linux-kernel - Re: [00/17] Large Blocksize Support V3

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Date:	Thu, 26 Apr 2007 18:55:01 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Mel Gorman <mel@...net.ie>
CC:	David Chinner <dgc@....com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>, clameter@....com,
	linux-kernel@...r.kernel.org,
	William Lee Irwin III <wli@...omorphy.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>
Subject: Re: [00/17] Large Blocksize Support V3

Mel Gorman wrote:
> On (26/04/07 16:50), Nick Piggin didst pronounce:

>>Fragmentation is the problem. The anti-frag patches don't actually
>>guarantee anything about fragmentation, and even if they did, then
> 
> 
> The grouping pages by mobility do not guarantee anything but the memory
> partition (kernelcore= boot parameter) does give hard guarantees about
> the amount of memory that is "movable". Of course, the partition requires
> configuration at boot-time so it's less than ideal but it does give hard
> guarantees.

For the hugepages people, I can understand that's a solution. But that's
the last thing you want to do on a system with a limited amount of memory,
or a regular Joe's desktop/server.

> Indeed but then you have to deal with internal fragmentation 
> for pages-larger-than-TLB-page. I'm not saying it's wrong but it does
> come with it's own set of issues.

None of them is perfect (the ways to increase the size of pagecache pages,
that is).

I think in the long term, TLB page sizes will probably increase a little
bit... but if a given page size is "good enough" for a CPU, they really
should be good enough for other hardware. I mean, come on, the CPU's TLB
has to have a good hit ratio and handle several lookups per cycle with a
3-cycle latency on 3GHz+ hardware... surely a an IO controller's
scatter-gather engine or IOMMU that has to do a few lookups per disk IO
is nowhere near so critical as a CPU's datapath: just add a few more
entries to it, they've already got hundreds of megs of cache, so that
isn't an issue either.

-- 
SUSE Labs, Novell Inc.
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