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

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

Andrea Righi wrote:
> 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，

I means If the task exceeds write limit, this task will be put to sleep, right?
So It doesn't get a chance to issue read requests.

Gui

> 
> -Andrea
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-- 
Regards
Gui Jianfeng

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