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From: | Paolo Bonzini |
Subject: | Re: [Qemu-devel] Stalls on Live Migration of VMs with a lot of memory |
Date: | Wed, 04 Jan 2012 13:28:30 +0100 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:9.0) Gecko/20111222 Thunderbird/9.0 |
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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