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

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Date:   Mon, 21 Jan 2019 16:47:15 -0500
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Andrea Righi <righi.andrea@...il.com>
Cc:     Josef Bacik <josef@...icpanda.com>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jens Axboe <axboe@...nel.dk>, 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 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?

Vivek