Re: [Qemu-devel] [PATCH v2 00/41] postcopy live migration

[Juan Quintela <quintela@redhat.com>]

Avi Kivity <avi@redhat.com> wrote:
> On 06/08/2012 01:16 PM, Juan Quintela wrote:
>> Anthony Liguori <aliguori@us.ibm.com> wrote:
>>
>> Once told that, we need to measure what is the time of an async page
>> fault over the network.  If it is too high, post copy just don't work.
>>
>> And no, I haven't seen any measurement that told us that this is going
>> to be fast enough, but there is always hope.
>
> At 10Gb/sec, the time to transfer one page is 4 microseconds.  At
> 40Gb/sec this drops to a microsecond, plus the latency.  This is on par
> with the time to handle a write protection fault that precopy uses.  But
> this can *only* be achieved with RDMA, otherwise the overhead of
> messaging and copying will dominate.
>
> Note this does not mean we should postpone merging until RDMA support is
> ready.  However we need to make sure the kernel interface is RDMA friendly.

Fully agree here.  I always thought that postcopy will work with RDMA or
something like that, any other thing would just add too much latency.

Later, Juan.