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? - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/