Message-ID: <m1irbjknxn.fsf@ebiederm.dsl.xmission.com>
Date:	Thu, 26 Apr 2007 12:09:24 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	William Lee Irwin III <wli@...omorphy.com>
Cc:	Christoph Lameter <clameter@....com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	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

William Lee Irwin III <wli@...omorphy.com> writes:

> On Thu, 26 Apr 2007, Nick Piggin wrote:
>>> OK, I would like to see them. And also discussions of things like why
>>> we shouldn't increase PAGE_SIZE instead.
>
> On Thu, Apr 26, 2007 at 12:34:50AM -0700, Christoph Lameter wrote:
>> Because 4k is a good page size that is bound to the binary format? Frankly 
>> there is no point in having my text files in large page sizes. However, 
>> when I read a dvd then I may want to transfer 64k chunks or when use my 
>> flash drive I may want to transfer 128k chunks. And yes if a scientific 
>> application needs to do data dump then it should be able to use very high 
>> page sizes (megabytes, gigabytes) to be able to continue its work while 
>> the huge dumps runs at full I/O speed ...
>
> It's possible to divorce PAGE_SIZE from the binary formats, though I
> found it difficult to keep up with the update treadmill. 

On x86_64 the sizes is actually 64K for executable binaries if I
recall correctly.  It certainly is not PAGE_SIZE, so we have some
flexibility there.

> Maybe it's
> like hch says and I just needed to find more and better API cleanups.
> I've only not tried to resurrect it because it's too much for me to do
> on my own. I essentially collapsed under the weight of it and my 2.5.x
> codebase ended up worse than Katrina as a disaster, which I don't want
> to repeat and think collaborators or a different project lead from
> myself are needed to avoid that happening again.

But we still have some issues with mmap. But since we could increase
PAGE_SIZE on x86_64 and not have to even worry about sub PAGE_SIZE
mmaps.  It is being suggested that if people really need larger
physical pages that they just fix PAGE_SIZE.  The everything just
works.

Thinking about it changing PAGE_SIZE on x86_64 should be about as
hard as doing the 3-level vs 2-level page table format.  We say
we have a different page table format that uses a larger PAGE_SIZE.
All arch code, all code in paths that we expect to change.

Boom all done.

It might be worth implementing just so people can play with different
PAGE_SIZE values for benchmarking.

I don't think the larger physical page size is really the issue here
though. 

> It's unclear how much the situation has changed since 32-bit workload
> feasibility issues have been relegated to ignorable or deliberate
> "f**k 32-bit" status. The effect is doubtless to make it easier, though
> to what degree I'm not sure.

Perhaps.

> Anyway, if that's being kicked around as an alternative, it could be
> said that I have some insight into the issues surrounding it.

Partially but also partially they are very much suggesting going down
the same path.  Currently mmap doesn't work with order >0 pages because
they are not yet addressing these issues at all.

This looks like a more flexible version of the old PAGE_CACHE_SIZE >
PAGE_SIZE code.  Which makes seriously question the whole idea.

Eric
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