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    SubjectRe: [patch 2/2] mm: add dirty_background_bytes and dirty_bytes sysctls
    On Sun, 2008-11-23 at 10:55 -0800, David Rientjes wrote:
    > This change introduces two new sysctls to /proc/sys/vm:
    > dirty_background_bytes and dirty_bytes.
    > dirty_background_bytes is the counterpart to dirty_background_ratio and
    > dirty_bytes is the counterpart to dirty_ratio.
    > With growing memory capacities of individual machines, it's no longer
    > sufficient to specify dirty thresholds as a percentage of the amount of
    > dirtyable memory over the entire system.
    > dirty_background_bytes and dirty_bytes specify quantities of memory, in
    > bytes, that represent the dirty limits for the entire system. If either
    > of these values is set, its value represents the amount of dirty memory
    > that is needed to commence either background or direct writeback.
    > When a `bytes' or `ratio' file is written, its counterpart becomes a
    > function of the written value. For example, if dirty_bytes is written to
    > be 8096, 8K of memory is required to commence direct writeback.
    > dirty_ratio is then functionally equivalent to 8K / the amount of
    > dirtyable memory:
    > dirtyable_memory = free pages + mapped pages + file cache
    > dirty_background_bytes = dirty_background_ratio * dirtyable_memory
    > -or-
    > dirty_background_ratio = dirty_background_bytes / dirtyable_memory
    > AND
    > dirty_bytes = dirty_ratio * dirtyable_memory
    > -or-
    > dirty_ratio = dirty_bytes / dirtyable_memory
    > Only one of dirty_background_bytes and dirty_background_ratio may be
    > specified at a time, and only one of dirty_bytes and dirty_ratio may be
    > specified. When one sysctl is written, the other appears as 0 when read.
    > The `bytes' files operate on a page size granularity since dirty limits
    > are compared with ZVC values, which are in page units.
    > Prior to this change, the minimum dirty_ratio was 5 as implemented by
    > get_dirty_limits() although /proc/sys/vm/dirty_ratio would show any user
    > written value between 0 and 100. This restriction is maintained, but
    > dirty_bytes has a lower limit of only one page.
    > Also prior to this change, the dirty_background_ratio could not equal or
    > exceed dirty_ratio. This restriction is maintained in addition to
    > restricting dirty_background_bytes. If either background threshold equals
    > or exceeds that of the dirty threshold, it is implicitly set to half the
    > dirty threshold.
    > Cc: Peter Zijlstra <>
    > Cc: Dave Chinner <>
    > Cc: Christoph Lameter <>
    > Signed-off-by: David Rientjes <>

    I don't really like the dual interface, but I guess given the
    restrictions this is the best one can do... So

    Acked-by: Peter Zijlstra <>

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