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    SubjectRe: [patch 4/5] mm: writeback: throttle __GFP_WRITE on per-zone dirty limits
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    Hi Andi,

    On Tue, Jul 26, 2011 at 5:37 AM, Andi Kleen <ak@linux.intel.com> wrote:
    >> The global dirty limits are put in proportion to the respective zone's
    >> amount of dirtyable memory and the allocation denied when the limit of
    >> that zone is reached.
    >>
    >> Before the allocation fails, the allocator slowpath has a stage before
    >> compaction and reclaim, where the flusher threads are kicked and the
    >> allocator ultimately has to wait for writeback if still none of the
    >> zones has become eligible for allocation again in the meantime.
    >>
    >
    > I don't really like this. It seems wrong to make memory
    > placement depend on dirtyness.
    >
    > Just try to explain it to some system administrator or tuner: her
    > head will explode and for good reasons.
    >
    > On the other hand I like doing round-robin in filemap by default
    > (I think that is what your patch essentially does)
    > We should have made  this default long ago. It avoids most of the
    > "IO fills up local node" problems people run into all the time.
    >
    > So I would rather just change the default in filemap allocation.
    >
    > That's also easy to explain.

    Just out of curiosity.
    Why do you want to consider only filemap allocation, not IO(ie,
    filemap + sys_[read/write]) allocation?

    --
    Kind regards,
    Minchan Kim
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