On 01/04/2012 12:42 PM, Peter Lieven wrote:
ok, then i misunderstood the ram blocks thing. i thought the guest ram
would consist of a collection of ram blocks.
then let me describe it differntly. would it make sense to process
bigger portions of memory (e.g. 1M) in stage 2 to reduce the number of
calls to cpu_physical_memory_reset_dirty and instead run it on bigger
portions of memory. we might loose a few dirty pages but they will be
tracked in the next iteration in stage 2 or in stage 3 at least. what
would be necessary is that nobody marks a page dirty
while i copy the dirty information for the portion of memory i want to
process.
Dirty memory tracking is done by the hypervisor and must be done at page
granularity.
- in stage 3 the vm is stopped, right? so there can't be any more dirty
blocks after scanning the whole memory once?
No, stage 3 is entered when there are very few dirty memory pages
remaining. This may happen after scanning the whole memory many
times. It may even never happen if migration does not converge
because of low bandwidth or too strict downtime requirements.
ok, is there a chance that i lose one final page if it is modified just
after i walked over it and i found no other page dirty (so bytes_sent = 0).
No, of course not. Stage 3 will send all missing pages while the VM is
stopped. There is a chance that the guest will go crazy and start
touching lots of pages at exactly the wrong time, and thus the downtime
will be longer than expected. However, that's a necessary evil; if you
cannot accept that, post-copy migration would provide a completely
different set of tradeoffs.
(BTW, bytes_sent = 0 is very rare).
Paolo
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