linux-kernel - Re: [00/41] Large Blocksize Support V7 (adds memmap support)

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Date:	Wed, 12 Sep 2007 10:20:11 +0200
From:	Andrea Arcangeli <andrea@...e.de>
To:	Christoph Lameter <clameter@....com>
Cc:	Jörn Engel <joern@...fs.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
	Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.com>,
	David Chinner <dgc@....com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Fengguang Wu <fengguang.wu@...il.com>,
	swin wang <wangswin@...il.com>, totty.lu@...il.com,
	hugh@...itas.com
Subject: Re: [00/41] Large Blocksize Support V7 (adds memmap support)

On Tue, Sep 11, 2007 at 05:04:41PM -0700, Christoph Lameter wrote:
> I would think that your approach would be slower since you always have to 
> populate 1 << N ptes when mmapping a file? Plus there is a lot of wastage 

I don't have to populate them, I could just map one at time. The only
reason I want to populate every possible pte that could map that page
(by checking vma ranges) is to _improve_ performance by decreasing the
number of page faults of an order of magnitude. Then with the 62th bit
after NX giving me a 64k tlb, I could decrease the frequency of the
tlb misses too.

> of memory because even a file with one character needs an order N page? So 
> there are less pages available for the same workload.

This is a known issue. The same is true for ppc64 64k. If that really
is an issue, that may need some generic solution with tail packing.

> Then you are breaking mmap assumptions of applications becaused the order 
> N kernel will no longer be able to map 4k pages.  You likely need a new 
> binary format that has pages correctly aligned. I know that we would need 
> one on IA64 if we go beyond the established page sizes.

No you misunderstood the whole design. My patch will be 100% backwards
compatible in all respects. If I could break backwards compatibility
70% of the complexity would go away...
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