[dm-devel] Re: [PATCH 18/23] io-controller: blkio_cgroup patches from Ryo to track async bios.

[Ryo Tsuruta <ryov valinux co jp>]

Hi,

> > Hi Rik,
> >
> > Thanks for reviewing the patches. I wanted to have better understanding of
> > where all does it help to associate a bio to the group of process who
> > created/owned the page. Hence few thoughts.
> >
> > When a bio is submitted to IO scheduler, it needs to determine the group
> > bio belongs to and group which should be charged to. There seem to be two
> > methods.
> >
> > - Attribute the bio to cgroup submitting process belongs to.
> > - For async requests, track the original owner hence cgroup of the page
> >  and charge that group for the bio.
> >
> > One can think of pros/cons of both the approaches.
> >
> > - The primary use case of tracking async context seems be that if a
> >  process T1 in group G1 mmaps a big file and then another process T2 in
> >  group G2, asks for memory and triggers reclaim and generates writes of
> >  the file pages mapped by T1, then these writes should not be charged to
> >  T2, hence blkio_cgroup pages.
> >
> >  But the flip side of this might be that group G2 is a low weight group
> >  and probably too busy also right now, which will delay the write out
> >  and possibly T2 will wait longer for memory to be allocated.

In order to avoid this wait, dm-ioband issues IO which has a page with
PG_Reclaim as early as possible.

> > - At one point of time Andrew mentioned that buffered writes are generally a
> >  big problem and one needs to map these to owner's group. Though I am not
> >  very sure what specific problem he was referring to. Can we attribute
> >  buffered writes to pdflush threads and move all pdflush threads in a
> >  cgroup to limit system wide write out activity?

I think that buffered writes also should be controlled per cgroup as
well as synchronous writes.

> > - Somebody also gave an example where there is a memory hogging process and
> >  possibly pushes out some processes to swap. It does not sound fair to
> >  charge those proccess for that swap writeout. These processes never
> >  requested swap IO.

I think that swap writeouts should be charged to the memory hogging
process, because the process consumes more resources and it should get
a penalty.

> > - If there are multiple buffered writers in the system, then those writers
> >  can also be forced to writeout some pages to disk before they are
> >  allowed to dirty more pages. As per the page cache design, any writer
> >  can pick any inode and start writing out pages. So it can happen a
> >  weight group task is writting out pages dirtied by a lower weight group
> >  task. If, async bio is mapped to owner's group, it might happen that
> >  higher weight group task might be made to sleep on lower weight group
> >  task because request descriptors are all consumed up.

As mentioned above, in dm-ioband, the bio is charged to the page owner
and issued immediately.

> > It looks like there does not seem to be a clean way which covers all the
> > cases without issues. I am just trying to think, what is a simple way
> > which covers most of the cases. Can we just stick to using submitting task
> > context to determine a bio's group (as cfq does). Which can result in
> > following.
> >
> > - Less code and reduced complexity.
> >
> > - Buffered writes will be charged to pdflush and its group. If one wish to
> >  limit buffered write activity for pdflush, one can move all the pdflush
> >  threads into a group and assign desired weight. Writes submitted in
> >  process context will continue to be charged to that process irrespective
> >  of the fact who dirtied that page.
> 
> What if we wanted to control buffered write activity per group? If a
> group keeps dirtying pages, we wouldn't want it to dominate the disk
> IO capacity at the expense of other cgroups (by dominating the writes
> sent down by pdflush).

Yes, I think that is true.

> > - swap activity will be charged to kswapd and its group. If swap writes
> >  are coming from process context, it gets charged to process and its
> >  group.
> >
> > - If one is worried about the case of one process being charged for write
> >  out of file mapped by another process during reclaim, then we can
> >  probably make use of memory controller and mount memory controller and
> >  io controller together on same hierarchy. I am told that with memory
> >  controller, group's memory will be reclaimed by the process requesting
> >  more memory. If that's the case, then IO will automatically be charged
> >  to right group if we use submitting task context.
> >
> > I just wanted to bring this point forward for more discussions to know
> > what is the right thing to do? Use bio tracking or not.

Thanks for bringing it forward.

Thanks,
Ryo Tsuruta