Il 19/03/2013 15:10, Michael R. Hines ha scritto: > On 03/19/2013 09:45 AM, Paolo Bonzini wrote: >> 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. > > No, we don't want to flush on "chunk full" - that has a different meaning. > We want to have as many chunks submitted to the hardware for transmission > as possible to keep the bytes moving.
That however gives me an idea... Instead of the full drain at the end of an iteration, does it make sense to do a "partial" drain at every chunk full, so that you don't have > N bytes pending and the downtime is correspondingly limited? >>>>> 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? > > I don't understand the question: the flushing we've been discussing > is *only* for RAM migration - not for the non-live state. Yes. But I would like to piggyback the final, full drain on the switch from RAM migration to device migration. >> 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. > > That still doesn't solve the problem: If there is nothing in flight, > then there is no reason to call qemu_fflush() in the first place. If there is no RAM migration in flight. So you have migrate RAM ... RAM migration finished, device migration start put_buffer <<<<< QEMUFileRDMA triggers drain put_buffer put_buffer put_buffer ... > The flushes we need are only for RAM, not the rest of it...... > > Make sense? Paolo
