From: Paolo Bonzini <pbonzini@redhat.com>
To: Peter Lieven <pl@dlh.net>
Cc: Shu Ming <shuming@linux.vnet.ibm.com>,
qemu-devel@nongnu.org, kvm@vger.kernel.org
Subject: Re: [Qemu-devel] Stalls on Live Migration of VMs with a lot of memory
Date: Wed, 04 Jan 2012 13:28:30 +0100 [thread overview]
Message-ID: <4F0445EE.9010905@redhat.com> (raw)
In-Reply-To: <4F043B12.60501@dlh.net>
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
next prev parent reply other threads:[~2012-01-04 12:28 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-01-03 18:04 [Qemu-devel] Stalls on Live Migration of VMs with a lot of memory Peter Lieven
2012-01-04 1:38 ` Shu Ming
2012-01-04 9:11 ` Peter Lieven
2012-01-04 10:53 ` Peter Lieven
2012-01-04 11:05 ` Paolo Bonzini
2012-01-04 11:22 ` Peter Lieven
2012-01-04 11:28 ` Paolo Bonzini
2012-01-04 11:42 ` Peter Lieven
2012-01-04 12:28 ` Paolo Bonzini [this message]
2012-01-04 13:08 ` Peter Lieven
2012-01-04 14:14 ` Paolo Bonzini
2012-01-04 14:17 ` Peter Lieven
2012-01-04 14:21 ` Peter Lieven
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