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    SubjectRe: mmap dirty limits on 32 bit kernels (Was: [BUG] New Kernel Bugs)

    On Thu, 15 Nov 2007, Linus Torvalds wrote:
    > The problem with HIGHMEM is that it causes various metadata (dentries,
    > inodes, page struct tables etc) to eat up memory "prime real estate" under
    > the same kind of conditions that also dirty a lot of memory. So the reason
    > we disallow HIGHMEM from dirty limits is only *partly* the per-device or
    > mapping DMA limits, and to a large degree the fact that non-highmem memory
    > is special in general, and it is usually the non-highmem areas that are
    > constrained - and need to be protected.

    Final note on this (promise):

    I'd really be very interested to hear if the patch I *do* think makes
    sense (ie the removal of the old "unmapped_ratio" logic) actually already
    solves most of Bron's problems.

    It may well be that that unmapped_ratio logic effectively undid the system
    configuration changes that Bron has done. It doesn't matter if Bron has

    >From our sysctl.conf:
    # This should help reduce flushing on Cache::FastMmap files
    vm.dirty_background_ratio = 50
    vm.dirty_expire_centisecs = 9000
    vm.dirty_ratio = 80
    vm.dirty_writeback_centisecs = 3000

    if it turns out that the "unmapped_ratio" logic turns the 80% back down to

    It may well be that 80% of the non-highmem memory is plenty good enough!

    Sure, older kernels allowed even more of memory to be dirty (since they
    didn't count dirty mappings at all), but we may have a case where the fact
    that we discount the HIGHMEM stuff isn't the major problem in itself, and
    that the dirty_ratio sysctl should be ok - but just gets screwed over by
    that unmapped_ratio logic.

    So Bron, if you can test that patch, I'd love to hear if it matters. It
    may not make any difference (maybe you don't actually trigger the
    unmapped_ratio logic at all), but I think it has the potential for being
    totally broken for you.

    People that don't change the dirty_ratio from the default values would
    generally never care, because the default dirty-ratio is *already* so low
    that even if the unmapped_ratio logic triggers, it won't much matter!

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      Last update: 2007-11-15 22:29    [W:0.021 / U:17.924 seconds]
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