Re: Likley stupid question on "throttle_vm_writeout"

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> On Mon, Nov 09, 2009 at 04:26:33PM +0100, Peter Zijlstra wrote:
> > On Mon, 2009-11-09 at 07:15 -0800, Martin Knoblauch wrote:
> > > Hi, (please CC me on replies)
> > > 
> > >  I have a likely stupid question on the function "throttle_vm_writeout". Looking at the code I find:
> > > 
> > >                 if (global_page_state(NR_UNSTABLE_NFS) +
> > >                         global_page_state(NR_WRITEBACK) <= dirty_thresh)
> > >                                 break;
> > >                 congestion_wait(WRITE, HZ/10);
> > > 
> > > Shouldn't the NR_FILE_DIRTY pages be considered as well?
> > 
> > Ha, you just trod onto a piece of ugly I'd totally forgotten about ;-)
> > 
> > The intent of throttle_vm_writeout() is to limit the total pages in
> > writeout and to wait for them to go-away.
> 
> Like this:
> 
>         vmscan fast => large NR_WRITEBACK => throttle vmscan based on it
> 
> > Everybody hates the function, nobody managed to actually come up with
> > anything better.
> 
> btw, here is another reason to limit NR_WRITEBACK: I saw many
> throttle_vm_writeout() waits if there is no wait queue to limit
> NR_WRITEBACK (eg. NFS). In that case the (steadily) big NR_WRITEBACK
> is _not_ caused by fast vmscan..

btw, this explanation point out why we should remove other bare congestion_wait()
in reclaim path.
At least, I observed the congestion_wait() in do_try_to_free_pages() decrease
reclaim performance very much.