Andrea Righi wrote: > On Wed, Mar 02, 2011 at 04:47:05PM -0500, Vivek Goyal wrote: >> On Wed, Mar 02, 2011 at 02:28:30PM +0100, Andrea Righi wrote: >>> On Mon, Feb 28, 2011 at 06:01:14PM -0500, Vivek Goyal wrote: >>>> On Mon, Feb 28, 2011 at 11:15:02AM +0100, Andrea Righi wrote: >>>>> Overview >>>>> ======== >>>>> Currently the blkio.throttle controller only support synchronous IO requests. >>>>> This means that we always look at the current task to identify the "owner" of >>>>> each IO request. >>>>> >>>>> However dirty pages in the page cache can be wrote to disk asynchronously by >>>>> the per-bdi flusher kernel threads or by any other thread in the system, >>>>> according to the writeback policy. >>>>> >>>>> For this reason the real writes to the underlying block devices may >>>>> occur in a different IO context respect to the task that originally >>>>> generated the dirty pages involved in the IO operation. This makes the >>>>> tracking and throttling of writeback IO more complicate respect to the >>>>> synchronous IO from the blkio controller's perspective. >>>>> >>>>> Proposed solution >>>>> ================= >>>>> In the previous patch set http://lwn.net/Articles/429292/ I proposed to resolve >>>>> the problem of the buffered writes limitation by tracking the ownership of all >>>>> the dirty pages in the system. >>>>> >>>>> This would allow to always identify the owner of each IO operation at the block >>>>> layer and apply the appropriate throttling policy implemented by the >>>>> blkio.throttle controller. >>>>> >>>>> This solution makes the blkio.throttle controller to work as expected also for >>>>> writeback IO, but it does not resolve the problem of faster cgroups getting >>>>> blocked by slower cgroups (that would expose a potential way to create DoS in >>>>> the system). >>>>> >>>>> In fact, at the moment critical IO requests (that have dependency with other IO >>>>> requests made by other cgroups) and non-critical requests are mixed together at >>>>> the filesystem layer in a way that throttling a single write request may stop >>>>> also other requests in the system, and at the block layer it's not possible to >>>>> retrieve such informations to make the right decision. >>>>> >>>>> A simple solution to this problem could be to just limit the rate of async >>>>> writes at the time a task is generating dirty pages in the page cache. The >>>>> big advantage of this approach is that it does not need the overhead of >>>>> tracking the ownership of the dirty pages, because in this way from the blkio >>>>> controller perspective all the IO operations will happen from the process >>>>> context: writes in memory and synchronous reads from the block device. >>>>> >>>>> The drawback of this approach is that the blkio.throttle controller becomes a >>>>> little bit leaky, because with this solution the controller is still affected >>>>> by the IO spikes during the writeback of dirty pages executed by the kernel >>>>> threads. >>>>> >>>>> Probably an even better approach would be to introduce the tracking of the >>>>> dirty page ownership to properly account the cost of each IO operation at the >>>>> block layer and apply the throttling of async writes in memory only when IO >>>>> limits are exceeded. >>>> Andrea, I am curious to know more about it third option. Can you give more >>>> details about accouting in block layer but throttling in memory. So say >>>> a process starts IO, then it will still be in throttle limits at block >>>> layer (because no writeback has started), then the process will write >>>> bunch of pages in cache. By the time throttle limits are crossed at >>>> block layer, we already have lots of dirty data in page cache and >>>> throttling process now is already late? >>> Charging the cost of each IO operation at the block layer would allow >>> tasks to write in memory at the maximum speed. Instead, with the 3rd >>> approach, tasks are forced to write in memory at the rate defined by the >>> blkio.throttle.write_*_device (or blkio.throttle.async.write_*_device). >>> >>> When we'll have the per-cgroup dirty memory accounting and limiting >>> feature, with this approach each cgroup could write to its dirty memory >>> quota at the maximum rate. >> Ok, so this is option 3 which you have already implemented in this >> patchset. >> >> I guess then I am confused with option 2. Can you elaborate a little >> more there. > > With option 3, we can just limit the rate at which dirty pages are > generated in memory. And this can be done introducing the files > blkio.throttle.async.write_bps/iops_device. > > At the moment in blk_throtl_bio() we charge the dispatched bytes/iops > _and_ we check if the bio can be dispatched. These two distinct > operations are now done by the same function. > > With option 2, I'm proposing to split these two operations and place > throtl_charge_io() at the block layer in __generic_make_request() and an > equivalent of tg_may_dispatch_bio() (maybe a better name would be > blk_is_throttled()) at the page cache layer, in > balance_dirty_pages_ratelimited_nr(): > > A prototype for blk_is_throttled() could be the following: > > bool blk_is_throttled(void); > > This means in balance_dirty_pages_ratelimited_nr() we won't charge any > bytes/iops to the cgroup, but we'll just check if the limits are > exceeded. And stop it in that case, so that no more dirty pages can be > generated by this cgroup. > > Instead at the block layer WRITEs will be always dispatched in > blk_throtl_bio() (tg_may_dispatch_bio() will always return true), but > the throtl_charge_io() would charge the cost of the IO operation to the > right cgroup. > > To summarize: > > __generic_make_request(): > blk_throtl_bio(q, &bio); > > balance_dirty_pages_ratelimited_nr(): > if (blk_is_throttled()) > // add the current task into a per-group wait queue and > // wake up once this cgroup meets its quota > > What do you think? Hi Andrea, This means when you throttle writes, the reads issued by this task are also throttled? Thanks, Gui > > Thanks, > -Andrea > -- 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/ . 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