Re: swapping and the value of /proc/sys/vm/swappiness

[Con Kolivas]

Marcelo Tosatti wrote:
> On Wed, Sep 15, 2004 at 08:53:21AM +1000, Con Kolivas wrote:
>
> > Marcelo Tosatti wrote:
> >
> > > On Tue, Sep 14, 2004 at 11:31:53AM -0700, Florin Andrei wrote:
> > >
> > >
> > > > On Mon, 2004-09-06 at 16:27, Andrew Morton wrote:
> > > >
> > > >
> > > > > Con Kolivas <kernel@xxxxxxxxxxx> wrote:
> > > >
> > > > > > The change was not deliberate but there have been some other people 
> > > > > > report significant changes in the swappiness behaviour as well (see 
> > > > > > archives). It has usually been of the increased swapping variety 
> > > > > > lately. It has been annoying enough to the bleeding edge desktop users 
> > > > > > for a swag of out-of-tree hacks to start appearing (like mine).
> > > > >
> > > > > All of which is largely wasted effort.
> > > >
> > > > > From a highly-theoretical, ivory-tower perspective, maybe; i am not the
> > > > one to pass judgement.
> > > > > From a realistic, "fix it 'cause it's performing worse than MSDOS
> > > > without a disk cache" perspective, definitely not true.
> > > >
> > > > I've found a situation where the vanilla kernel has a behaviour that
> > > > makes no sense:
> > > >
> > > > http://marc.theaimsgroup.com/?l=linux-kernel&m=109237941331221&w=2
> > > > http://marc.theaimsgroup.com/?l=linux-kernel&m=109237959719868&w=2
> > > > http://marc.theaimsgroup.com/?l=linux-kernel&m=109238126314192&w=2
> > > >
> > > > A patch by Con Kolivas fixed it:
> > > >
> > > > http://marc.theaimsgroup.com/?l=linux-kernel&m=109410526607990&w=2
> > > >
> > > > I cannot offer more details, i have no time for experiments, i just need
> > > > a system that works. The vanilla kernel does not.
> > >
> > >
> > > Have you tried to decrease the value of /proc/sys/vm/swappiness 
> > > to say 30 and see what you get?
> > >
> > > Andrew's point is that we should identify the problem - Con's patch
> > > rewrites swapping policy.  
> >
> > I already answered this. That hard swappiness patch does not really 
> > rewrite swapping policy. It identifies exactly what has changed because 
> > it does not count "distress in the swap tendency". Therefore if the 
> > swappiness value is the same, the mapped ratio is the same (in the 
> > workload) yet the vm is swappinig more, it is getting into more 
> > "distress". The mapped ratio is the same but the "distress" is for some 
> > reason much higher in later kernels, meaning the priority of our 
> > scanning is getting more and more intense. This should help direct your 
> > searches.
>
>
> > These are the relevant lines of code _from mainline_:
> >
> > distress = 100 >> zone->prev_priority
> > mapped_ratio = (sc->nr_mapped * 100) / total_memory;
> > swap_tendency = mapped_ratio / 2 + distress + vm_swappiness
> > if (swap_tendency >= 100)
> > -		reclaim_mapped = 1;
> >
> >
> > That hard swappiness patch effectively made "distress == 0" always.
>
> So isnt it true that decreasing vm_swappiness should compensate 
> distress and have the same effect of your patch? 

Nope. We swap large amounts with the wrong workload at swappiness==0 
where we wouldn't before at swappiness==60. ie there is no workaround 
possible without changing the code in some way.

> To be fair I'm just arguing, haven't really looked at the code.

Thats cool ;)

Cheers,
Con
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