Message-ID: <20110306155247.GA1687@linux.develer.com>
Date:	Sun, 6 Mar 2011 16:52:47 +0100
From:	Andrea Righi <arighi@...eler.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Greg Thelen <gthelen@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>,
	Ryo Tsuruta <ryov@...inux.co.jp>,
	Hirokazu Takahashi <taka@...inux.co.jp>,
	Jens Axboe <axboe@...nel.dk>, Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	containers@...ts.linux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] blk-throttle: async write throttling

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?

Thanks,
-Andrea
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