Re: [PATCH 1/9] io-throttle documentation

[Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Tue, Apr 14, 2009 at 10:21:12PM +0200, Andrea Righi wrote:

[..]
> +4.2. Buffered I/O (write-back) tracking
> +
> +For buffered writes the scenario is a bit more complex, because the writes in
> +the page cache are processed asynchronously by kernel threads (pdflush), using
> +a write-back policy. So the real writes to the underlying block devices occur
> +in a different I/O context respect to the task that originally generated the
> +dirty pages.
> +
> +The I/O bandwidth controller uses the following solution to resolve this
> +problem.
> +
> +If the operation is a buffered write, we can charge the right cgroup looking at
> +the owner of the first page involved in the I/O operation, that gives the
> +context that generated the I/O activity at the source. This information can be
> +retrieved using the page_cgroup functionality originally provided by the cgroup
> +memory controller [4], and now provided specifically by the bio-cgroup
> +controller [5].
> +
> +In this way we can correctly account the I/O cost to the right cgroup, but we
> +cannot throttle the current task in this stage, because, in general, it is a
> +different task (e.g., pdflush that is processing asynchronously the dirty
> +page).
> +
> +For this reason, all the write-back requests that are not directly submitted by
> +the real owner and that need to be throttled are not dispatched immediately in
> +submit_bio(). Instead, they are added into an rbtree and processed
> +asynchronously by a dedicated kernel thread: kiothrottled.
> +

Hi Andrea,

I am trying to go through your patches now and also planning to test it
out. While reading the documentation async write handling interested
me. IIUC, looks like you are throttling writes once they are being 
written to the disk (either by pdflush or in the context of the process
because vm_dirty_ratio crossed etc).

If that's the case, will a process not see an increased rate of writes
till we are not hitting dirty_background_ratio?

Secondly, if above is giving acceptable performance resutls, then we
should be able to provide max bw control at IO scheduler level (along
with proportional bw control)?

So instead of doing max bw and proportional bw implementation in two
places with the help of different controllers, I think we can do it
with the help of one controller at one place. 

Please do have a look at my patches also to figure out if that's possible
or not. I think it should be possible.

Keeping both at single place should simplify the things.

Thanks
Vivek