Re: [RFC/T/D][PATCH 2/2] Linux/Guest cooperative unmapped page cache control

Balbir Singh
2010-06-14T17:16:51Z

* Dave Hansen <dave@linux.vnet.ibm.com> [2010-06-14 10:09:31]:

> On Mon, 2010-06-14 at 22:28 +0530, Balbir Singh wrote:
> > If you've got duplicate pages and you know
> > that they are duplicated and can be retrieved at a lower cost, why
> > wouldn't we go after them first?
>
> I agree with this in theory. But, the guest lacks the information about
> what is truly duplicated and what the costs are for itself and/or the
> host to recreate it. "Unmapped page cache" may be the best proxy that
> we have at the moment for "easy to recreate", but I think it's still too
> poor a match to make these patches useful.
>

That is why the policy (in the next set) will come from the host. As
to whether the data is truly duplicated, my experiments show up to 60%
of the page cache is duplicated. The first patch today is again
enabled by the host. Both of them are expected to be useful in the
cache != none case.

The data I have shows more details including the performance and
overhead.

--
Three Cheers,
Balbir