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    SubjectRe: [patch 00/14] Page cache cleanup in anticipation of Large Blocksize support
    > On Thu, 14 Jun 2007 15:22:46 -0700 (PDT) Christoph Lameter <clameter@sgi.com> wrote:
    > On Thu, 14 Jun 2007, Andrew Morton wrote:
    >
    > > With 64k pagesize the amount of memory required to hold a kernel tree (say)
    > > will go from 270MB to 1400MB. This is not an optimisation.
    >
    > I do not think that the 100% users will do kernel compiles all day like
    > we do. We likely would prefer 4k page size for our small text files.

    There are many, many applications which use small files.

    > > Several 64k pagesize people have already spent time looking at various
    > > tail-packing schemes to get around this serious problem. And that's on
    > > _server_ class machines. Large ones. I don't think
    > > laptop/desktop/samll-server machines would want to go anywhere near this.
    >
    > I never understood the point of that exercise. If you have variable page
    > size then the 64k page size can be used specific to files that benefit
    > from it. Typically usage scenarios are video audio streaming I/O, large
    > picture files, large documents with embedded images. These are the major
    > usage scenarioes today and we suck the. Our DVD/CD subsystems are
    > currently not capable of directly reading from these devices into the page
    > cache since they do not do I/O in 4k chunks.

    So with sufficient magical kernel heuristics or operator intervention, some
    people will gain some benefit from 64k pagesize. Most people with most
    workloads will remain where they are: shoving zillions of physically
    discontiguous pages into fixed-size sg lists.

    Whereas with contig-pagecache, all users on all machines with all workloads
    will benefit from the improved merging.

    > > > fsck times etc etc are becoming an issue for desktop
    > > > systems
    > >
    > > I don't see what fsck has to do with it.
    > >
    > > fsck is single-threaded (hence no locking issues) and operates against the
    > > blockdev pagecache and does a _lot_ of small reads (indirect blocks,
    > > especially). If the memory consumption for each 4k read jumps to 64k, fsck
    > > is likely to slow down due to performing a lot more additional IO and due
    > > to entering page reclaim much earlier.
    >
    > Every 64k block contains more information and the number of pages managed
    > is reduced by a factor of 16. Less seeks , less tlb pressure , less reads,
    > more cpu cache and cpu cache prefetch friendly behavior.

    argh. Everything you say is just wrong. A fsck involves zillions of
    discontiguous small reads. It is largely seek-bound, so there is no
    benefit to be had here. Your proposed change will introduce regressions by
    causing larger amounts of physical reading and large amounts of memory
    consumption.


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