linux-kernel - Re: [PATCH 0/3] Virtual huge zero page

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Message-ID: <20121001163118.GC18051@redhat.com>
Date:	Mon, 1 Oct 2012 18:31:18 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH 0/3] Virtual huge zero page

On Mon, Oct 01, 2012 at 08:34:28AM -0700, H. Peter Anvin wrote:
> On 09/29/2012 06:48 AM, Andrea Arcangeli wrote:
> > 
> > There would be a small cache benefit here... but even then some first
> > level caches are virtually indexed IIRC (always physically tagged to
> > avoid the software to notice) and virtually indexed ones won't get any
> > benefit.
> > 
> 
> Not quite.  The virtual indexing is limited to a few bits (e.g. three
> bits on K8); the right way to deal with that is to color the zeropage,
> both the regular one and the virtual one (the virtual one would circle
> through all the colors repeatedly.)
> 
> The cache difference, therefore, is *huge*.

Kirill measured the cache benefit and it provided a 6% gain, not very
huge but certainly significant.

> It's a performance tradeoff, and it can, and should, be measured.

I now measured the other side of the trade, by touching only one
character every 4k page in the range to simulate a very seeking load,
and doing so the physical huge zero page wins with a 600% margin, so
if the cache benefit is huge for the virtual zero page, the TLB
benefit is massive for the physical zero page.

Overall I think picking the solution that risks to regress the least
(also compared to current status of no zero page) is the safest.

Thanks!
Andrea
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