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    SubjectRe: [RFC] [PATCH -mm 0/2] memcg: per cgroup dirty_ratio
    Michael Rubin wrote:
    > On Fri, Sep 12, 2008 at 1:18 PM, Andrew Morton
    > <> wrote:
    >> One thing to think about please: Michael Rubin is hitting problems with
    >> the existing /proc/sys/vm/dirty-ratio. Its present granularity of 1%
    >> is just too coarse for really large machines, and as
    >> memory-size/disk-speed ratios continue to increase, this will just get
    >> worse.
    > Re-sending since I top-posted before. Never again. Also adding more
    > thoughts on a byte based interface.
    > Currently the problem we are hitting is that we cannot specify pdflush
    > to have background limits less than 1% of memory. I am currently
    > finishing up a patch right now that adds a dirty_ratio_millis
    > interface. I hope to submit the patch to LKML by the end of the week.
    > The idea is that we don't want to break backwards compatibility and we
    > also don't want to have two conflicting knobs in the sysctl or
    > /proc/sys/vm/ space. I thought adding a new knob for those who want to
    > specify finer grained functionality was a compromise. So the patch has
    > a vm_dirty_ratio and a vm_dirty_ratio_millis interface. The first to
    > specify 0-100% and the second to specify .0 to .999%.
    > So to represent 0.125% of RAM we set
    > vm_dirty_ratio = 0
    > vm_dirty_ratio_millis = 125
    > The same for the background_ratio.
    > I would also prefer using a bytes interface but I am not sure how to
    > offer that without either removing the legacy interface of the ratios
    > or by offering a concurrent interface that might be confusing such as
    > when users are looking at the old one and not aware of a new one.
    > Any feedback?
    > mrubin

    I think using millis is ok today, but it may not scale well to systems
    with 1TB of memory (in this case the min granularity would be 10MB).

    A bytes/pages interface would resolve such problem also for tomorrow

    Moreover, wouldn't it be safer to set them mutually exclusive? I mean,
    writing a value != 0 to vm_dirty_millis automatically sets
    vm_dirty_ratio to 0 (disabled) and vice versa (this could be implemented
    using an appropriate .proc_handler for example).

    OK, I would like to set percentages like 12.456%, but if we don't do so
    a simple "sysctl -p" could create unexpected behaviours, reconfiguring
    the vm_dirty_ratio and not vm_dirty_ratio_millis for example.

    The same should be valid also for a bytes/pages interface, so setting
    vm_dirty_bytes != 0 (or vm_dirty_pages) should "disable" vm_dirty_ratio
    and vice versa.


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      Last update: 2008-09-23 14:53    [W:0.026 / U:5.584 seconds]
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