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:[EMAIL PROTECTED] 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
