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