[Devel] Re: [PATCH 9/9] ext3: do not throttle metadata and journal IO

[Theodore Tso]

On Tue, Apr 21, 2009 at 11:44:29PM +0530, Balbir Singh wrote:
> 
> That would be true in general, but only the process writing to the
> file will dirty it. So dirty already accounts for the read/write
> split. I'd assume that the cost is only for the dirty page, since we
> do IO only on write in this case, unless I am missing something very
> obvious.

Maybe I'm missing something, but the (in development) patches I saw
seemed to use the existing infrastructure designed for RSS cost
tracking (which is also not yet in mainline, unless I'm mistaken ---
but I didn't see page_get_page_cgroup() in the mainline tree yet).

Right?  So if process A in cgroup A reads touches the file first by
reading from it, then the pages read by process A will be assigned as
being "owned" by cgroup A.   Then when the patch described at

      http://lkml.org/lkml/2008/9/9/245

... tries to charge a write done by process B in cgroup B, the code
will call page_get_page_cgroup(), see that it is "owned" by cgroup A,
and charge the dirty page to cgroup A.  If process A and all of the
other processes in cgroup A only access this file read-only, and
process B is updating this file very heavily --- and it is a large
file --- then cgroup B will get a completely free pass as far as
dirtying pages to this file, since it will be all charged 100% to
cgroup A, incorrectly.

So what am I missing?

						- Ted
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