Messages in this thread | | | Date | Wed, 23 Sep 2009 10:26:22 +0800 | From | Wu Fengguang <> | Subject | Re: regression in page writeback |
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On Wed, Sep 23, 2009 at 09:59:41AM +0800, Andrew Morton wrote: > On Wed, 23 Sep 2009 09:45:00 +0800 Wu Fengguang <fengguang.wu@intel.com> wrote: > > > On Wed, Sep 23, 2009 at 09:28:32AM +0800, Andrew Morton wrote: > > > On Wed, 23 Sep 2009 09:17:58 +0800 Wu Fengguang <fengguang.wu@intel.com> wrote: > > > > > > > On Wed, Sep 23, 2009 at 08:54:52AM +0800, Andrew Morton wrote: > > > > > On Wed, 23 Sep 2009 08:22:20 +0800 Wu Fengguang <fengguang.wu@intel.com> wrote: > > > > > > > > > > > Jens' per-bdi writeback has another improvement. In 2.6.31, when > > > > > > superblocks A and B both have 100000 dirty pages, it will first > > > > > > exhaust A's 100000 dirty pages before going on to sync B's. > > > > > > > > > > That would only be true if someone broke 2.6.31. Did they? > > > > > > > > > > SYSCALL_DEFINE0(sync) > > > > > { > > > > > wakeup_pdflush(0); > > > > > sync_filesystems(0); > > > > > sync_filesystems(1); > > > > > if (unlikely(laptop_mode)) > > > > > laptop_sync_completion(); > > > > > return 0; > > > > > } > > > > > > > > > > the sync_filesystems(0) is supposed to non-blockingly start IO against > > > > > all devices. It used to do that correctly. But people mucked with it > > > > > so perhaps it no longer does. > > > > > > > > I'm referring to writeback_inodes(). Each invocation of which (to sync > > > > 4MB) will do the same iteration over superblocks A => B => C ... So if > > > > A has dirty pages, it will always be served first. > > > > > > > > So if wbc->bdi == NULL (which is true for kupdate/background sync), it > > > > will have to first exhaust A before going on to B and C. > > > > > > But that works OK. We fill the first device's queue, then it gets > > > congested and sync_sb_inodes() does nothing and we advance to the next > > > queue. > > > > So in common cases "exhaust" is a bit exaggerated, but A does receive > > much more opportunity than B. Computation resources for IO submission > > are unbalanced for A, and there are pointless overheads in rechecking A. > > That's unquantified handwaving. One CPU can do a *lot* of IO.
Yes.. I had the impression that the writeback submission can be pretty slow. It should be because of the congestion_wait. Now that it is removed, things are going faster when queue is not full.
> > > If a device has more than a queue's worth of dirty data then we'll > > > probably leave some of that dirty memory un-queued, so there's some > > > lack of concurrency in that situation. > > > > Good insight. > > It was wrong. See the other email.
No your first insight is correct. Because the (unnecessary) teeny sleeps is independent of the A=>B=>C traversing order. Only queue congestion could help skip A.
> > That possibly explains one major factor of the > > performance gains of Jens' per-bdi writeback. > > I've yet to see any believable and complete explanation for these > gains. I've asked about these things multiple times and nothing happened.
The per-bdi writeback threads does make things more straight. But given that Jens also piggy backed some other behavior changes, it's hard to judge the pure gain of the per-bdi writeback.
> I suspect that what happened over time was that previously-working code > got broken, then later people noticed the breakage but failed to > analyse and fix it in favour of simply ripping everything out and > starting again. > > So for the want of analysing and fixing several possible regressions, > we've tossed away some very sensitive core kernel code which had tens > of millions of machine-years testing. I find this incredibly rash.
Sorry..
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