Re: Bug: Buffer cache is not scan resistant

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.

- Luke

Msg is shrt cuz m on ma treo

-----Original Message-----
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

Hi Simon,

> 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.

Thanks,
Sherry
--
Sherry Moore, Solaris Kernel Development http://blogs.sun.com/sherrym