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

[Josef Bacik]

On Fri, Jan 18, 2019 at 06:07:45PM +0100, Paolo Valente wrote:
> 
> 
> > Il giorno 18 gen 2019, alle ore 17:35, Josef Bacik <josef@xxxxxxxxxxxxxx> ha scritto:
> > 
> > On Fri, Jan 18, 2019 at 11:31:24AM +0100, Andrea Righi wrote:
> >> 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.
> > 
> > How so?  The writeback threads are per-cgroup and have the cgroup stuff set
> > properly.  So if you dirty a bunch of pages, they are associated with your
> > cgroup, and then writeback happens and it's done in the writeback thread
> > associated with your cgroup and then that is throttled.  Then you are throttled
> > at balance_dirty_pages() because the writeout is taking longer.
> > 
> 
> IIUC, Andrea described this problem: certain processes in a certain group dirty a
> lot of pages, causing write back to start.  Then some other blameless
> process in the same group experiences very high latency, in spite of
> the fact that it has to do little I/O.
> 

In that case the io controller isn't doing it's job and needs to be fixed (or
reconfigured).  io.latency guards against this, I assume io.max would keep this
from happening if it were configured properly.

> Does your blk_cgroup_congested() stuff solves this issue?
> 
> Or simply I didn't get what Andrea meant at all :)
> 

I _think_ Andrea is talking about the fact that we can generate IO indirectly
and never get throttled for it, which is what blk_cgroup_congested() is meant to
address.  I added it specifically because some low prio task was just allocating
all of the memory on the system and causing a lot of pressure because of
swapping, but there was no direct feedback loop there.  blk_cgroup_congested()
provides that feedback loop.

Course I could be wrong too and we're all just talking past each other ;).
Thanks,

Josef