Re: [RFC] respect the referenced bit of KVM guest pages?

[Wu Fengguang]

On Thu, Aug 06, 2009 at 07:44:01PM +0800, Avi Kivity wrote:
> On 08/06/2009 01:59 PM, Wu Fengguang wrote:

scheme KEEP_MOST:

>> How about, for every N pages that you scan, evict at least 1 page,
>> regardless of young bit status?  That limits overscanning to a N:1
>> ratio.  With N=250 we'll spend at most 25 usec in order to locate one
>> page to evict.

scheme DROP_CONTINUOUS:

> > This is a quick hack to materialize the idea. It remembers roughly
> > the last 32*SWAP_CLUSTER_MAX=1024 active (mapped) pages scanned,
> > and if _all of them_ are referenced, then the referenced bit is
> > probably meaningless and should not be taken seriously.

> I don't think we should ignore the referenced bit. There could still be 
> a large batch of unreferenced pages later on that we should 
> preferentially swap. If we swap at least 1 page for every 250 scanned, 
> after 4K swaps we will have traversed 1M pages, enough to find them.

I guess both schemes have unacceptable flaws.

For JVM/BIGMEM workload, most pages would be found referenced _all the time_.
So the KEEP_MOST scheme could increase reclaim overheads by N=250 times;
while the DROP_CONTINUOUS scheme is effectively zero cost.

However, the DROP_CONTINUOUS scheme does bring more _indeterminacy_.
It can behave vastly different on single active task and multi ones.
It is short sighted and can be cheated by bursty activities.

> > As a refinement, the static variable 'recent_all_referenced' could be
> > moved to struct zone or made a per-cpu variable.
> 
> Definitely this should be made part of the zone structure, consider the 
> original report where the problem occurs in a 128MB zone (where we can 
> expect many pages to have their referenced bit set).

Good point. Here the cgroup list is highly stressed, while the global
zones are idling.

Thanks,
Fengguang
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