From: Vivek Goyal <vgoyal@redhat.com>
To: Andrea Righi <righi.andrea@gmail.com>
Cc: Josef Bacik <josef@toxicpanda.com>, Tejun Heo <tj@kernel.org>,
Li Zefan <lizefan@huawei.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Jens Axboe <axboe@kernel.dk>, Dennis Zhou <dennis@kernel.org>,
cgroups@vger.kernel.org, linux-block@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 0/3] cgroup: fsio throttle controller
Date: Mon, 28 Jan 2019 14:26:20 -0500 [thread overview]
Message-ID: <20190128192620.GB10240@redhat.com> (raw)
In-Reply-To: <20190128174129.GB8272@xps-13>
On Mon, Jan 28, 2019 at 06:41:29PM +0100, Andrea Righi wrote:
> Hi Vivek,
>
> sorry for the late reply.
>
> On Mon, Jan 21, 2019 at 04:47:15PM -0500, Vivek Goyal wrote:
> > On Sat, Jan 19, 2019 at 11:08:27AM +0100, Andrea Righi wrote:
> >
> > [..]
> > > Alright, let's skip the root cgroup for now. I think the point here is
> > > if we want to provide sync() isolation among cgroups or not.
> > >
> > > According to the manpage:
> > >
> > > sync() causes all pending modifications to filesystem metadata and cached file data to be
> > > written to the underlying filesystems.
> > >
> > > And:
> > > According to the standard specification (e.g., POSIX.1-2001), sync() schedules the writes, but
> > > may return before the actual writing is done. However Linux waits for I/O completions, and
> > > thus sync() or syncfs() provide the same guarantees as fsync called on every file in the sys‐
> > > tem or filesystem respectively.
> > >
> > > Excluding the root cgroup, do you think a sync() issued inside a
> > > specific cgroup should wait for I/O completions only for the writes that
> > > have been generated by that cgroup?
> >
> > Can we account I/O towards the cgroup which issued "sync" only if write
> > rate of sync cgroup is higher than cgroup to which page belongs to. Will
> > that solve problem, assuming its doable?
>
> Maybe this would mitigate the problem, in part, but it doesn't solve it.
>
> The thing is, if a dirty page belongs to a slow cgroup and a fast cgroup
> issues "sync", the fast cgroup needs to wait a lot, because writeback is
> happening at the speed of the slow cgroup.
Hi Andrea,
But that's true only for I/O which has already been submitted to block
layer, right? Any new I/O yet to be submitted could still be attributed
to faster cgroup requesting sync.
Until and unless cgroups limits are absurdly low, it should not take very
long for already submitted I/O to finish. If yes, then in practice, it
might not be a big problem?
Vivek
>
> Ideally in this case we should bump up the writeback speed, maybe even
> temporarily inherit the write rate of the sync cgroup, similarly to a
> priority-inversion locking scenario, but I think it's not doable at the
> moment without applying big changes.
>
> Or we could isolate the sync domain, meaning that a cgroup issuing a
> sync will only wait for the syncing of the pages that belong to that
> sync cgroup. But probably also this method requires big changes...
>
> -Andrea
next prev parent reply other threads:[~2019-01-28 19:26 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-18 10:31 [RFC PATCH 0/3] cgroup: fsio throttle controller Andrea Righi
2019-01-18 10:31 ` [RFC PATCH 1/3] fsio-throttle: documentation Andrea Righi
2019-01-18 10:31 ` [RFC PATCH 2/3] fsio-throttle: controller infrastructure Andrea Righi
2019-01-18 10:31 ` [RFC PATCH 3/3] fsio-throttle: instrumentation Andrea Righi
2019-01-18 11:04 ` [RFC PATCH 0/3] cgroup: fsio throttle controller Paolo Valente
2019-01-18 11:10 ` Andrea Righi
2019-01-18 11:11 ` Paolo Valente
2019-01-18 16:35 ` Josef Bacik
2019-01-18 17:07 ` Paolo Valente
2019-01-18 17:12 ` Josef Bacik
2019-01-18 19:02 ` Andrea Righi
2019-01-18 18:44 ` Andrea Righi
2019-01-18 19:46 ` Josef Bacik
2019-01-19 10:08 ` Andrea Righi
2019-01-21 21:47 ` Vivek Goyal
2019-01-28 17:41 ` Andrea Righi
2019-01-28 19:26 ` Vivek Goyal [this message]
2019-01-29 18:39 ` Andrea Righi
2019-01-29 18:50 ` Josef Bacik
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