linux-kernel - Re: [patch 50/52] mm: implement per-zone shrinker

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Date:	Fri, 25 Jun 2010 02:00:52 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, John Stultz <johnstul@...ibm.com>,
	Frank Mayhar <fmayhar@...gle.com>
Subject: Re: [patch 50/52] mm: implement per-zone shrinker

On Thu, Jun 24, 2010 at 12:06:50PM +0200, Andi Kleen wrote:
> npiggin@...e.de writes:
> 
> > Allow the shrinker to do per-zone shrinking. This means it is called for
> > each zone scanned. The shrinker is now completely responsible for calculating
> > and batching (given helpers), which provides better flexibility.
> 
> Beyond the scope of this patch, but at some point this probably needs
> to be even more fine grained. With large number of cores/threads in 
> each socket a "zone" is actually shared by quite a large number 
> of CPUs now and this can cause problems.

Yes, possibly. At least it is a much better step than the big dumb
global list.

 
> > +void shrinker_add_scan(unsigned long *dst,
> > +			unsigned long scanned, unsigned long total,
> > +			unsigned long objects, unsigned int ratio)
> > +{
> > +	unsigned long long delta;
> > +
> > +	delta = (unsigned long long)scanned * objects * ratio;
> > +	do_div(delta, total + 1);
> > +	delta /= (128ULL / 4ULL);
> 
> Again I object to the magic numbers ...
> 
> > +		nr += shrink_slab(zone, 1, 1, 1, GFP_KERNEL);
> > +	if (nr >= 10)
> > +		goto again;
> 
> And here.

I don't like them either -- problem is they were inherited from the
old code (actually 128 is the fixed point scale, I do have a define
for it just forgot to use it).

I don't know where 4 came from. And 10 is just a random number someone
picked out of a hat :P

 
> Overall it seems good, but I have not read all the shrinker callback
> changes in all subsystems.

Thanks for looking over it Andi.

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