Re: Page host virtual assist patches.

[Nick Piggin <nickpiggin@yahoo.com.au>]

Martin Schwidefsky wrote:
> On Tue, 2006-04-25 at 18:26 +1000, Nick Piggin wrote:

>>I don't think there is any beauty in this scheme, to be honest.
> 
> 
> Beauty lies in the eye of the beholder. From my point of view there is
> benefit to the method.

That's 'cause you have an s390.

> 
> 
>>I don't see why calling into the host is bad - won't it be able to
>>make better reclaim decisions? If starting IO is the wrong thing to
>>do under a hypervisor, why is it the right thing to do on bare metal?
> 
> 
> First some assumptions about the environment. We are talking about a
> paging hypervisor that runs several hundreds of guest Linux images. The
> memory is overcommited, the sum of the guest memory sizes is larger than
> the host memory by a factor of 2-3. Usually a large percentage of the
> guests memory is paged out by the hypervisor.
> 
> Both the host and the guest follow an LRU strategy. That means that the
> host will pick the oldest page from the idlest guest. Almost the same
> would happen if you call into the idlest guest to let the guest free its
> oldest page. But the catch is that the guest will touch a lot of page
> doing its vmscan operation, if that causes a single additional host i/o
> because a guest page needs to be retrieved from the host swap device,
> you are already in negative territory.

Why would most guest memory be paged out if the host reclaims by first
asking guests to reclaim, *then* paging them out?

I can understand that you observe most guest memory to be paged out
under pressure with the present scheme, but the dynamics will completely
change I think... You'll be left with shrunk guests, which you could
then mark as unreclaimable, stop asking them to reclaim, then page the
rest of their memory out from the host.

 > It does attempt to keep some memory free. But lets say 1000 guest images
 > generate a lot of memory pressure. You will run out of memory, and
 > anything that speeds up the host reclaim will improve the situation. And

I believe that, and I'm sure there are lots of really invasive things you
could do to make it even faster...

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