Re: [PATCH] remove throttle_vm_writeout()

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 04 Oct 2007 20:10:10 +0200
Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> 
> On Thu, 2007-10-04 at 10:46 -0700, Andrew Morton wrote:
> > On Thu, 04 Oct 2007 18:47:07 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 
> > > static int may_write_to_queue(struct backing_dev_info *bdi)
> > > {
> > > 	if (current->flags & PF_SWAPWRITE)
> > > 		return 1;
> > > 	if (!bdi_write_congested(bdi))
> > > 		return 1;
> > > 	if (bdi == current->backing_dev_info)
> > > 		return 1;
> > > 	return 0;
> > > }
> > > 
> > > Which will write to congested queues. Anybody know why?
> 
> OK, I guess I could have found that :-/

Nice changelog, if I do say so myself ;)

> >     One fix for this would be to add an additional "really congested"
> >     threshold in the request queues, so kswapd can still perform
> >     nonblocking writeout.  This gives kswapd priority over pdflush while
> >     allowing kswapd to feed many disk queues.  I doubt if this will be
> >     called for.
> 
> I could do that.

I guess first you'd need to be able to reproduce the problem which that
patch fixed, then check that it remains fixed.

Sigh.  That problem was fairly subtle.  We could re-break reclaim in
this way and not find out about it for six months.  There's a lesson here. 
Several.