linux-kernel - Re: Large stack usage in fs code (especially for PPC64)

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Message-Id: <200811182124.33141.nickpiggin@yahoo.com.au>
Date:	Tue, 18 Nov 2008 21:24:32 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, linuxppc-dev@...abs.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Large stack usage in fs code (especially for PPC64)

On Tuesday 18 November 2008 13:08, Linus Torvalds wrote:
> On Tue, 18 Nov 2008, Paul Mackerras wrote:
> > Also, you didn't respond to my comments about the purely software
> > benefits of a larger page size.
>
> I realize that there are benefits. It's just that the downsides tend to
> swamp the upsides.
>
> The fact is, Intel (and to a lesser degree, AMD) has shown how hardware
> can do good TLB's with essentially gang lookups, giving almost effective
> page sizes of 32kB with hardly any of the downsides. Couple that with

It's much harder to do this with powerpc I think because they would need
to calculate 8 hashes and touch 8 cachelines to prefill 8 translations,
wouldn't they?


> low-latency fault handling (for not when you miss in the TLB, but when
> something really isn't in the page tables), and it seems to be seldom the
> biggest issue.
>
> (Don't get me wrong - TLB's are not unimportant on x86 either. But on x86,
> things are generally much better).

The per-page processing costs are interesting too, but IMO there is more
work that should be done to speed up order-0 pages. The patches I had to
remove the sync instruction for smp_mb() in unlock_page sped up pagecache
throughput (populate, write(2), reclaim) on my G5 by something really
crazy like 50% (most of that's in, but I'm still sitting on that fancy
unlock_page speedup to remove the final smp_mb).

I suspect some of the costs are also in powerpc specific code to insert
linux ptes into their hash table. I think some of the synchronisation for
those could possibly be shared with generic code so you don't need the
extra layer of locks there.
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