Re: [Lsf-pc] [LSF/MM TOPIC] Support for 1GB THP

[Rik van Riel <riel@redhat.com>]

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On Tue, 2016-03-01 at 16:44 -0500, Matthew Wilcox wrote:
> On Tue, Mar 01, 2016 at 11:25:41AM +0100, Jan Kara wrote:
> > On Tue 01-03-16 02:09:11, Matthew Wilcox wrote:
> > > There are a few issues around 1GB THP support that I've come up
> > > against
> > > while working on DAX support that I think may be interesting to
> > > discuss
> > > in person.
> > > 
> > >  - Do we want to add support for 1GB THP for anonymous
> > > pages?  DAX support
> > >    is driving the initial 1GB THP support, but would anonymous
> > > VMAs also
> > >    benefit from 1GB support?  I'm not volunteering to do this
> > > work, but
> > >    it might make an interesting conversation if we can identify
> > > some users
> > >    who think performance would be better if they had 1GB THP
> > > support.
> > 
> > Some time ago I was thinking about 1GB THP and I was wondering:
> > What is the
> > motivation for 1GB pages for persistent memory? Is it the savings
> > in memory
> > used for page tables? Or is it about the cost of fault?
> 
> I think it's both.  I heard from one customer who calculated that
> with
> a 6TB server, mapping every page into a process would take ~24MB of
> page tables.  Multiply that by the 50,000 processes they expect to
> run
> on a server of that size consumes 1.2TB of DRAM.  Using 1GB pages
> reduces
> that by a factor of 512, down to 2GB.

Given the amounts of memory in systems, and the fact
that 1TB (or even 2MB) page sizes will not always be
possible, even with DAX on persistent memory, I
suspect it may be time to implement the reclaiming of
page tables that only map file pages.

-- 
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