linux-ext4 - Re: Directories

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Message-ID: <20061012002646.GP19345@melbourne.sgi.com>
Date:	Thu, 12 Oct 2006 10:26:47 +1000
From:	David Chinner <dgc@....com>
To:	Steve Lord <lord@....org>
Cc:	David Chinner <dgc@....com>, Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, xfs@....sgi.com
Subject: Re: Directories > 2GB

On Wed, Oct 11, 2006 at 11:49:10AM -0500, Steve Lord wrote:
> David Chinner wrote:
> >On Tue, Oct 10, 2006 at 10:19:04AM +0100, Christoph Hellwig wrote:
> >>On Mon, Oct 09, 2006 at 09:15:28PM -0500, Steve Lord wrote:
> >>>Hi Dave,
> >>>
> >>>My recollection is that it used to default to on, it was disabled
> >>>because it needs to map the buffer into a single contiguous chunk
> >>>of kernel memory. This was placing a lot of pressure on the memory
> >>>remapping code, so we made it not default to on as reworking the
> >>>code to deal with non contig memory was looking like a major
> >>>effort.
> >>Exactly.  The code works but tends to go OOM pretty fast at least
> >>when the dir blocksize code is bigger than the page size.  I should
> >>give the code a spin on my ppc box with 64k pages if it works better
> >>there.
> >
> >The pagebuf code doesn't use high-order allocations anymore; it uses
> >scatter lists and remapping to allow physically discontiguous pages
> >in a multi-page buffer. That is, the pages are sourced via
> >find_or_create_page() from the address space of the backing device,
> >and then mapped via vmap() to provide a virtually contigous mapping
> >of the multi-page buffer.
> >
> >So I don't think this problem exists anymore...
> 
> I was not referring to high order allocations here, but the overhead
> of doing address space remapping every time a directory is accessed.

Ah - ok. contig -> non-contig and OOM is usually discussed in the
context of higher order allocations failing. FWIW, I've not noticed
any extra overhead - the CPU usage seems to grow roughly linearly
with the increase in directory operations done as a result of
higher throughput for the same number of I/Os. I'll have a look at
the Vm stats, though, next time I run a comparison to see how bad
this is.

Thanks for the clarification, Steve.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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