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    SubjectRe: [PATCH 10/10] mm: per device dirty threshold
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    On Tue, 2007-04-24 at 11:14 +0200, Miklos Szeredi wrote:

    > > > I'm still not quite sure what purpose the above "soft" limiting
    > > > serves. It seems to just give advantage to writers, which managed to
    > > > accumulate lots of dirty pages, and then can convert that into even
    > > > more dirtyings.
    > >
    > > The queues only limit the actual in-flight writeback pages,
    > > balance_dirty_pages() considers all pages that might become writeback as
    > > well as those that are.
    > >
    > > > Would it make sense to remove this behavior, and ensure that
    > > > balance_dirty_pages() doesn't return until the per-queue limits have
    > > > been complied with?
    > >
    > > I don't think that will help, balance_dirty_pages drives the queues.
    > > That is, it converts pages from mere dirty to writeback.
    >
    > Yes. But current logic says, that if you convert "write_chunk" dirty
    > to writeback, you are allowed to dirty "ratelimit" more.
    >
    > D: number of dirty pages
    > W: number of writeback pages
    > L: global limit
    > C: write_chunk = ratelimit_pages * 1.5
    > R: ratelimit
    >
    > If D+W >= L, then R = 8
    >
    > Let's assume, that D == L and W == 0. And that all of the dirty pages
    > belong to a single device. Also for simplicity, lets assume an
    > infinite length queue, and a slow device.
    >
    > Then while converting the dirty pages to writeback, D / C * R new
    > dirty pages can be created. So when all existing dirty have been
    > converted:
    >
    > D = L / C * R
    > W = L
    >
    > D + W = L * (1 + R / C)
    >
    > So we see, that we're now even more above the limit than before the
    > conversion. This means, that we starve writers to other devices,
    > which don't have as many dirty pages, because until the slow device
    > doesn't finish these writes they will not get to do anything.
    >
    > Your patch helps this in that if the other writers have an empty queue
    > and no dirty, they will be allowed to slowly start writing. But they
    > will not gain their full share until the slow dirty-hog goes below the
    > global limit, which may take some time.
    >
    > So I think the logical thing to do, is if the dirty-hog is over it's
    > queue limit, don't let it dirty any more until it's dirty+writeback go
    > below the limit. That allowes other devices to more quickly gain
    > their share of dirty pages.

    Ahh, now I see; I had totally blocked out these few lines:

    pages_written += write_chunk - wbc.nr_to_write;
    if (pages_written >= write_chunk)
    break; /* We've done our duty */

    yeah, those look dubious indeed... And reading back Neil's comments, I
    think he agrees.

    Shall we just kill those?

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