Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755477Ab1CGMOa (ORCPT ); Mon, 7 Mar 2011 07:14:30 -0500 Received: from cn.fujitsu.com ([222.73.24.84]:51178 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1752201Ab1CGMO3 convert rfc822-to-8bit (ORCPT ); Mon, 7 Mar 2011 07:14:29 -0500 Message-ID: <4D74CC5F.9070002@cn.fujitsu.com> Date: Mon, 07 Mar 2011 20:15:27 +0800 From: Gui Jianfeng User-Agent: Thunderbird 2.0.0.24 (Windows/20100228) MIME-Version: 1.0 To: Andrea Righi CC: Vivek Goyal , Balbir Singh , Daisuke Nishimura , KAMEZAWA Hiroyuki , Greg Thelen , Wu Fengguang , Ryo Tsuruta , Hirokazu Takahashi , Jens Axboe , Jonathan Corbet , Andrew Morton , containers@lists.linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/3] blk-throttle: async write throttling References: <1298888105-3778-1-git-send-email-arighi@develer.com> <20110228230114.GB20845@redhat.com> <20110302132830.GB2061@linux.develer.com> <20110302214705.GD2547@redhat.com> <20110306155247.GA1687@linux.develer.com> <4D7489BF.9030808@cn.fujitsu.com> <20110307113411.GA4485@linux.develer.com> <4D74C531.7070305@cn.fujitsu.com> <20110307115918.GB4485@linux.develer.com> In-Reply-To: <20110307115918.GB4485@linux.develer.com> X-MIMETrack: Itemize by SMTP Server on mailserver/fnst(Release 8.5.1FP4|July 25, 2010) at 2011-03-07 20:13:14, Serialize by Router on mailserver/fnst(Release 8.5.1FP4|July 25, 2010) at 2011-03-07 20:13:15 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7720 Lines: 157 Andrea Righi wrote: > On Mon, Mar 07, 2011 at 07:44:49PM +0800, Gui Jianfeng wrote: >> 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. > > Oh yes, you're right. This could be a problem. OTOH I wouldn't like to > introduce an additional queue to submit the write requests in the page > cache and dispatch them asyncrhonously. > > mmh... ideas? > hmm, dispatching asynchronously will make things more complicated. But writes blocking reads goes against the idea of page cache. I'm not sure how to solve this... Gui > -Andrea > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/