linux-kernel - Re: [QUICKLIST 0/4] Arch independent quicklists V2

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Message-Id: <20070313035250.f908a50e.akpm@linux-foundation.org>
Date:	Tue, 13 Mar 2007 03:52:50 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	clameter@....com, linux-mm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [QUICKLIST 0/4] Arch independent quicklists V2

> On Tue, 13 Mar 2007 19:03:38 +1100 Nick Piggin <nickpiggin@...oo.com.au> wrote:
> Andrew Morton wrote:
> >>On Tue, 13 Mar 2007 00:13:25 -0700 (PDT) Christoph Lameter <clameter@....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.

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