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

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Date:	Thu, 26 Apr 2007 08:19:03 -0700
From:	William Lee Irwin III <wli@...omorphy.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Andy Whitcroft <apw@...dowen.org>,
	Christoph Lameter <clameter@....com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, Mel Gorman <mel@...net.ie>,
	David Chinner <dgc@....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

Andy Whitcroft wrote:
>> To my knowledge, moving to a higher base page size has its advantages in
>> TLB reach, but brings with it some pretty serious downsides.  Especially
>> in caching small files.  Internal fragmentation in the page cache
>> significantly affecting system performance.  So much so that development
>> is ongoing to see if supporting sub-base-page objects in the buffer
>> cache could be beneficial.

On Fri, Apr 27, 2007 at 01:08:17AM +1000, Nick Piggin wrote:
> I think 16K would be pretty reasonable (ia64 tends to use it). I guess
> powerpc went to 64k either because that's what databases want or because
> their TLB refills are too slow, so the internal fragmentation bites
> them a lot harder.
> But that was more of a side comment, because I still think io controllers
> should be easily capable of operation on 4K pages. Graphics cards are,
> aren't they?

Base page size need only be a power-of-two multiple of the MMU pagesize.
ISTR the 64KB PAGE_SIZE in ppc64 being motivated by TLB considerations.

There is not going to be a way to improve how well IO controllers deal
with 4K pages. The scatter/gather lists for smaller fragments will
require more device communication overhead no matter what, and
furthermore, the more discontiguity, the more seekiness.


-- wli
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