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 -- 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
