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

From: Avi Kivity
Date: Mon Jun 14 2010 - 13:35:44 EST

On 06/14/2010 06:55 PM, Dave Hansen wrote:
On Mon, 2010-06-14 at 18:44 +0300, Avi Kivity wrote:
On 06/14/2010 06:33 PM, Dave Hansen wrote:
At the same time, I see what you're trying to do with this. It really
can be an alternative to ballooning if we do it right, since ballooning
would probably evict similar pages. Although it would only work in idle
guests, what about a knob that the host can turn to just get the guest
to start running reclaim?
Isn't the knob in this proposal the balloon? AFAICT, the idea here is
to change how the guest reacts to being ballooned, but the trigger
itself would not change.
I think the patch was made on the following assumptions:
1. Guests will keep filling their memory with relatively worthless page
cache that they don't really need.
2. When they do this, it hurts the overall system with no real gain for

In the case of a ballooned guest, they _won't_ keep filling memory. The
balloon will prevent them. So, I guess I was just going down the path
of considering if this would be useful without ballooning in place. To
me, it's really hard to justify _with_ ballooning in place.

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.

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

My issue is that changing the type of object being preferentially
reclaimed just changes the type of workload that would prematurely
suffer from reclaim. In this case, workloads that use a lot of unmapped
pagecache would suffer.

btw, aren't /proc/sys/vm/swapiness and vfs_cache_pressure similar knobs?
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.

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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