linux-kernel - Re: [RFC PATCH 0/3] cgroup: fsio throttle controller

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Date:   Fri, 18 Jan 2019 12:10:08 +0100
From:   Andrea Righi <righi.andrea@...il.com>
To:     Paolo Valente <paolo.valente@...aro.org>
Cc:     Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jens Axboe <axboe@...nel.dk>, Vivek Goyal <vgoyal@...hat.com>,
        Josef Bacik <josef@...icpanda.com>,
        Dennis Zhou <dennis@...nel.org>, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] cgroup: fsio throttle controller

On Fri, Jan 18, 2019 at 12:04:17PM +0100, Paolo Valente wrote:
> 
> 
> > Il giorno 18 gen 2019, alle ore 11:31, Andrea Righi <righi.andrea@...il.com> ha scritto:
> > 
> > This is a redesign of my old cgroup-io-throttle controller:
> > https://lwn.net/Articles/330531/
> > 
> > I'm resuming this old patch to point out a problem that I think is still
> > not solved completely.
> > 
> > = Problem =
> > 
> > The io.max controller works really well at limiting synchronous I/O
> > (READs), but a lot of I/O requests are initiated outside the context of
> > the process that is ultimately responsible for its creation (e.g.,
> > WRITEs).
> > 
> > Throttling at the block layer in some cases is too late and we may end
> > up slowing down processes that are not responsible for the I/O that
> > is being processed at that level.
> > 
> > = Proposed solution =
> > 
> > The main idea of this controller is to split I/O measurement and I/O
> > throttling: I/O is measured at the block layer for READS, at page cache
> > (dirty pages) for WRITEs, and processes are limited while they're
> > generating I/O at the VFS level, based on the measured I/O.
> > 
> 
> Hi Andrea,
> what the about the case where two processes are dirtying the same
> pages?  Which will be charged?
> 
> Thanks,
> Paolo

Hi Paolo,

in this case only the first one will be charged for the I/O activity
(the one that changes a page from clean to dirty). This is probably not
totally fair in some cases, but I think it's a good compromise, at the
end rewriting the same page over and over while it's already dirty
doesn't actually generate I/O activity, until the page is flushed back
to disk.

Obviously I'm open to other better ideas and suggestions.

Thanks!
-Andrea