Re: Page host virtual assist patches.

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

Martin Schwidefsky wrote:
> On Mon, 2006-04-24 at 18:01 -0700, Andrew Morton wrote:
> 
>>Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:
>>
>>> The basic idea of host virtual assist (hva) is to give a host system
>>> which virtualizes the memory of its guest systems on a per page basis
>>> usage information for the guest pages. The host can then use this
>>> information to optimize the management of guest pages, in particular
>>> the paging. This optimizations can be used for unused (free) guest
>>> pages, for clean page cache pages, and for clean swap cache pages.
>>
>>This is pretty significant stuff.  It sounds like something which needs to
>>be worked through with other possible users - UML, Xen, vware, etc.
>>
>>How come the reclaim has to be done in the host?  I'd have thought that a
>>much simpler approach would be to perform a host->guest upcall saying
>>either "try to free up this many pages" or "free this page" or "free this
>>vector of pages"?
> 
> 
> Because calling into the guest is too slow. You need to schedule a cpu,
> the code that does the allocation needs to run, which might need other
> pages, etc. The beauty of the scheme is that the host can immediately
> remove a page that is mark as volatile or unused. No i/o, no scheduling,
> nothing. Consider what that does to the latency of the hosts memory
> allocation. Even if the percentage of discardable pages is small, lets
> say 25% of the guests memory, the host will quickly find reusable
> memory. If the vmscan of the host attempts to evict 100 pages, on
> average it will start i/o for 75 of them, the other 25 are immediately
> free for reuse.
> 

I don't think there is any beauty in this scheme, to be honest.

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?

As for latency of host's memory allocation, it should attempt to
keep some buffer of memory free.

-- 
SUSE Labs, Novell Inc.
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