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

[Vivek Goyal <vgoyal@redhat.com>]

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