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    SubjectRe: [RFC] [PATCH -mm 0/2] memcg: per cgroup dirty_ratio
    Balbir Singh wrote:
    > 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.
    > Just provide a vm_dirty_ration_in_bytes interface and keep it in sync with
    > vm_dirty_ratio (they are just two representations of the same internal value)
    > and for higher resolution propose that users use the bytes interface.

    Hi Balbir,

    now that I read carefully the documentation, the description in
    Documentation/filesystems/proc.txt seems to be a bit misleading. In
    proc.txt we say that dirty_ratio and dirty_background_ratio are "a
    percentage of total system memory", but in mm/page-writeback.c we apply
    the percentages to the dirtyable memory: free pages + reclaimable pages.
    So, first of all I think we should clarify this in the documentation...

    Saying that, keeping in sync the vm_dirty_amount_in_bytes according to
    dirty_ratio_in_percentage is not a trivial task. One is a static value,
    the other depends on the dirtyable memory in the system. If we want to
    preserve the same behaviour we should do the following:

    dirty_ratio = x => dirty_amount_in_bytes = x * dirtyable_memory / 100

    dirty_amount_in_bytes = y => dirty_ratio = y / dirtyable_memory * 100

    But anytime the dirtyable memory (or the total memory in the system)
    changes we should update both values accordingly to preserve the
    coherency between them (ouch!).

    Possible solutions:

    1) introduce fine-grained dirty_ratio handling decimals by an opportune
    parser (disadvantage: this would break the compatibility with all the
    userspace apps that expect to read an int from vm_dirty_ratio)

    2) introduce dirty_ratio + dirty_ratio_millis (disadvantage: can
    generate unexpected behaviours when something is written to
    dirty_ratio ignoring the existence of dirty_ratio_millis)

    3) introduce dirty_ratio + dirty_amount_in_bytes mutually exclusive,
    writing to one automatically "disable" the other (disadvantage:
    writing to dirty_ratio ignoring dirty_amount_in_bytes can cause
    unexpected behaviours)

    4) introduce dirty_ratio + dirty_amount_in_bytes and change the
    old behaviour: when something is written to dirty_ratio,
    dirty_amount_in_bytes is evaluated in function of totalram_pages (or
    the memcg limit) and then we always use this static value, instead of
    something that depends on the dirtyable memory - we can easily update
    dirty_amount_in_bytes also when totalram_pages or the memcg limit
    changes (disadvantage: change an old - working - behaviour).

    5) handle fine-grained dirty_ratio decimals by an opportune parser when
    writing something to dirty_ratio; export the percentage units via
    dirty_ratio, and the decimals via dirty_ratio_decimals; writing to
    dirty_ratio_decimals is not allowed.

    I tend to choose 5. The same for dirty_background_ratio.


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