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Date: Mon, 20 Aug 2012 12:00:04 +1000 From: Dave Chinner <david@...morbit.com> To: Fengguang Wu <fengguang.wu@...el.com> Cc: Namjae Jeon <linkinjeon@...il.com>, akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, Namjae Jeon <namjae.jeon@...sung.com>, linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org Subject: Re: [PATCH 3/3] writeback: add dirty_ratio_time per bdi variable (NFS write performance) On Sun, Aug 19, 2012 at 10:57:24AM +0800, Fengguang Wu wrote: > On Sat, Aug 18, 2012 at 05:50:02AM -0400, Namjae Jeon wrote: > > From: Namjae Jeon <namjae.jeon@...sung.com> > > > > This patch is based on suggestion by Wu Fengguang: > > https://lkml.org/lkml/2011/8/19/19 > > > > kernel has mechanism to do writeback as per dirty_ratio and dirty_background > > ratio. It also maintains per task dirty rate limit to keep balance of > > dirty pages at any given instance by doing bdi bandwidth estimation. > > > > Kernel also has max_ratio/min_ratio tunables to specify percentage of writecache > > to control per bdi dirty limits and task throtelling. > > > > However, there might be a usecase where user wants a writeback tuning > > parameter to flush dirty data at desired/tuned time interval. > > > > dirty_background_time provides an interface where user can tune background > > writeback start time using /sys/block/sda/bdi/dirty_background_time > > > > dirty_background_time is used alongwith average bdi write bandwidth estimation > > to start background writeback. > > Here lies my major concern about dirty_background_time: the write > bandwidth estimation is an _estimation_ and will sure become wildly > wrong in some cases. So the dirty_background_time implementation based > on it will not always work to the user expectations. > > One important case is, some users (eg. Dave Chinner) explicitly take > advantage of the existing behavior to quickly create & delete a big > 1GB temp file without worrying about triggering unnecessary IOs. It's a fairly common use case - short term temp files are used by lots of applications and avoiding writing them - especially on NFS - is a big performance win. Forcing immediate writeback will definitely cause unprdictable changes in performance for many people... > > Results are:- > > ========================================================== > > Case:1 - Normal setup without any changes > > ./performancetest_arm ./100MB write > > > > RecSize WriteSpeed RanWriteSpeed > > > > 10485760 7.93MB/sec 8.11MB/sec > > 1048576 8.21MB/sec 7.80MB/sec > > 524288 8.71MB/sec 8.39MB/sec > > 262144 8.91MB/sec 7.83MB/sec > > 131072 8.91MB/sec 8.95MB/sec > > 65536 8.95MB/sec 8.90MB/sec > > 32768 8.76MB/sec 8.93MB/sec > > 16384 8.78MB/sec 8.67MB/sec > > 8192 8.90MB/sec 8.52MB/sec > > 4096 8.89MB/sec 8.28MB/sec > > > > Average speed is near 8MB/seconds. > > > > Case:2 - Modified the dirty_background_time > > ./performancetest_arm ./100MB write > > > > RecSize WriteSpeed RanWriteSpeed > > > > 10485760 10.56MB/sec 10.37MB/sec > > 1048576 10.43MB/sec 10.33MB/sec > > 524288 10.32MB/sec 10.02MB/sec > > 262144 10.52MB/sec 10.19MB/sec > > 131072 10.34MB/sec 10.07MB/sec > > 65536 10.31MB/sec 10.06MB/sec > > 32768 10.27MB/sec 10.24MB/sec > > 16384 10.54MB/sec 10.03MB/sec > > 8192 10.41MB/sec 10.38MB/sec > > 4096 10.34MB/sec 10.12MB/sec > > > > we can see, average write speed is increased to ~10-11MB/sec. > > ============================================================ > > The numbers are impressive! All it shows is that avoiding the writeback delay writes a file a bit faster. i.e. 5s delay + 10s @ 10MB/s vs no delay and 10s @10MB/s. That's pretty obvious, really, and people have been trying to make this "optimisation" for NFS clients for years in the misguided belief that short-cutting writeback caching is beneficial to application performance. What these numbers don't show that is whether over-the-wire writeback speed has improved at all. Or what happens when you have a network that is faster than the server disk, or even faster than the client can write into memory? What about when there are multiple threads, or the network is congested, or the server overloaded? In those cases the performance differential will disappear and there's a good chance that the existing code will be significantly faster because it places less imediate load on the server and network.D... If you need immediate dispatch of your data for single threaded performance then sync_file_range() is your friend. > FYI, I tried another NFS specific approach > to avoid big NFS COMMITs, which achieved similar performance gains: > > nfs: writeback pages wait queue > https://lkml.org/lkml/2011/10/20/235 Which is basically controlling the server IO latency when commits occur - smaller ranges mean the commit (fsync) is faster, and more frequent commits mean the data goes to disk sooner. This is something that will have a positive impact on writeback speeds because it modifies the NFs client writeback behaviour to be more server friendly and not stall over the wire. i.e. improving NFS writeback performance is all about keeping the wire full and the server happy, not about reducing the writeback delay before we start writing over the wire. Cheers, Dave. -- Dave Chinner david@...morbit.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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