On Wed, Feb 23, 2011 at 4:40 PM, KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote: > On Wed, 23 Feb 2011 19:10:33 -0500 > Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > >> On Thu, Feb 24, 2011 at 12:14:11AM +0100, Andrea Righi wrote: >> > On Wed, Feb 23, 2011 at 10:23:54AM -0500, Vivek Goyal wrote: >> > > > > Agreed. Granularity of per inode level might be accetable in many >> > > > > cases. Again, I am worried faster group getting stuck behind slower >> > > > > group. >> > > > > >> > > > > I am wondering if we are trying to solve the problem of ASYNC write throttling >> > > > > at wrong layer. Should ASYNC IO be throttled before we allow task to write to >> > > > > page cache. The way we throttle the process based on dirty ratio, can we >> > > > > just check for throttle limits also there or something like that.(I think >> > > > > that's what you had done in your initial throttling controller implementation?) >> > > > >> > > > Right. This is exactly the same approach I've used in my old throttling >> > > > controller: throttle sync READs and WRITEs at the block layer and async >> > > > WRITEs when the task is dirtying memory pages. >> > > > >> > > > This is probably the simplest way to resolve the problem of faster group >> > > > getting blocked by slower group, but the controller will be a little bit >> > > > more leaky, because the writeback IO will be never throttled and we'll >> > > > see some limited IO spikes during the writeback. >> > > >> > > Yes writeback will not be throttled. Not sure how big a problem that is. >> > > >> > > - We have controlled the input rate. So that should help a bit. >> > > - May be one can put some high limit on root cgroup to in blkio throttle >> > > controller to limit overall WRITE rate of the system. >> > > - For SATA disks, try to use CFQ which can try to minimize the impact of >> > > WRITE. >> > > >> > > It will atleast provide consistent bandwindth experience to application. >> > >> > Right. >> > >> > > >> > > >However, this is always >> > > > a better solution IMHO respect to the current implementation that is >> > > > affected by that kind of priority inversion problem. >> > > > >> > > > I can try to add this logic to the current blk-throttle controller if >> > > > you think it is worth to test it. >> > > >> > > At this point of time I have few concerns with this approach. >> > > >> > > - Configuration issues. Asking user to plan for SYNC ans ASYNC IO >> > > separately is inconvenient. One has to know the nature of workload. >> > > >> > > - Most likely we will come up with global limits (atleast to begin with), >> > > and not per device limit. That can lead to contention on one single >> > > lock and scalability issues on big systems. >> > > >> > > Having said that, this approach should reduce the kernel complexity a lot. >> > > So if we can do some intelligent locking to limit the overhead then it >> > > will boil down to reduced complexity in kernel vs ease of use to user. I >> > > guess at this point of time I am inclined towards keeping it simple in >> > > kernel. >> > > >> > >> > BTW, with this approach probably we can even get rid of the page >> > tracking stuff for now. >> >> Agreed. >> >> > If we don't consider the swap IO, any other IO >> > operation from our point of view will happen directly from process >> > context (writes in memory + sync reads from the block device). >> >> Why do we need to account for swap IO? Application never asked for swap >> IO. It is kernel's decision to move soem pages to swap to free up some >> memory. What's the point in charging those pages to application group >> and throttle accordingly? >> > > I think swap I/O should be controlled by memcg's dirty_ratio. > But, IIRC, NEC guy had a requirement for this... > > I think some enterprise cusotmer may want to throttle the whole speed of > swapout I/O (not swapin)...so, they may be glad if they can limit throttle > the I/O against a disk partition or all I/O tagged as 'swapio' rather than > some cgroup name. > > But I'm afraid slow swapout may consume much dirty_ratio and make things > worse ;) > > > >> > >> > However, I'm sure we'll need the page tracking also for the blkio >> > controller soon or later. This is an important information and also the >> > proportional bandwidth controller can take advantage of it. >> >> Yes page tracking will be needed for CFQ proportional bandwidth ASYNC >> write support. But until and unless we implement memory cgroup dirty >> ratio and figure a way out to make writeback logic cgroup aware, till >> then I think page tracking stuff is not really useful. >> > > I think Greg Thelen is now preparing patches for dirty_ratio. > > Thanks, > -Kame > > Correct. I am working on the memcg dirty_ratio patches with latest mmotm memcg. I am running some test cases which should be complete tomorrow. Once testing is complete, I will sent the patches for review. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
