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

[Andrew Morton <akpm@osdl.org>]

> > > > 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?

No, not at all.

If we restore the total_scanned logic then kswapd will throttle itself, as
designed.  Regardless of laptop_mode.  I did that, and monitored the page
scanning and reclaim rates under various workloads.  I observed that with
the fix in place, kswapd performed less page reclaim and direct-reclaim
performed more reclaim.  And I wasn't able to demonstrate any benchmark
improvements with the fix in place, so things are left as they are.

> How can that have any effect if may_writepage is ignored if !laptop_mode? 

This is to do with kswapd throttling.  If we put kswapd to sleep more
often, it does less scanning and reclaiming.

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

yup.

	cat /proc/vmstat > /tmp/1
	run workload
	cat /proc/vmstat > /tmp/2
	analyse /tmp/1 and /tmp/2

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

Would be nice.

> > > 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).

sure.