From: Andrew Morton <akpm@linux-foundation.org>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: clameter@sgi.com, linux-mm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [QUICKLIST 0/4] Arch independent quicklists V2
Date: Tue, 13 Mar 2007 03:52:50 -0800 [thread overview]
Message-ID: <20070313035250.f908a50e.akpm@linux-foundation.org> (raw)
In-Reply-To: <45F65ADA.9010501@yahoo.com.au>
> On Tue, 13 Mar 2007 19:03:38 +1100 Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> Andrew Morton wrote:
> >>On Tue, 13 Mar 2007 00:13:25 -0700 (PDT) Christoph Lameter <clameter@sgi.com> wrote:
> >>Page table pages have the characteristics that they are typically zero
> >>or in a known state when they are freed.
> >
> >
> > Well if they're zero then perhaps they should be released to the page allocator
> > to satisfy the next __GFP_ZERO request. If that request is for a pagetable
> > page, we break even (except we get to remove special-case code). If that
> > __GFP_ZERO allocation was or some application other than for a pagetable, we
> > win.
> >
> > iow, can we just nuke 'em?
>
> Page allocator still requires interrupts to be disabled, which this doesn't.
Bah. How many cli/sti statements fit into a single cachemiss?
> Considering there isn't much else that frees known zeroed pages, I wonder if
> it is worthwhile.
If you want a zeroed page for pagecache and someone has just stuffed a
known-zero, cache-hot page into the pagetable quicklists, you have good
reason to be upset.
In fact, if you want a _non_-zeroed page and someone has just stuffed a
known-zero, cache-hot page into the pagetable quicklists, you still have
reason to be upset. You *want* that cache-hot page.
Generally, all these little private lists of pages (such as the ones which
slab had/has) are a bad deal. Cache effects preponderate and I do think
we're generally better off tossing the things into a central pool.
Plus, we can get in a situation where take a cache-cold, known-zero page
from the pte quicklist when there is a cache-hot, non-zero page sitting in
the page allocator. I suspect that zeroing the cache-hot page would take a
similar amount of time to a single miss agains the cache-cold page.
I'm not saying that I _know_ that the quicklists are pointless, but I don't
think it's established that they are pointful.
ISTR that experiments with removing the i386 quicklists made zero
difference, but that was an awfully long time ago. Significantly, it
predated per-cpu-pages..
> Last time the zeroidle discussion came up was IIRC not actually real performance
> gain, just cooking the 1024 CPU threaded pagefault numbers ;)
Maybe, dunno. It was apparently a win on powerpc many years ago. I had a
fiddle with it 5-6 years ago on x86 using a cache-disabled mapping of the
page. But it needed too much support in core VM to bother. Since then
we've grown per-cpu page magazines and __GFP_ZERO. Plus I'm not aware of
anyone having tried doing it on x86 with non-temporal stores.
next prev parent reply other threads:[~2007-03-13 10:52 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-03-13 7:13 [QUICKLIST 0/4] Arch independent quicklists V2 Christoph Lameter
2007-03-13 7:13 ` [QUICKLIST 1/4] Generic quicklist implementation Christoph Lameter
2007-03-13 9:05 ` Paul Mundt
2007-03-15 20:51 ` Christoph Lameter
2007-03-13 7:13 ` [QUICKLIST 2/4] Quicklist support for i386 Christoph Lameter
2007-03-13 7:13 ` [QUICKLIST 3/4] Quicklist support for x86_64 Christoph Lameter
2007-03-13 7:13 ` [QUICKLIST 4/4] Quicklist support for sparc64 Christoph Lameter
2007-03-13 8:53 ` [QUICKLIST 0/4] Arch independent quicklists V2 Andrew Morton
2007-03-13 8:03 ` Nick Piggin
2007-03-13 11:52 ` Andrew Morton [this message]
2007-03-13 11:06 ` Nick Piggin
2007-03-13 12:15 ` Andrew Morton
2007-03-13 11:20 ` Christoph Lameter
2007-03-13 12:30 ` Andrew Morton
2007-03-15 20:23 ` Christoph Lameter
2007-03-13 11:30 ` Nick Piggin
2007-03-13 12:47 ` Andrew Morton
2007-03-13 12:01 ` Nick Piggin
2007-03-13 13:11 ` Andrew Morton
2007-03-13 12:18 ` Nick Piggin
2007-03-13 17:30 ` Jeremy Fitzhardinge
2007-03-13 20:03 ` Matt Mackall
2007-03-13 20:17 ` Jeremy Fitzhardinge
2007-03-13 20:21 ` Matt Mackall
2007-03-13 21:07 ` David Miller
2007-03-13 21:14 ` Matt Mackall
2007-03-13 21:36 ` Jeremy Fitzhardinge
2007-03-13 21:46 ` Peter Chubb
2007-03-13 21:48 ` David Miller
2007-03-14 1:12 ` William Lee Irwin III
2007-03-15 23:12 ` William Lee Irwin III
2007-03-13 23:58 ` Paul Mackerras
2007-03-13 11:17 ` Christoph Lameter
2007-03-13 12:27 ` Andrew Morton
2007-03-15 20:28 ` Christoph Lameter
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