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    SubjectRe: Possible Idea with filesystem buffering.

    On Monday, January 21, 2002 05:07:30 PM +0300 Hans Reiser
    <> wrote:

    > Rik van Riel wrote:
    >> On Mon, 21 Jan 2002, Hans Reiser wrote:
    >>> Pressure received is not equal to pages yielded. ... The number of
    >>> pages yielded should depend on the interplay of pressure received and
    >>> accesses made.

    Ah, once the FS starts counting accesses, we get in trouble. The FS should
    strive to know only these 3 things:

    How to read useful data into a page
    How to flush a dirty page
    How to free a pinned page

    The VM records everything else, including how often a page is accessed, and
    which pages should be freed in response to memory pressure. Of course, the
    FS might have details on many more things such as write clustering, delayed
    allocations, or which pinned pages require tons of extra work to write out.
    This fools us into thinking the FS might be the best place to decide how to
    react under memory pressure, leading to a little VM in each FS.

    Everything gets cleaner if we push this info up to the VM in a generic
    fashion, instead of trying to push bits of the VM down into each
    The FS should have no idea of what memory pressure is, down that path lies
    pain, suffering, and deadlocks against the journal ;-)

    If the VM is telling the FS to write a pinned page when there are unpinned
    pages that can be written with less cost, then we need to give the VM
    better hints about the actual cost of writing the pinned page.

    For periodic group flushes (delayed allocation, journal commits, etc), we
    need better throttling on dirty pages instead of just dirty buffers like we
    do now.

    I'm not delusional enough to think this will make all the vm<->journal
    nastiness go away, but it hopefully should be less painful than adding
    extra VM intelligence into each FS.


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