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    SubjectRe: 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.


    Dave Chinner
    Principal Engineer
    SGI Australian Software Group
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