Re: [PATCH] remove throttle_vm_writeout()

Andrew Morton
2007-10-04T18:56:00Z

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.