On Tue, Nov 30, 2010 at 12:26:37PM +1100, Neil Brown wrote: > On Mon, 29 Nov 2010 19:48:52 -0500 Ric Wheeler <rwheeler@xxxxxxxxxx> wrote: > > > On 11/29/2010 07:39 PM, Neil Brown wrote: > > > On Mon, 29 Nov 2010 14:05:36 -0800 "Darrick J. Wong"<djwong@xxxxxxxxxx> > > > wrote: > > > > > >> On certain types of hardware, issuing a write cache flush takes a considerable > > >> amount of time. Typically, these are simple storage systems with write cache > > >> enabled and no battery to save that cache after a power failure. When we > > >> encounter a system with many I/O threads that write data and then call fsync > > >> after more transactions accumulate, ext4_sync_file performs a data-only flush, > > >> the performance of which is suboptimal because each of those threads issues its > > >> own flush command to the drive instead of trying to coordinate the flush, > > >> thereby wasting execution time. > > >> > > >> Instead of each fsync call initiating its own flush, there's now a flag to > > >> indicate if (0) no flushes are ongoing, (1) we're delaying a short time to > > >> collect other fsync threads, or (2) we're actually in-progress on a flush. > > >> > > >> So, if someone calls ext4_sync_file and no flushes are in progress, the flag > > >> shifts from 0->1 and the thread delays for a short time to see if there are any > > >> other threads that are close behind in ext4_sync_file. After that wait, the > > >> state transitions to 2 and the flush is issued. Once that's done, the state > > >> goes back to 0 and a completion is signalled. > > > I haven't seen any of the preceding discussion do I might be missing > > > something important, but this seems needlessly complex and intrusive. > > > In particular, I don't like adding code to md to propagate these timings up > > > to the fs, and I don't the arbitrary '2ms' number. > > > > > > Would it not be sufficient to simply gather flushes while a flush is pending. > > > i.e > > > - if no flush is pending, set the 'flush pending' flag, submit a flush, > > > then clear the flag. > > > - if a flush is pending, then wait for it to complete, and then submit a > > > single flush on behalf of all pending flushes. > > > > > > That way when flush is fast, you do a flush every time, and when it is slow > > > you gather multiple flushes together. > > > I think it would issues a few more flushes than your scheme, but it would be > > > a much neater solution. Have you tried that and found it to be insufficient? > > > > > > Thanks, > > > NeilBrown > > > > > > > The problem with that is that you can introduce a wait for the next flush longer > > than it would take to complete the flush. Having the wait adjust itself > > according to the speed of the device is much better I think.... > > > > Hi Ric, > > You certainly can introduce a wait longer than the flush would take, but > the proposed code does that too. > > The proposed code waits "average flush time", then submits a flush for > all threads that are waiting. > My suggestion submits a flush (Which on average takes 'average flush time') > and then submits a flush for all threads that started waiting during that > time. > > So we do get two flushes rather than one, but also the flush starts > straight away, so will presumably finish sooner. > > I do have the wait 'adjust itself according to the speed of the device' by > making flushes wait for one flush to complete. > > > I'm guessing that the situation where this is an issue is when you have a > nearly constant stream of flush requests - is that right? Yes. --D > In that case: > - the current code would submit lots of over-lapping flush requests, > reducing the opportunity for optimising multiple requests in the > device, > - my proposal would submit a steady sequence of flushes with minimal > gaps > - the code from Darrick would submit a steady sequence of flush requests > which would be separated by pauses of 'average flush time'. > This would increase latency, but might increase throughput too, I > don't know. > > So it seems to me that the performance differences between my suggesting > and Darrick's proposal are not obvious. So some testing would be > interesting. > > I was going to suggest changes to Darrick's code to demonstrate my idea, but > I think that code is actually wrong, so it wouldn't be a good base to start. > > In particular, the usage of a continuation seems racy. > As soon as one thread sets EXT4_FLUSH_IDLE, another thread could call > INIT_COMPLETION, before some other threads has had a chance to wake up and > test the completion status in wait_for_completion. > The effect is that the other thread would wait for an extra time+flush which > it shouldn't have to. So it isn't a serious race, but it looks wrong. > > NeilBrown > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
