Messages in this thread | | | Subject | Re: regression in page writeback | From | Richard Kennedy <> | Date | Tue, 22 Sep 2009 09:52:48 +0100 |
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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.
I tried Peter's suggestion of using bdi_thresh/2 but I didn't see any difference on my desktop hardware, but it may help RAID setups. I don't think anyone tried it though.
Since Jens Axboe's per-bdi code got merged in the latest kernel tree, there's a lot of change in these code paths so I'm not sure how that reacts and if this change is still needed or relevant.
regards Richard
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