linux-kernel - Re: [QUICKLIST 1/5] Quicklists for page table pages V3

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Message-Id: <20070319175524.812b2f5d.akpm@linux-foundation.org>
Date:	Mon, 19 Mar 2007 17:55:24 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	linux-mm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [QUICKLIST 1/5] Quicklists for page table pages V3

On Mon, 19 Mar 2007 17:44:28 -0700 (PDT)
Christoph Lameter <clameter@....com> wrote:

> On Mon, 19 Mar 2007, Andrew Morton wrote:
> 
> > Please provide proof that quicklists are superior to simply going direct to
> > the page allocator for these pages.
> 
> See the patch. We are only touching 2 cachelines instead of 32. So even 
> without considering the page allocator overhead and the slab allocator 
> overhead (which will make the situation even better) its superior.

That's not proof, it is handwaving.  I could wave right back at you and
claim that the benefit from returning a cache-hot pte page back to the page
allocator for reuse exceeds the benefit which you waved at me above.

You may well be right, but nothing is proven, afaict.

> > > I doubt it. The zeroing is a by product of our way of serializing pte 
> > > handling. Its going to be difficult to change that.
> > 
> > Nick didn't think so, and I don't see the problem either.
> 
> You do not think that our current way of handling ptes is okay? If we do 
> not zero the ptes then we need to separate munmap from process shutdown.

Yep.  It's possible that process shutdown is a sufficiently common and
costly special-case for it to be worth special-casing.

> > We'll save on some bus traffic by avoiding the writeback, but how much
> > effect that will have we don't know.  Presumably little.
> 
> The advantage of the quicklists is that it does not require a rework of 
> the pte serialization.

No, these are unrelated.  We can get pte pages from the page allocator and
zero them without touching the munmap handling.

But it's possible that if we _were_ to optimise the munmap handling as
suggested, the end result would be superior.

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