linux-kernel - Re: [PATCH, RFC 0/9] Introduce huge zero page

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Message-ID: <502D7D6E.8090303@linux.intel.com>
Date:	Thu, 16 Aug 2012 16:08:30 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-mm@...ck.org, Andi Kleen <ak@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH, RFC 0/9] Introduce huge zero page

On 08/16/2012 12:40 PM, Andrea Arcangeli wrote:
> Hi Andrew,
> 
> On Thu, Aug 16, 2012 at 12:20:23PM -0700, Andrew Morton wrote:
>> That's a pretty big improvement for a rather fake test case.  I wonder
>> how much benefit we'd see with real workloads?
> 
> The same discussion happened about the zero page in general and
> there's no easy answer. I seem to recall that it was dropped at some
> point and then we reintroduced the zero page later.
> 
> Most of the time it won't be worth it, it's just a few pathological
> compute loads that benefits IIRC. So I'm overall positive about it
> (after it's stable).
> 
> Because this is done the right way (i.e. to allocate an hugepage at
> the first wp fault, and to fallback exclusively if compaction fails)
> it will help much less than the 4k zero pages if the zero pages are
> scattered over the address space and not contiguous (it only helps if
> there are 512 of them in a row). OTOH if they're contiguous, the huge
> zero pages will perform better than the 4k zero pages.
> 

One thing that I asked for testing a "virtual zero page" where the same
page (or N pages for N-way page coloring) is reused across a page table.
 It would have worse TLB performance but likely *much* better cache
behavior.

	-hpa


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