linux-kernel - Re: [RFC] de-asmify the x86-64 system call slowpath

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Date:	Thu, 6 Feb 2014 06:42:09 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Peter Anvin <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ning Qu <quning@...gle.com>
Subject: Re: [RFC] de-asmify the x86-64 system call slowpath


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> [...]
> 
> No, I was thinking "try to optimistically map 8 adjacent aligned 
> pages at a time" - that would be the same cacheline in the page 
> tables, so it would be fairly cheap if we couple it with a 
> gang-lookup of the pages in the page cache (or, for anonymous pages, 
> by just optimistically trying to do an order-3 page allocation, and 
> if that works, just map the 32kB allocation you got as eight 
> individual pages).
> 
> I know it's been discussed at some point, and I even have a dim 
> memory of having seen some really ugly patches.

I have a dim memory of having written such group-prefaulting patches 
myself a decade ago or so - IIRC the main problem was that at that 
time we never found a common load where it really mattered, and it was 
easy to spend more time doing all this extra work and not see the 
prefaulted pages used.

But the cost/benefit balance has indeed changed so IMO it's worth a 
try.

Thanks,

	Ingo
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