Re: [Qemu-devel] [RFC PATCH RDMA support v4: 08/10] introduce QEMUFileRDMA

[Paolo Bonzini]

Il 19/03/2013 14:40, Michael R. Hines ha scritto:
> On 03/19/2013 09:25 AM, Paolo Bonzini wrote:
>> Yes, the drain needs to happen in a few places already:
>>
>> 1. During save_rdma_page (if the current "chunk" is full of pages)
>> Ok, this is internal to RDMA so no problem.
>>
>>> 2. During the end of each iteration (now using qemu_fflush in my current
>>> patch)
>> Why?
> 
> This is because of downtime: You have to drain the queue anyway at the
> very end, and if you don't drain it in advance after each iteration, then
> the queue will have lots of bytes in it waiting for transmission and the
> Virtual Machine will be stopped for a much longer period of time during
> the last iteration waiting for RDMA card to finish transmission of all
> those
> bytes.

Shouldn't the "current chunk full" case take care of it too?

Of course if you disable chunking you have to add a different condition,
perhaps directly into save_rdma_page.

> If you wait till the last iteration to do this, then all of that waiting time 
> gets
> counted as downtime, causing the VCPUs to be unnecessarily stopped.
> 
>>> 3. And also during qemu_savem_state_complete(), also using qemu_fflush.
>> This would be caught by put_buffer, but (2) would not.
>>
> 
> I'm not sure this is good enough either - we don't want to flush
> the queue *frequently*..... only when it's necessary for performance
> .... we do want the queue to have some meat to it so the hardware
> can write bytes as fast as possible.....
> 
> If we flush inside put_buffer (which is called very frequently):

Is it called at any time during RAM migration?

> then we have no way to distinquish *where* put buffer was called from
> (either from qemu_savevm_state_complete() or from a device-level
> function call that's using QEMUFile).

Can you make drain a no-op if there is nothing in flight?  Then every
call to put_buffer after the first should not have any overhead.

Paolo

>>>> Yes, I am suggesting to modify f->pos in save_rdma_page instead.
>>>>
>>>> Paolo
>>>>
>>> Would that not confuse the other QEMUFile users?
>>> If I change that pointer (without actually putting bytes
>>> in into QEMUFile), won't the f->pos pointer be
>>> incorrectly updated?
>> f->pos is never used directly by QEMUFile, it is almost an opaque value.
>>   It is accumulated on every qemu_fflush (so that it can be passed to the
>> ->put_buffer function), and returned by qemu_ftell; nothing else.
>>
>> If you make somehow save_rdma_page a new op, returning a value from that
>> op and adding it to f->pos would be a good way to achieve this.
> 
> Ok, great - I'll take advantage of that........Thanks.
>