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. >
