Hi Jan, On Wed, Mar 23, 2011 at 05:43:14AM +0800, Jan Kara wrote: > Hello Fengguang, > > On Fri 18-03-11 22:30:01, Wu Fengguang wrote: > > On Wed, Mar 09, 2011 at 06:31:10AM +0800, Jan Kara wrote: > > > > > > Hello, > > > > > > I'm posting second version of my IO-less balance_dirty_pages() patches. This > > > is alternative approach to Fengguang's patches - much simpler I believe (only > > > 300 lines added) - but obviously I does not provide so sophisticated control. > > > > Well, it may be too early to claim "simplicity" as an advantage, until > > you achieve the following performance/feature comparability (most of > > them are not optional ones). AFAICS this work is kind of heavy lifting > > that will consume a lot of time and attention. You'd better find some > > more fundamental needs before go on the reworking. > > > > (1) latency > > (2) fairness > > (3) smoothness > > (4) scalability > > (5) per-task IO controller > > (6) per-cgroup IO controller (TBD) > > (7) free combinations of per-task/per-cgroup and bandwidth/priority controllers > > (8) think time compensation > > (9) backed by both theory and tests > > (10) adapt pause time up on 100+ dirtiers > > (11) adapt pause time down on low dirty pages > > (12) adapt to new dirty threshold/goal > > (13) safeguard against dirty exceeding > > (14) safeguard against device queue underflow > I think this is a misunderstanding of my goals ;). My main goal is to > explore, how far we can get with a relatively simple approach to IO-less > balance_dirty_pages(). I guess what I have is better than the current > balance_dirty_pages() but it sure does not even try to provide all the > features you try to provide. OK. > I'm thinking about tweaking ratelimiting logic to reduce latencies in some > tests, possibly add compensation when we waited for too long in > balance_dirty_pages() (e.g. because of bumpy IO completion) but that's > about it... > > Basically I do this so that we can compare and decide whether what my > simple approach offers is OK or whether we want some more complex solution > like your patches... Yeah, now both results are on the website. Let's see whether they are acceptable for others. > > > The basic idea (implemented in the third patch) is that processes throttled > > > in balance_dirty_pages() wait for enough IO to complete. The waiting is > > > implemented as follows: Whenever we decide to throttle a task in > > > balance_dirty_pages(), task adds itself to a list of tasks that are throttled > > > against that bdi and goes to sleep waiting to receive specified amount of page > > > IO completions. Once in a while (currently HZ/10, in patch 5 the interval is > > > autotuned based on observed IO rate), accumulated page IO completions are > > > distributed equally among waiting tasks. > > > > > > This waiting scheme has been chosen so that waiting time in > > > balance_dirty_pages() is proportional to > > > number_waited_pages * number_of_waiters. > > > In particular it does not depend on the total number of pages being waited for, > > > thus providing possibly a fairer results. > > > > When there comes no IO completion in 1 second (normal in NFS), the > > tasks will all get stuck. It is fixable based on your v2 code base > > (detailed below), however will likely bring the same level of > > complexity as the base bandwidth solution. > I have some plans how to account for bumpy IO completion when we wait for > a long time and then get completion of much more IO than we actually need. > But in case where processes use all the bandwidth and the latency of the > device is high, sure they take the penalty and have to wait for a long time > in balance_dirty_pages(). No, I don't think it's good to block for long time in balance_dirty_pages(). This seems to be our biggest branch point. > > As for v2, there are still big gap to fill. NFS dirtiers are > > constantly doing 20-25 seconds long delays > > > > http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/jan-bdp-v2b/3G/nfs-1dd-1M-8p-2945M-20%25-2.6.38-rc8-jan-bdp+-2011-03-10-16-31/balance_dirty_pages-pause.png > > http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/jan-bdp-v2b/3G/nfs-10dd-1M-8p-2945M-20%25-2.6.38-rc8-jan-bdp+-2011-03-10-16-38/balance_dirty_pages-pause.png > Yeah, this is because they want lots of pages each > (3/2*MAX_WRITEBACK_PAGES). I'll try to change ratelimiting to make several > shorter sleeps. But ultimately you have to wait this much. Just you can > split those big sleeps in more of smaller ones. Ideally I prefer less than 100ms sleep time for achieving smooth responsiveness and IO pipeline. > > and the tasks are bumping forwards > > > > http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/jan-bdp-v2b/3G/nfs-10dd-1M-8p-2945M-20%25-2.6.38-rc8-jan-bdp+-2011-03-10-16-38/balance_dirty_pages-task-bw.png > Yeah, that's a result of bumpy NFS writeout and basically the consequence > of the above. Maybe it can be helped but I don't find this to be a problem on > its own... ditto. > > > Since last version I've implemented cleanups as suggested by Peter Zilstra. > > > The patches undergone more throughout testing. So far I've tested different > > > filesystems (ext2, ext3, ext4, xfs, nfs), also a combination of a local > > > filesystem and nfs. The load was either various number of dd threads or > > > fio with several threads each dirtying pages at different speed. > > > > > > Results and test scripts can be found at > > > http://beta.suse.com/private/jack/balance_dirty_pages-v2/ > > > See README file for some explanation of test framework, tests, and graphs. > > > Except for ext3 in data=ordered mode, where kjournald creates high > > > fluctuations in waiting time of throttled processes (and also high latencies), > > > the results look OK. Parallel dd threads are being throttled in the same way > > > (in a 2s window threads spend the same time waiting) and also latencies of > > > individual waits seem OK - except for ext3 they fit in 100 ms for local > > > filesystems. They are in 200-500 ms range for NFS, which isn't that nice but > > > to fix that we'd have to modify current ratelimiting scheme to take into > > > account on which bdi a page is dirtied. Then we could ratelimit slower BDIs > > > more often thus reducing latencies in individual waits... > > > > Yes the per-cpu rate limit is a problem, so I'm switching to per-task > > rate limit. > BTW: Have you considered per-bdi ratelimiting? Both per-task and per-bdi > make sense just they are going to have slightly different properties... > Current per-cpu ratelimit counters tend to behave like per-task > ratelimiting at least for fast dirtiers because once a task is blocked in > balance_dirty_pages() another task runs on that cpu and uses the counter > for itself. So I wouldn't expect big differences from per-task > ratelimiting... Good point. It should be enough to have per-bdi rate limiting threshold. However I still need to keep per-task nr_dirtied for doing per-task throttling bandwidth. The per-cpu bdp_ratelimits mix dirtied pages from different tasks and bdis, which is unacceptable for me. > > The direct input from IO completion is another issue. It leaves the > > dirty tasks at the mercy of low layer (VFS/FS/bdev) fluctuations and > > latencies. So I'm introducing the base bandwidth as a buffer layer. > > You may employ the similar technique: to simulate a more smooth flow > > of IO completion events based on the average write bandwidth. Then it > > naturally introduce the problem of rate mismatch between > > simulated/real IO completions, and the need to do more elaborated > > position control. > Exacttly, that's why I don't want to base throttling on some computed > value (well, I also somehow estimate necessary sleep time but that's more a > performance optimization) but rather leave tasks "at the mercy of lower > layers" as you write ;) I don't think it's necessarily a bad thing. Again, the same branch point :) > > > The results for different bandwidths fio load is interesting. There are 8 > > > threads dirtying pages at 1,2,4,..,128 MB/s rate. Due to different task > > > bdi dirty limits, what happens is that three most aggresive tasks get > > > throttled so they end up at bandwidths 24, 26, and 30 MB/s and the lighter > > > dirtiers run unthrottled. > > > > The base bandwidth based throttling can do better and provide almost > > perfect fairness, because all tasks writing to one bdi derive their > > own throttle bandwidth based on the same per-bdi base bandwidth. So > > the heavier dirtiers will converge to equal dirty rate and weight. > So what do you consider a perfect fairness in this case and are you sure > it is desirable? I was thinking about this and I'm not sure... Perfect fairness could be 1, 2, 4, 8, N, N, N MB/s, where N = (write_bandwidth - 1 - 2 - 4 - 8) / 3. I guess its usefulness is largely depending on the user space applications. Most of them should not be sensible to it. Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
