Re: Some thoughts about cache and swap

[John Bradford]

Quote from Matt Mackall <mpm@xxxxxxxxxxx>:
> On Sun, Jun 06, 2004 at 10:38:25AM +0200, Christian Borntraeger wrote:
> > John Bradford wrote:
> > > Quote from Rik van Riel <riel@xxxxxxxxxx>:
> > > > I wonder if we should just bite the bullet and implement
> > > > LIRS, ARC or CART for Linux.  These replacement algorithms
> > > > should pretty much detect by themselves which pages are
> > > > being used again (within a reasonable time) and which pages
> > > > aren't.
> > > Is there really much performance to be gained from tuning the 'limited'
> > > cache space, or will it just hurt as many or more systems than it helps?
> > 
> > Thats a very good question. 
> > Most of the time the current algorithm works quite well.
> > On the other hand, I definitely know what people mean when they complain 
> > about cachingand all this stuff. By just copying a big file that I dont use 
> > afterwards or watching an video I have 2 wonderful scenarios.
> 
> Perhaps people should read about the referenced algorithms. LRU
> (including the hybrid LRU that Linux uses) is vulnerable to
> "scanning" of the sort you're describing, while the above algorithms
> have varying degrees of scan-resistance. As lack of scan-resistance
> seems to be "the big problem" in the current VM, this looks like an
> interesting direction to go in.

Does "the big problem" really exist though?

Despite all of this discussion about swap and memory management, I _never_
reproduce any of the problems mentioned in normal use.  In my experience,
extreme VM problems almost always stem from mis-configured swap.

John.
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