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

From: Avi Kivity
Date: Tue Jun 15 2010 - 02:59:21 EST

On 06/14/2010 08:45 PM, Balbir Singh wrote:

There are two decisions that need to be made:

- how much memory a guest should be given
- given some guest memory, what's the best use for it

The first question can perhaps be answered by looking at guest I/O
rates and giving more memory to more active guests. The second
question is hard, but not any different than running non-virtualized
- except if we can detect sharing or duplication. In this case,
dropping a duplicated page is worthwhile, while dropping a shared
page provides no benefit.
I think there is another way of looking at it, give some free memory

1. Can the guest run more applications or run faster

That's my second question. How to best use this memory. More applications == drop the page from cache, faster == keep page in cache.

All we need is to select the right page to drop.

2. Can the host potentially get this memory via ballooning or some
other means to start newer guest instances

Well, we already have ballooning. The question is can we improve the eviction algorithm.

I think the answer to 1 and 2 is yes.

How the patch helps answer either question, I'm not sure. I don't
think preferential dropping of unmapped page cache is the answer.

Preferential dropping as selected by the host, that knows about the
setup and if there is duplication involved. While we use the term
preferential dropping, remember it is still via LRU and we don't
always succeed. It is a best effort (if you can and the unmapped pages
are not highly referenced) scenario.

How can the host tell if there is duplication? It may know it has some pagecache, but it has no idea whether or to what extent guest pagecache duplicates host pagecache.

Those tell you how to balance going after the different classes of
things that we can reclaim.

Again, this is useless when ballooning is being used. But, I'm thinking
of a more general mechanism to force the system to both have MemFree
_and_ be acting as if it is under memory pressure.
If there is no memory pressure on the host, there is no reason for
the guest to pretend it is under pressure. If there is memory
pressure on the host, it should share the pain among its guests by
applying the balloon. So I don't think voluntarily dropping cache
is a good direction.

There are two situations

1. Voluntarily drop cache, if it was setup to do so (the host knows
that it caches that information anyway)

It doesn't, really. The host only has aggregate information about itself, and no information about the guest.

Dropping duplicate pages would be good if we could identify them. Even then, it's better to drop the page from the host, not the guest, unless we know the same page is cached by multiple guests.

But why would the guest voluntarily drop the cache? If there is no memory pressure, dropping caches increases cpu overhead and latency even if the data is still cached on the host.

2. Drop the cache on either a special balloon option, again the host
knows it caches that very same information, so it prefers to free that
up first.

Dropping in response to pressure is good. I'm just not convinced the patch helps in selecting the correct page to drop.

error compiling committee.c: too many arguments to function

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