Re: [PATCH 0/5] blk-throttle: writeback and swap IO control

From: Vivek Goyal
Date: Thu Feb 24 2011 - 11:19:54 EST

On Thu, Feb 24, 2011 at 09:40:39AM +0900, KAMEZAWA Hiroyuki wrote:

> > > If we don't consider the swap IO, any other IO
> > > operation from our point of view will happen directly from process
> > > context (writes in memory + sync reads from the block device).
> >
> > Why do we need to account for swap IO? Application never asked for swap
> > IO. It is kernel's decision to move soem pages to swap to free up some
> > memory. What's the point in charging those pages to application group
> > and throttle accordingly?
> >
> I think swap I/O should be controlled by memcg's dirty_ratio.
> But, IIRC, NEC guy had a requirement for this...
> I think some enterprise cusotmer may want to throttle the whole speed of
> swapout I/O (not swapin), they may be glad if they can limit throttle
> the I/O against a disk partition or all I/O tagged as 'swapio' rather than
> some cgroup name.

If swap is on a separate disk, then one can control put write throttling rules
on systemwide swapout. Though I still don't understand how that can help.

> But I'm afraid slow swapout may consume much dirty_ratio and make things
> worse ;)

Exactly. So I think focus should be controlling things earlier and stop
applications early before they can either write too much data in page
cache etc.

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