On Thu, Jan 13, 2011 at 01:38:55PM +0800, Shaohua Li wrote: > 2011/1/13 Darrick J. Wong <djwong@xxxxxxxxxx>: > > On certain types of storage hardware, flushing the write cache takes a > > considerable amount of time. Typically, these are simple storage systems with > > write cache enabled and no battery to save that cache during a power failure. > > When we encounter a system with many I/O threads that try to flush the cache, > > performance is suboptimal because each of those threads issues its own flush > > command to the drive instead of trying to coordinate the flushes, thereby > > wasting execution time. > > > > Instead of each thread initiating its own flush, we now try to detect the > > situation where multiple threads are issuing flush requests. The first thread > > to enter blkdev_issue_flush becomes the owner of the flush, and all threads > > that enter blkdev_issue_flush before the flush finishes are queued up to wait > > for the next flush. When that first flush finishes, one of those sleeping > > threads is woken up to perform the next flush and then wake up the other > > threads which are asleep waiting for the second flush to finish. > > > > In the single-threaded case, the thread will simply issue the flush and exit. > > > > To test the performance of this latest patch, I created a spreadsheet > > reflecting the performance numbers I obtained with the same ffsb fsync-happy > > workload that I've been running all along: http://tinyurl.com/6xqk5bs > > > > The second tab of the workbook provides easy comparisons of the performance > > before and after adding flush coordination to the block layer. Variations in > > the runs were never more than about 5%, so the slight performance increases and > > decreases are negligible. It is expected that devices with low flush times > > should not show much change, whether the low flush times are due to the lack of > > write cache or the controller having a battery and thereby ignoring the flush > > command. > > > > Notice that the elm3b231_ipr, elm3b231_bigfc, elm3b57, elm3c44_ssd, > > elm3c44_sata_wc, and elm3c71_scsi profiles showed large performance increases > > from flush coordination. These 6 configurations all feature large write caches > > without battery backups, and fairly high (or at least non-zero) average flush > > times, as was discovered when I studied the v6 patch. > > > > Unfortunately, there is one very odd regression: elm3c44_sas. This profile is > > a couple of battery-backed RAID cabinets striped together with raid0 on md. I > > suspect that there is some sort of problematic interaction with md, because > > running ffsb on the individual hardware arrays produces numbers similar to > > elm3c71_extsas. elm3c71_extsas uses the same type of hardware array as does > > elm3c44_sas, in fact. > > > > FYI, the flush coordination patch shows performance improvements both with and > > without Christoph's patch that issues pure flushes directly. The spreadsheet > > only captures the performance numbers collected without Christoph's patch. > Hi, > can you explain why there is improvement with your patch? If there are > multiple flush, blk_do_flush already has queue for them (the > ->pending_flushes list). With the current code, if we have n threads trying to issue flushes, the block layer will issue n flushes one after the other. I think the point of Christoph's pure-flush patch is to skip the serialization step and allow issuing of the n pure flushes in parallel. The point of this patch is optimize even more aggressively, such that as soon as the system becomes free to process a pure flush (at time t), all the requests for a pure flush that were created since the last time a pure flush was actually issued can be covered with a single flush issued at time t. In other words, this patch tries to reduce the number of pure flushes issued. --D -- 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
