Re: writeout stalls in current -git

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

On Fri, 2 Nov 2007 18:33:29 +0800
Fengguang Wu <wfg@mail.ustc.edu.cn> wrote:

> On Fri, Nov 02, 2007 at 11:15:32AM +0100, Peter Zijlstra wrote:
> > On Fri, 2007-11-02 at 10:21 +0800, Fengguang Wu wrote:
> > 
> > > Interestingly, no background_writeout() appears, but only
> > > balance_dirty_pages() and wb_kupdate.  Obviously wb_kupdate won't
> > > block the process.
> > 
> > Yeah, the background threshold is not (yet) scaled. So it can happen
> > that the bdi_dirty limit is below the background limit.
> > 
> > I'm curious though as to these stalls, though, I can't seem to think of
> > what goes wrong.. esp since most writeback seems to happen from pdflush.
> 
> Me confused too. The new debug patch will confirm whether emerge is
> waiting in balance_dirty_pages().
> 
> > (or I'm totally misreading it - quite a possible as I'm still recovering
> > from a serious cold and not all the green stuff has yet figured out its
> > proper place wrt brain cells 'n stuff)
> 
> Do take care of yourself.
> 
> > 
> > I still have this patch floating around:
> 
> I think this patch is OK for 2.6.24 :-)
> 
> Reviewed-by: Fengguang Wu <wfg@mail.ustc.edu.cn> 

I would prefer Tested-by: :(

> > 
> > ---
> > Subject: mm: speed up writeback ramp-up on clean systems
> > 
> > We allow violation of bdi limits if there is a lot of room on the
> > system. Once we hit half the total limit we start enforcing bdi limits
> > and bdi ramp-up should happen. Doing it this way avoids many small
> > writeouts on an otherwise idle system and should also speed up the
> > ramp-up.

Given the problems we're having in there I'm a bit reluctant to go tossing
hastily put together and inadequately tested stuff onto the fire.  And
that's what this patch looks like to me.

Wanna convince me otherwise?