Am 06.03.2018 um 17:35 schrieb Peter Lieven: > Am 06.03.2018 um 17:07 schrieb Stefan Hajnoczi: >> On Mon, Mar 05, 2018 at 02:52:16PM +0000, Dr. David Alan Gilbert wrote: >>> * Peter Lieven (p...@kamp.de) wrote: >>>> Am 05.03.2018 um 12:45 schrieb Stefan Hajnoczi: >>>>> On Thu, Feb 22, 2018 at 12:13:50PM +0100, Peter Lieven wrote: >>>>>> I stumbled across the MAX_INFLIGHT_IO field that was introduced in 2015 >>>>>> and was curious what was the reason >>>>>> to choose 512MB as readahead? The question is that I found that the >>>>>> source VM gets very unresponsive I/O wise >>>>>> while the initial 512MB are read and furthermore seems to stay >>>>>> unreasponsive if we choose a high migration speed >>>>>> and have a fast storage on the destination VM. >>>>>> >>>>>> In our environment I modified this value to 16MB which seems to work >>>>>> much smoother. I wonder if we should make >>>>>> this a user configurable value or define a different rate limit for the >>>>>> block transfer in bulk stage at least? >>>>> I don't know if benchmarks were run when choosing the value. From the >>>>> commit description it sounds like the main purpose was to limit the >>>>> amount of memory that can be consumed. >>>>> >>>>> 16 MB also fulfills that criteria :), but why is the source VM more >>>>> responsive with a lower value? >>>>> >>>>> Perhaps the issue is queue depth on the storage device - the block >>>>> migration code enqueues up to 512 MB worth of reads, and guest I/O has >>>>> to wait? >>>> That is my guess. Especially if the destination storage is faster we >>>> basically alsways have >>>> 512 I/Os in flight on the source storage. >>>> >>>> Does anyone mind if the reduce that value to 16MB or do we need a better >>>> mechanism? >>> We've got migration-parameters these days; you could connect it to one >>> of those fairly easily I think. >>> Try: grep -i 'cpu[-_]throttle[-_]initial' for an example of one that's >>> already there. >>> Then you can set it to whatever you like. >> It would be nice to solve the performance problem without adding a >> tuneable. >> >> On the other hand, QEMU has no idea what the queue depth of the device >> is. Therefore it cannot prioritize guest I/O over block migration I/O. >> >> 512 parallel requests is much too high. Most parallel I/O benchmarking >> is done at 32-64 queue depth. >> >> I think that 16 parallel requests is a reasonable maximum number for a >> background job. >> >> We need to be clear though that the purpose of this change is unrelated >> to the original 512 MB memory footprint goal. It just happens to touch >> the same constant but the goal is now to submit at most 16 I/O requests >> in parallel to avoid monopolizing the I/O device. > I think we should really look at this. The variables that control if we stay > in the while loop or not are incremented and decremented > at the following places: > > mig_save_device_dirty: > mig_save_device_bulk: > block_mig_state.submitted++; > > blk_mig_read_cb: > block_mig_state.submitted--; > block_mig_state.read_done++; > > flush_blks: > block_mig_state.read_done--; > > The condition of the while loop is: > (block_mig_state.submitted + > block_mig_state.read_done) * BLOCK_SIZE < > qemu_file_get_rate_limit(f) && > (block_mig_state.submitted + > block_mig_state.read_done) < > MAX_INFLIGHT_IO) > > At first I wonder if we ever reach the rate-limit because we put the read > buffers onto f AFTER we exit the while loop? > > And even if we reach the limit we constantly maintain 512 I/Os in parallel > because we immediately decrement read_done > when we put the buffers to f in flush_blks. In the next iteration of the > while loop we then read again until we have 512 in-flight I/Os. > > And shouldn't we have a time limit to limit the time we stay in the while > loop? I think we artificially delay sending data to f?
Thinking about it for a while I would propose the following: a) rename MAX_INFLIGHT_IO to MAX_IO_BUFFERS b) add MAX_PARALLEL_IO with a value of 16 c) compare qemu_file_get_rate_limit only with block_mig_state.read_done This would yield in the following condition for the while loop: (block_mig_state.read_done * BLOCK_SIZE < qemu_file_get_rate_limit(f) && (block_mig_state.submitted + block_mig_state.read_done) < MAX_IO_BUFFERS && block_mig_state.submitted < MAX_PARALLEL_IO) Sounds that like a plan? Peter