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

[Borzenkov, Andrey]

> -----Original Message-----
> From: Avi Kivity [mailto:avi@xxxxxxxxxx]
> Sent: Wednesday, January 11, 2012 3:27 PM
> To: Borzenkov, Andrey
> Cc: Eric Dumazet; linux-kernel@xxxxxxxxxxxxxxx
> 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? 
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