Messages in this thread | | | Date | Tue, 22 Sep 2009 19:41:43 +0800 | From | Shaohua Li <> | Subject | Re: regression in page writeback |
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On Tue, Sep 22, 2009 at 05:05:01PM +0800, Wu, Fengguang wrote: > On Tue, Sep 22, 2009 at 04:52:48PM +0800, Richard Kennedy wrote: > > On Tue, 2009-09-22 at 10:32 +0200, Peter Zijlstra wrote: > > > On Tue, 2009-09-22 at 16:24 +0800, Wu Fengguang wrote: > > > > On Tue, Sep 22, 2009 at 04:09:25PM +0800, Peter Zijlstra wrote: > > > > > On Tue, 2009-09-22 at 16:05 +0800, Wu Fengguang wrote: > > > > > > > > > > > > I'm not sure how this patch stopped the "overshooting" behavior. > > > > > > Maybe it managed to not start the background pdflush, or the started > > > > > > pdflush thread exited because it found writeback is in progress by > > > > > > someone else? > > > > > > > > > > > > - if (bdi_nr_reclaimable) { > > > > > > + if (bdi_nr_reclaimable > bdi_thresh) { > > > > > > > > > > The idea is that we shouldn't move more pages from dirty -> writeback > > > > > when there's not actually that much dirty left. > > > > > > > > IMHO this makes little sense given that pdflush will move all dirty > > > > pages anyway. pdflush should already be started to do background > > > > writeback before the process is throttled, and it is designed to sync > > > > all current dirty pages as quick as possible and as much as possible. > > > > > > Not so, pdflush (or now the bdi writer thread thingies) should not > > > deplete all dirty pages but should stop writing once they are below the > > > background limit. > > > > > > > > Now, I'm not sure about the > bdi_thresh part, I've suggested to maybe > > > > > use bdi_thresh/2 a few times, but it generally didn't seem to make much > > > > > of a difference. > > > > > > > > One possible difference is, the process may end up waiting longer time > > > > in order to sync write_chunk pages and quit the throttle. This could > > > > hurt the responsiveness of the throttled process. > > > > > > Well, that's all because this congestion_wait stuff is borken.. > > > > > > > The problem occurred as pdflush stopped when the number of dirty pages > > reached the background threshold but balance_dirty_pages kept moving > > pages to writeback because the total of dirty + writeback was over the > > limit. > > Ah yes it is possible. The pdflush started by balance_dirty_pages() > does stop at the background threshold (sorry for the confusion!), > and then balance_dirty_pages() continue to sync pages in _smaller_ > chunk sizes, which should be suboptimal.. This is possible. Without the patch, balance_dirty_pages() can move some pages to writeback and don't need do congestion_wait(), so the task can continue doing write. The patch seems to break this. I tried to set dirty_exceeded only when bdi_nr_reclaimable > bdi_thresh, this helps a little in my test, but still not reach the best.
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