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

From: Avi Kivity
Date: Tue Jun 15 2010 - 03:09:57 EST

On 06/14/2010 08:58 PM, Dave Hansen wrote:
On Mon, 2010-06-14 at 19:34 +0300, Avi Kivity wrote:
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.
I can think of quite a few places where this would be beneficial.

Ballooning is dangerous. I've OOMed quite a few guests by
over-ballooning them. Anything that's voluntary like this is safer than
things imposed by the host, although you do trade of effectiveness.

That's a bug that needs to be fixed. Eventually the host will come under pressure and will balloon the guest. If that kills the guest, the ballooning is not effective as a host memory management technique.

Trying to defer ballooning by voluntarily dropping cache is simply trying to defer being bitten by the bug.

If all the guests do this, then it leaves that much more free memory on
the host, which can be used flexibly for extra host page cache, new
guests, etc...

If the host detects lots of pagecache misses it can balloon guests down. If pagecache is quiet, why change anything?

If the host wants to start new guests, it can balloon guests down. If no new guests are wanted, why change anything?


A system in this state where everyone is proactively
keeping their footprints down is more likely to be able to handle load

That is true. But from the guest's point of view, voluntarily giving up memory means dropping the guest's cushion vs load spikes.

Reclaim is an expensive, costly activity, and this ensures that
we don't have to do that when we're busy doing other things like
handling load spikes.

The guest doesn't want to reclaim memory from the host when it's under a load spike either.

This was one of the concepts behind CMM2: reduce
the overhead during peak periods.

Ah, but CMM2 actually reduced work being done by sharing information between guest and host.

It's also handy for planning. Guests exhibiting this behavior will
_act_ as if they're under pressure. That's a good thing to approximate
how a guest will act when it _is_ under pressure.

If a guest acts as if it is under pressure, then it will be slower and consume more cpu. Bad for both guest and host.

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.
I think we're trying to consider things slightly outside of ballooning
at this point. If ballooning was the end-all solution, I'm fairly sure
Balbir wouldn't be looking at this stuff. Just trying to keep options
open. :)

I see this as an extension to ballooning - perhaps I'm missing the big picture. I would dearly love to have CMM2 where decisions are made on a per-page basis instead of using heuristics.

error compiling committee.c: too many arguments to function

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