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

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Message-ID: <Pine.LNX.4.64.0703130413080.5720@schroedinger.engr.sgi.com>
Date:	Tue, 13 Mar 2007 04:17:26 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	linux-mm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [QUICKLIST 0/4] Arch independent quicklists V2

On Tue, 13 Mar 2007, 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.

Nope that wont work.

1. We need to support other states of pages other than zeroed.

2. Prezeroing does not make much sense if a large portion of the
   page is being used. Performance is better if the whole page 
   is zeroed directly before use.Prezeroing only makes sense for sparse
   allocations like the page table pages.

> (Will require some work in the page allocator)
> (That work will open the path to using the idle thread to prezero pages)

I already tried that 3 years ago and there was *no* benefit for usual
users of the a page allocator. The advantage exists only if a small
portion of the page is used. F.e. For one cacheline there was a 4x 
improvement. See lkml archives for prezeroing.

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