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Date: Tue, 21 Aug 2012 20:57:36 +0800 From: Fengguang Wu <fengguang.wu@...el.com> To: Namjae Jeon <linkinjeon@...il.com> Cc: "J. Bruce Fields" <bfields@...ldses.org>, Dave Chinner <david@...morbit.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 Tue, Aug 21, 2012 at 02:48:35PM +0900, Namjae Jeon wrote: > 2012/8/21, J. Bruce Fields <bfields@...ldses.org>: > > On Mon, Aug 20, 2012 at 12:00:04PM +1000, Dave Chinner wrote: > >> 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. > > > > Wait, aren't we confusing client and server side here? > > > > If I read Namjae Jeon's post correctly, I understood that it was the > > *server* side he was modifying to start writeout sooner, to improve > > response time to eventual expected commits from the client. The > > responses above all seem to be about the client. > > > > Maybe it's all the same at some level, but: naively, starting writeout > > early would seem a better bet on the server side. By the time we get > > writes, the client has already decided they're worth sending to disk. > Hi Bruce. > > Yes, right, I have not changed writeback setting on NFS client, It was > changed on NFS Server. Ah OK, I'm very supportive to lower the NFS server's background writeback threshold. This will obviously help reduce disk idle time as well as turning a good amount of SYNC writes to ASYNC ones. > So writeback behaviour on NFS client will work at default. So There > will be no change in data caching behaviour > at NFS client. It will reduce server side wait time for NFS COMMIT by > starting early writeback. Agreed. > > > > And changes to make clients and applications friendlier to the server > > are great, but we don't always have that option--there are more clients > > out there than servers and the latter may be easier to upgrade than the > > former. > I agree about your opinion.. Agreed. Thanks, Fengguang -- 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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