linux-kernel - Re: [PATCh] x86: overmapped fix when 4K pages on tail

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Date:	Sun, 13 Jul 2008 17:04:20 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Ingo Molnar <mingo@...e.hu>, Yinghai Lu <yhlu.kernel@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCh] x86: overmapped fix when 4K pages on tail - 64bit

Andi Kleen wrote:
> 
> First I was only commenting on one specific patch, nothing more.
> 
> My point is full rounding to 4K on all corners is wasteful because the
> CPUs have to handle that case anyways and every split costs precious
> TLB entries in direct mapping accesses.
>

Well, the CPU *does* handle them... by splitting the larger pages into 
smaller pages.  They still end up in the small-page TLB, so there is no 
real difference if done in the CPU or in software.

> And I might be old fashioned, but I still think minimizing TLB misses
> in the kernel is still quite important since the TLBs of modern x86
> CPUs are still comparatively small.
> 
> btw that is why I was  also quite disappointed that the new cpa eliminated
> reassembly. It means that on a long uptime system even with moderate
> traffic of CPA page allocation/free eventually the completely direct mapping
> will be all 4K. And there will be TLB miss galore on each system call
> when user space is TLB intensive.
> 
> Ok in that light Yinghai's patch is perhaps not so bad after longer
> uptime in that scenario. Still performance directly after boot up is
> also something that shouldn't be ignored and I'm still hopefully that
> reassembly will be readded at some point anyways.

Memory state transitions are (fortunately) relatively rare and 
long-lived, but of course having reassembly is a nice thing to have in 
the long run.  Such reassembly also would rather naturally handle any 
small-page effects of boundary cases.

	-hpa
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