From: Andrea Arcangeli <aarcange@redhat.com>
To: "H. Peter Anvin" <hpa@linux.intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, Andi Kleen <ak@linux.intel.com>,
linux-kernel@vger.kernel.org,
"Kirill A. Shutemov" <kirill@shutemov.name>,
Arnd Bergmann <arnd@arndb.de>, Ingo Molnar <mingo@kernel.org>,
linux-arch@vger.kernel.org
Subject: Re: [PATCH 0/3] Virtual huge zero page
Date: Mon, 1 Oct 2012 18:31:18 +0200 [thread overview]
Message-ID: <20121001163118.GC18051@redhat.com> (raw)
In-Reply-To: <5069B804.6040902@linux.intel.com>
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
next prev parent reply other threads:[~2012-10-01 16:31 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-09-28 23:37 [PATCH 0/3] Virtual huge zero page Kirill A. Shutemov
2012-09-28 23:37 ` [PATCH 1/3] asm-generic: introduce pmd_special() and pmd_mkspecial() Kirill A. Shutemov
2012-09-28 23:37 ` [PATCH 2/3] mm, thp: implement virtual huge zero page Kirill A. Shutemov
2012-09-28 23:37 ` [PATCH 3/3] x86: implement HAVE_PMD_SPECAIL Kirill A. Shutemov
2012-09-29 13:48 ` [PATCH 0/3] Virtual huge zero page Andrea Arcangeli
2012-09-29 14:30 ` Andi Kleen
2012-09-29 14:37 ` Andrea Arcangeli
2012-10-01 13:49 ` Kirill A. Shutemov
2012-10-01 16:14 ` Andrea Arcangeli
2012-10-01 17:18 ` Kirill A. Shutemov
2012-10-01 15:34 ` H. Peter Anvin
2012-10-01 16:31 ` Andrea Arcangeli [this message]
2012-10-01 17:03 ` H. Peter Anvin
2012-10-01 17:15 ` Kirill A. Shutemov
2012-10-01 18:03 ` Andrea Arcangeli
2012-10-01 17:26 ` Andrea Arcangeli
2012-10-01 17:33 ` H. Peter Anvin
2012-10-01 17:36 ` Kirill A. Shutemov
2012-10-01 17:37 ` H. Peter Anvin
2012-10-01 17:44 ` Kirill A. Shutemov
2012-10-01 17:52 ` H. Peter Anvin
2012-10-01 18:56 ` Kirill A. Shutemov
2012-10-01 18:05 ` Andrea Arcangeli
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