linux-kernel - RE: Extermeley large PageTables over 500G on kernel 2.6.32.49 (SLES11 SP1)

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Message-ID: <CB1B361E45A85849902B25342E88900501CFDCE5C966@ABGEX70E.FSC.NET>
Date:	Fri, 13 Jan 2012 06:20:23 +0100
From:	"Borzenkov, Andrey" <andrey.borzenkov@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: Extermeley large PageTables over 500G on kernel 2.6.32.49
 (SLES11 SP1)

> -----Original Message-----
> From: Avi Kivity [mailto:avi@...hat.com]
> Sent: Wednesday, January 11, 2012 3:27 PM
> To: Borzenkov, Andrey
> Cc: Eric Dumazet; linux-kernel@...r.kernel.org
> Subject: Re: Extermeley large PageTables over 500G on kernel 2.6.32.49
> (SLES11 SP1)
> 
> On 01/11/2012 12:52 PM, Borzenkov, Andrey wrote:
> > > If not, its normal to eat so much memory for page tables
> > >
> > > grep VmPTE /proc/*/status
> > >
> > >
> >
> > Forgive my ignorance. I thought that
> >
> > 1. PTE is 8 bytes per page, which is 4K which gives 2K per 1M of memory
> > 2. All processes sharing the same shared memory share the same page
> table
> >
> > So page table for Oracle SGA 500G would be around 1G and shared by all
> Oracle clients. Is my assumption incorrect?
> >
> 
> The second assumption is incorrect.  So fully populated the 2000
> processes would consume 2T; they just haven't accessed all the SGA yet.
> 

Does kernel ever free allocated page tables, or once allocated they stick for the process lifetime?