[dm-devel] Re: [PATCH] io-controller: Add io group reference handling for request

[Andrea Righi <righi andrea gmail com>]

On Wed, May 27, 2009 at 03:56:31PM +0900, Ryo Tsuruta wrote:
> > I think that only putting the hook in try_to_unmap() doesn't work
> > correctly, because IOs will be charged to reclaiming processes or
> > kswapd. These IOs should be charged to processes which cause memory
> > pressure.
> 
> Consider the following case:
> 
>   (1) There are two processes Proc-A and Proc-B.
>   (2) Proc-A maps a large file into many pages by mmap() and writes
>       many data to the file.
>   (3) After (2), Proc-B try to get a page, but there are no available
>       pages because Proc-A has used them.
>   (4) kernel starts to reclaim pages, call try_to_unmap() to unmap
>       a page which is owned by Proc-A, then blkio_cgroup_set_owner()
>       sets Proc-B's ID on the page because the task's context is Proc-B.
>   (5) After (4), kernel writes the page out to a disk. This IO is
>       charged to Proc-B.
> 
> In the above case, I think that the IO should be charged to a Proc-A,
> because the IO is caused by Proc-A's memory pressure. 
> I think we should consider in the case without memory and swap
> isolation.

mmmh.. even if they're strictly related I think we're mixing two
different problems in this way: memory pressure control and IO control.

It seems you're proposing something like the badness() for OOM
conditions to charge swap IO depending on how bad is a cgroup in terms
of memory consumption. I don't think this is the right way to proceed,
also because we already have the memory and swap control.

-Andrea