[Ksummit-discuss] [TECH TOPIC] Memory thrashing, was Re: Self nomination

[Johannes Weiner]

On Mon, Aug 01, 2016 at 09:11:32AM -0700, Dave Hansen wrote:
> On 08/01/2016 09:06 AM, James Bottomley wrote:
> >>  With persistent memory devices you might actually run out of CPU 
> >> > capacity while performing basic page aging before you saturate the 
> >> > storage device (which is why Andi Kleen has been suggesting to 
> >> > replace LRU reclaim with random replacement for these devices). So 
> >> > storage device saturation might not be the final answer to this
> >> > problem.
> > We really wouldn't want this.  All cloud jobs seem to have memory they
> > allocate but rarely use, so we want the properties of the LRU list to
> > get this on swap so we can re-use the memory pages for something else. 
> >  A random replacement algorithm would play havoc with that.
> 
> I don't want to put words in Andi's mouth, but what we want isn't
> necessarily something that is random, but it's something that uses less
> CPU to swap out a given page.

Random eviction doesn't mean random outcome of what stabilizes in
memory and swap. The idea is to apply pressure on all pages equally
but in no particular order, and then the in-memory set forms based on
reference frequencies and refaults/swapins.

Our anon LRU approximation can be so inaccurate as to be doing that
already anyway, only with all the overhead of having an LRU list.