Re: [Qemu-devel] What's the advantages of POSTCOPY over CPU-THROTTLE?

["Jason J. Herne" <jjherne@linux.vnet.ibm.com>]

On 01/06/2016 04:57 AM, Dr. David Alan Gilbert wrote:
> * Zhangbo (Oscar) (oscar.zhangbo@huawei.com) wrote:
>> Hi all:
>> 　　Postcopy is suitable for migrating guests which have large page change rates. It
>>      1 makes the guest run at the destination ASAP.
>>      2 makes the downtime of the guest small enough.
>>      If we don't take the 1st advantage into account, then, its benefit seems similar with CPU-THROTTLE: both of them make the guest's downtime small during migration.
>>
>>      CPU-THROTTLE would make the guest's dirtypage rate *smaller than the network bandwidth*, in order to make the to_send_page_number in each iteration convergent and achieve the small-enough downtime during the last iteration.
>>      If we adopt POST-COPY here, the guest's dirtypage rate would *become equal to the bandwidth*, because we have to fetch its memory from the source side, via the network.
>>      Both of them would introduce performance degradations of the guest, which may in turn cause downtime larger.
>>
>>      So, here comes the question: If we just compare POSTCOPY with CPU-THROTTLE for their advantages in decreasing downtime, POSTCOPY seems has no pos over CPU-THROTTLE, is that right?
>>
>>      Meanwhile, Are there any other benifits of POSTCOPY besides the 2 mentioned above?
>
> It's a good question and they do both try and help solve the same problem.
> One problem with cpu-throttle is whether you can throttle the CPU enough to
> get the dirty-rate below the rate of the network, and the answer to that is
> very workload dependent.  On a large, many-core VM, even a little bit of CPU
> can dirty a lot of memory.  Postcopy is guaranteed to finish migration,
> irrespective of the workload.
>
> Postcopy is pretty fine-grained, in that only threads that are accessing
> pages that are still on the source are blocked, since it allows the use
> of async page faults, that means it's even finer grained than the vCPU level,
> so many threads come back up to full performance pretty quickly
> even if there are a few pages left.
>

Good answer Dave. FWIW, I completely agree. Using cpu throttling can 
help the situation depending on workload. Postcopy will *always* work. 
One possible side effect of Postcopy is loss of the guest if the network 
connection dies during the postcopy phase of migration. This should be a 
very rare occurrence however. So both methods have their uses.

-- 
-- Jason J. Herne (jjherne@linux.vnet.ibm.com)