Re: [Qemu-devel] [BUG] Regression of exec migration

[Anthony Liguori <anthony@codemonkey.ws>]

Pierre Riteau wrote:
> On 27 août 09, at 16:13, Anthony Liguori wrote:
>
>> Pierre Riteau wrote:
>>> [Sorry Chris, resending without the giant attachments.]
>>>
>>> Commit 907500095851230a480b14bc852c4e49d32cb16d makes exec migration 
>>> much slower than before.
>>> I'm running the latest HEAD of qemu, on Debian Lenny 5.0.2.
>>>
>>> I'm migrating a fully booted Linux VM (also running Lenny) with 
>>> 128MB of RAM to a file, using the following command: migrate "exec: 
>>> cat > vmimage". The resulting file has a size of 57MB (because we 
>>> save only what is allocated from the 128MB).
>>> With the current HEAD, it takes from 15 to 40 seconds (it's 
>>> variable) to perform the migration to the file.
>>> With commit 907500095851230a480b14bc852c4e49d32cb16d reverted (or 
>>> just commenting the "socket_set_nonblock(s->fd);" statement), it 
>>> takes about 3 seconds.
>>
>> Without that changeset, it wasn't a live migration.  The better way 
>> to compare would be to issue stop before doing the migrate and 
>> compare that time with the previous time.
>>
>> When a migration is live, it's iterative which means there's more 
>> work to do.
>
> I tried with stop too, and I get the same results. It's an idle VM so 
> only a small number of pages are being modified while the migration is 
> going on.
> I agree that the changeset seems good, the code it replaces was 
> obviously wrong.
> But I think there is something wrong somewhere else, unless it is 
> considered normal that it takes so much time for an exec migration.
> To compare, using the same setup with one more machine and a Gigabit 
> network, a tcp migration capped at 35m (the slowest speed I've 
> measured from the disk, it can be way faster) takes about the same 
> time, between 2 and 4 seconds.

I don't think the difference between 3 seconds and 15 seconds is 
significant.

Can you try a different workload that will result in a migration that 
takes much longer (say multiple minutes)?  That is, I'd like to know 
whether there's a fixed greater cost of exec: migration vs. factor of 5.

I expect exec: to be slower because there is more copying but not by a 
factor of 5.  I expect that it's going to be a combination of relatively 
small constant factor + relatively small constant fixed cost.

Regards,

Anthony Liguori