linux-kernel - Re: [PATCH 0/7] [RFC] hugetlb: pagetable

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Date:	Tue, 20 Mar 2007 16:50:13 -0700
From:	Dave Hansen <hansendc@...ibm.com>
To:	Adam Litke <agl@...ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	William Lee Irwin III <wli@...omorphy.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ken Chen <kenchen@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] [RFC] hugetlb: pagetable_operations API (V2)

On Mon, 2007-03-19 at 13:05 -0700, Adam Litke wrote:
> For the common case (vma->pagetable_ops == NULL), we do almost the
> same thing as the current code: load and test.  The third instruction
> is different in that we jump for the common case instead of jumping in
> the hugetlb case.  I don't think this is a big deal though.  If it is,
> would an unlikely() macro fix it? 

I wouldn't worry about micro-optimizing it at that level.  The CPU does
enough stuff under the covers that I wouldn't worry about it at all.

I wonder if the real differential impact (if any) is likely to come from
the pagetable_ops cacheline being hot or cold, since it is in a
different place in the structure than the flags.  But, from a quick
glance I see a few vm_ops references preceding pagetable_ops references,
so the pagetable_ops cacheline might already be hot most of the time.  

BTW, are there any other possible users for these things other than
large pages?

-- Dave

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