On Tue, Nov 30, 2010 at 11:39:06AM +1100, 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? Some time ago I actually did test the patchset with the schedule_hrtimeout removed, which I think is fairly close to what you've suggested. As I recall, it did help a bit, though not as much as also instituting the wait to limit the % of disk execution time spent on flushes. That said, I think you might be right about the completion races, so I'll try to code up your suggestion to see what happens with a newer kernel. --D > > Thanks, > NeilBrown > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
