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[dm-devel] Re: [Xen-devel] Re: [PATCH 7/9] blkio-cgroup-v9: Page tracking hooks



"KAMEZAWA Hiroyuki" <kamezawa hiroyu jp fujitsu com> wrote:
> Ryo Tsuruta wrote:
> > KAMEZAWA Hiroyuki <kamezawa hiroyu jp fujitsu com> wrote:
> 
> >> > dm-ioband gives high priority to I/O for swap-out by checking whethe=
> r
> >> > PG_swapcache flag is set on the I/O page, regardless of the assigned
> >> > I/O bandwidth, and the bandwidth consumed for swap-out is charged to
> >> > the owner of the pages as a debt.
> >> > How about this approach?
> >>
> >> I don't think it's reasonable. Why I/O device, scheduler should know
> >> about
> >> such mm-related information ? I think layering is wrong.
> >
> > I think that urgent I/O requests such as swap-out should be notified
> > by setting a special flag in the struct bio, but there is no such
> > mechanism at this time. That is why dm-ioband uses this approach.
> >
> >> And your approatch cannot be a workaround.
> >>
> >> In follwing _typical_ case,
> >>
> >>   - A process does small logging to /var/log/mylog, once in a sec.
> >>     but it uses some amount of cold memory or shmem.
> >>
> >> This process's logging will be delayed _unexpectedly_ by some buggy
> >> process
> >> which does memory leak.
> >
> > Do you mean that the delay in logging is caused since the small process
> > is swapped out unexpectedly by the buggy processes?
> I don't write "small process", "small logging".
> Buggy process does swap-out and cosumes someone else's bandwidth, then,
> loggind will be delayed. Important here is throttle bandwidth consumed by
> buggy prorcess, not other's.

Thank you for explaining it.

> > How about using memory cgroup to prevent the small process from swap-ou=
> t?
> It never be help if memcg is not configured.

blkio-cgroup is recommended to use with memcg. I think that it can be
a good solution to resolve such problem.

> My point is "don't allow anyone to use bandwidth of others."
> Considering job isolation, a thread who requests swap-out should be charg=
> ed
> against bandwidth.

>From another perspective, the swap-out is caused since the buggy
process uses a large amount of memory, so it can be considered as 
the bandwidth of logging process is used due to the buggy process.

Please consider the following case. If a thread who requests swap-out
is charged, the thread is charged other threads' I/O.

   (1)                          --------      (2)
   Process A                   |        |     Process B
   mmaps a large area in   --> | memory | <-- tries to allocate a page.
   the memory and writes       |        |
   data to there.               --------     (3)
                                   |         To get a free page,
                                   |         the data written by Proc.A
                                   |         is written out to the disk.
                                   V         The I/O is done by using
                                ---------    Proc.B's bandwidth.
                               |  disk   |   
                                ---------

Thus I think that page owners should be charged against bandwidth.

Thanks,
Ryo Tsuruta


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