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

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Date:	Wed, 11 Jan 2012 12:23:37 +0100
From:	"Borzenkov, Andrey" <andrey.borzenkov@...fujitsu.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"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: Eric Dumazet [mailto:eric.dumazet@...il.com]
> Sent: Wednesday, January 11, 2012 3:20 PM
> To: Borzenkov, Andrey
> Cc: linux-kernel@...r.kernel.org
> Subject: RE: Extermeley large PageTables over 500G on kernel 2.6.32.49
> (SLES11 SP1)
> 
> Le mercredi 11 janvier 2012 à 11:52 +0100, Borzenkov, Andrey a écrit :
> 
> > Colleagues responsible for Oracle are reconfiguring it now.
> >
> 
> OK. Since your hardware supports 1GB hugepages, you might try to use
> them as well. Not sure if your kernel is recent enough...
> 
> cat /proc/cmdline
> ro root=LABEL=/ hugepagesz=1GB hugepages=512
> 

It says 2M so it probably is the limit.

> > > 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
> 
> It depends how oracle maps its SGA.
> 
> It probably uses a method disallowing page table sharing.

Do you have any reference to description of these methods so we can check?

Thank you!

> 
> Anyway, hugetables for this kind of workload is a must.
>