linux-kernel - Re: [QUICKLIST 0/6] Arch independent quicklists V1

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Date:	Mon, 12 Mar 2007 04:12:32 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	David Miller <davem@...emloft.net>
cc:	linux-kernel@...r.kernel.org, ak@...e.com, holt@....com,
	linux-ia64@...r.kernel.org, mpm@...enic.com
Subject: Re: [QUICKLIST 0/6] Arch independent quicklists V1

On Sun, 11 Mar 2007, David Miller wrote:

> I'm going to make the radical declaration that it be perhaps often
> better to always initialize page table chunks to all zeros on
> allocation.

That is the case if most of the page is going to be used soon. If we have 
sparse access patterns then not zeroing can avoid uselessly bringing 
cachelines in.

> The reason is that every time I've monitored the allocation patterns
> of these things on SMP, the page table chunks always get released on a
> different cpu than where they were initialized.

But its even advantageous in that case for sparse allocs.
 
> The allocator side just does nothing but emit L2 cache line ownership
> transactions as the pte page is touched.  Especially on chips like
> PowerPC where zero initialization is absurdly cheap, we can avoid all
> of the cache line transfers if we just initialize it at allocation
> time.

We have no need to touch all the cache lines w/o initialization if we 
alloc from the quicklist. And that is a performance benefit.

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