Re: [RFC 1/3] non-resident page tracking

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2005-08-09 at 15:25 -0300, Marcelo Tosatti wrote:
> Hi Rik,
> 
> Two hopefully useful comments:
> 
> i) ARC and its variants requires additional information about page
> replacement (namely whether the page has been reclaimed from the L1 or
> L2 lists).
> 
> How costly would it be to add this information to the hash table?
> 
I've been thinking on reserving another word in the cache-line and use
that as a bit-array to keep that information; the only problems with
that would be atomicy of the {bucket,bit} tuple and very large
cachelines where NUM_NR > 32.

> ii) From my reading of the patch, the provided "distance" information is
> relative to each hash bucket. I'm unable to understand the distance metric
> being useful if measured per-hash-bucket instead of globally?

The assumption is that IFF the hash function has good distribution
properties the per bucket distance is a good approximation of
(distance >> nonres_shift).

> 
> PS: Since remember_page() is always called with the zone->lru_lock held,
> the preempt_disable/enable pair is unecessary at the moment... still, 
> might be better to leave it there for safety reasons.
> 

There being multiple zones; owning zone->lru_lock does not guarantee
uniqueness on the remember_page() path as its a global structure.

-- 
Peter Zijlstra <a.p.zijlstra@chello.nl>