Re: balance_pgdat(): where is total_scanned ever updated?

[Marcelo Tosatti <marcelo.tosatti@cyclades.com>]

On Tue, Nov 09, 2004 at 11:36:20AM -0800, Andrew Morton wrote:
> Marcelo Tosatti <marcelo.tosatti@cyclades.com> wrote:
> >
> > On Sat, Nov 06, 2004 at 04:11:14PM -0800, Andrew Morton wrote:
> > > Chuck Ebbert <76306.1226@compuserve.com> wrote:
> > > >
> > > > Kernel version is 2.6.9, but I see no updates to this function in BK-current.
> > > > How is total_scanned ever updated?  AFAICT it is always zero.
> > > 
> > > It's a bug which was introduced months ago when we added struct
> > > reclaim_state.
> > > 
> > > > In mm/vmscan.c:balance_pgdat(), there are these references to total_scanned
> > > > (missing whitepace indicated by "^"):
> > > > 
> > > > 
> > > >  977:        int total_scanned, total_reclaimed;
> > > > 
> > > >  983:        total_scanned = 0;
> > > > 
> > > > 1076:                        if (total_scanned > SWAP_CLUSTER_MAX * 2 &&
> > > > 1077:                            total_scanned > total_reclaimed+total_reclaimed/2)
> > > >                                                                ^ ^             ^ ^
> > > > 
> > > > 1088:                if (total_scanned && priority < DEF_PRIORITY - 2)
> > > > 
> > > > 
> > > > Could this be part of the problems with reclaim?  Or have I missed something?
> > > 
> > > I had a patch which fixes it in -mm for a while.  It does increase the
> > > number of pages which are reclaimed via direct reclaim and decreases the
> > > number of pages which are reclaimed by kswapd.  As one would expect from
> > > throttling kswapd.  This seems undesirable.
> > 
> > Hi Andrew,
> > 
> > Do you have any numbers to backup the claim "It does increase the
> > number of pages which are reclaimed via direct reclaim and decreases the
> > number of pages which are reclaimed by kswapd", please?
> 
> Run a workload and watch /proc/vmstat.  iirc, the one-line total_scanned
> fix takes the kswapd-vs-direct reclaim rate from 1:1 to 1:3 or thereabouts.

You're talking about laptop_mode ONLY, then?
How can that have any effect if may_writepage is ignored if !laptop_mode? 

About /proc/vmstat - each output is huge - do you actually read those?

We need a vmstat like tool for that information to be readable.

> > Because linux-2.6.10-rc1-mm2 (and 2.6.9) completly ignores sc->may_writepage 
> > under normal operation, its only used when laptop_mode is on:
> > 
> > 		if (laptop_mode && !sc->may_writepage)
> > 			goto keep_locked;
> > 
> > Is this intentional ???
> 
> yup.  In laptop mode we try to scan further to find a clean page rather
> than spinning up the disk for a writepage.

It might be interesting to use sc->may_writepage independantly of
laptop mode (ie make kswapd only writeout pages if the reclaim ratio 
is low).

> > > I'm leaving this alone until it can be demonstrated that fixing it improves
> > > kernel behaviour in some manner.
> > 
> > I dont see it working at all?
> > 
> 
> There's lots of useful info in /proc/vmstat.

I dont care much about laptop mode.