On Wed, Mar 7, 2018 at 7:55 AM, Peter Lieven <p...@kamp.de> wrote: > 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?
That sounds good to me. Stefan
