing in detail the specific case of L2 cache effects
> with specific kernel calls. All of the test results have been
> stand-alone, so we've not done any measurements in that area. I say this
> because you make the point that reducing the working set size of write
> workloads has no effect on the L2 cache issue, but ISTM its still
> potentially a cache spoiling issue.
What I wanted to point out was that (reiterating to avoid requoting),
- My test was simply to demonstrate that the observed performance
difference with VACUUM was caused by whether the size of the
user buffer caused L2 thrashing.
- In general, application should reduce the size of the working set
to reduce the penalty of TLB misses and cache misses.
- If the application access pattern meets the NTA trigger condition,
the benefit of reducing the working set size will be much smaller.
Whatever I said is probably orthogonal to the buffer cache issue you
guys have been discussing, but I haven't read all the email exchange
on the subject.
Sherry Moore, Solaris Kernel Development http://blogs.sun.com/sherrym
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