Re: [Qemu-block] block migration and MAX_IN_FLIGHT_IO

[Stefan Hajnoczi]

On Wed, Mar 7, 2018 at 7:55 AM, Peter Lieven <address@hidden> 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 (address@hidden) 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