Re: [PATCH 0/3] blk-throttle: async write throttling

[Andrea Righi <arighi@develer.com>]

On Mon, Mar 07, 2011 at 03:31:11PM +0800, Gui Jianfeng wrote:
> 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

Exactly, we're treating the throttling of READs and WRITEs in two
different ways.

READs will be always throttled synchronously in the
__generic_make_request() -> blk_throtl_bio() path.

-Andrea

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