linux-kernel - Re: [PATCH 1/4] x86_64: Switch to SPARSE

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Message-ID: <Pine.LNX.4.64.0704051119400.9800@schroedinger.engr.sgi.com>
Date:	Thu, 5 Apr 2007 11:27:47 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andy Whitcroft <apw@...dowen.org>
cc:	Martin Bligh <mbligh@...gle.com>,
	Dave Hansen <hansendc@...ibm.com>, Andi Kleen <ak@...e.de>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 1/4] x86_64: Switch to SPARSE_VIRTUAL

On Thu, 5 Apr 2007, Andy Whitcroft wrote:

> Christoph if you could let us know which benchmarks you are seeing gains
> with that would be a help.

You saw the numbers that Ken got with the pipe test right?

Then there are some minor improvements if you run AIM7.

I got into this because I saw the code that was generated by sparsemem and 
discontig. Sparsemem was doing a series of lookups and so did pure 
discontig on x86_64. Both were really too complex for inlining. 
Discontig/Vmemmap on IA64 was sort of cleaner but there was still a 
reference to a variable plus it was creating the needs for lots of TLBs.

These VM primitives are pretty critical and the code for those needs to be 
minimal and inline. So I removed the variable ref from discontig/Vmemmap 
and reduced the wokr to be done to the simple formula (that I also found 
in textbooks on memmap management). Avoided the TLB pressure by using the 
1-1 mappings page size.

I could get real some performance numbers for this by sticking in a 
performance counter before and after virt_to_page and page_address. But I 
am pretty sure about the result just looking at the code.
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