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Re: Report: Linux huge pages with Postgres

From: Martijn van Oosterhout <kleptog(at)svana(dot)org>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Simon Riggs <simon(at)2ndQuadrant(dot)com>, pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: Report: Linux huge pages with Postgres
Date: 2010-11-28 19:45:31
Message-ID: 20101128194531.GB3796@svana.org (view raw or flat)
Thread:
Lists: pgsql-hackers
On Sun, Nov 28, 2010 at 02:32:04PM -0500, Tom Lane wrote:
> > Sure, but 4MB of memory is enough to require 1000 TLB entries, which is
> > more than enough to blow the TLB even on a Nehalem.
> 
> That can't possibly be right.  I'm sure the chip designers have heard of
> programs using more than 4MB.

According to
http://www.realworldtech.com/page.cfm?ArticleID=RWT040208182719&p=8
on the Core 2 chip there wasn't even enough TLB to cover the entire
onboard cache. With Nehalem there are 2304 TLB entries on the chip,
which cover at least the whole onboard cache, but only just.

Memory access is expensive. I think if you got good statistics on how
much time your CPU is waiting for memory it'd be pretty depressing.

Have a nice day,
-- 
Martijn van Oosterhout   <kleptog(at)svana(dot)org>   http://svana.org/kleptog/
> Patriotism is when love of your own people comes first; nationalism,
> when hate for people other than your own comes first. 
>                                       - Charles de Gaulle

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