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Re: PostgreSQL and HugePage

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: daveg <daveg(at)sonic(dot)net>
Cc: Greg Stark <gsstark(at)mit(dot)edu>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Mark Kirkwood <mark(dot)kirkwood(at)catalyst(dot)net(dot)nz>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: PostgreSQL and HugePage
Date: 2010-10-20 20:13:55
Message-ID: AANLkTinwzLqxE0Q57cU3UE+jV5Ei-YgCk6SFUNg6Zwbn@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, Oct 20, 2010 at 3:47 PM, daveg <daveg(at)sonic(dot)net> wrote:
> On Wed, Oct 20, 2010 at 12:28:25PM -0700, Greg Stark wrote:
>> On Wed, Oct 20, 2010 at 12:17 PM, Greg Stark <gsstark(at)mit(dot)edu> wrote:
>> > I don't think it's a big cost once all the processes
>> > have been forked if you're reusing them beyond perhaps slightly more
>> > efficient cache usage.
>>
>> Hm, this site claims to get a 13% win just from the reduced tlb misses
>> using a preload hack with Pg 8.2. That would be pretty substantial.
>>
>> http://oss.linbit.com/hugetlb/
>
> That was my motivation in trying a patch. TLB misses can be a substantial
> overhead. I'm not current on the state of play, but working at Sun's
> benchmark lab on a DB TPC-B benchmark something for the first generation
> of MP systems, something like 30% of all bus traffic was TLB misses. The
> next iteration of the hardward had a much larger TLB.
>
> I have a client with 512GB memory systems, currently with 128GB configured
> as postgresql buffer cache. Which is 32M TLB entires trying to fit in the
> few dozed cpu TLB slots. I suspect there may be some contention.
>
> I'll benchmark of course.

Do you mean 128GB shared buffers, or shared buffers + OS cache?  I
think that the general wisdom is that performance tails off beyond
8-10GB of shared buffers anyway, so a performance improvement on 128GB
shared buffers might not mean much unless you can also show that 128GB
shared buffers actually performs better than some smaller amount.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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