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

[Con Kolivas]

Marcelo Tosatti wrote:
> On Tue, Sep 07, 2004 at 08:56:47PM +1000, Con Kolivas wrote:
>
> > Nick Piggin wrote:
> >
> > > Marcelo Tosatti wrote:
> > >
> > >
> > > > Hi kernel fellows,
> > > >
> > > > I volunteer. I'll try something tomorrow to compare swappiness of 
> > > > older kernels like  2.6.5 and 2.6.6, which were fine on SGI's Altix 
> > > > tests, up to current newer kernels (on small memory boxes of course).
> > >
> > > Hi Marcelo,
> > >
> > > Just a suggestion - I'd look at the thrashing control patch first.
> > > I bet that's the cause.
> >
> > Good point!
> >
> > I recall one of my users found his workload which often hit swap lightly 
> > was swapping much heavier and his performance dropped dramatically until 
> > I stopped including the swap thrash control patch. I informed Rik about 
> > it some time back so I'm not sure if he addressed it in the meantime.
>
>
> Swap thrashing code doesnt affect anything, at least on my simple contained test.
> With the same test, the amount of swapped out memory with 2.6.6/2.6.7 is 100-150MB,
>  while 2.6.8/2.6.9-mm* swaps out around 250MB.
>
> I tried 2.6.7's "vmscan.c" on 2.6.8 without noticeable difference, I wonder why. 
>
> What I've noticed before with the swap token code is total crap interactivity 
> when memory hog is running. Which doesnt happen without it.
>
> Con, I've seen your hard swappiness patch, why do you remove the current
> swap_tendency calculation? Can you give us some insight into it? 

Sure. It was painfully simple. The swap tendency worked basically the 
same but did not take into account distress. ie It made the "swappiness" 
knob purely dependant on mapped ratio. For whatever reason, if the 
swappiness value is the same in later kernels but swaps more, there is 
more "distress" meaning we are priority scanning much more aggressively.

Cheers,
Con

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