FIO performance regression in 4.11 kernel vs. 4.10 kernel observed on ARM64

[Jens Axboe]

On 05/08/2017 05:19 AM, Arnd Bergmann wrote:
> On Mon, May 8, 2017 at 1:07 PM, Will Deacon <will.deacon at arm.com> wrote:
>> Hi Scott,
>>
>> Thanks for the report.
>>
>> On Fri, May 05, 2017 at 06:37:55PM -0700, Scott Branden wrote:
>>> I have updated the kernel to 4.11 and see significant performance
>>> drops using fio-2.9.
>>>
>>> Using FIO the performanced drops from 281 KIOPS to 207 KIOPS using
>>> single core and task.
>>> Percent performance drop becomes even worse if multi-cores and multi-
>>> threads are used.
>>>
>>> Platform is ARM64 based A72.  Can somebody reproduce the results or
>>> know what may have changed to make such a dramatic change?
>>>
>>> FIO command and resulting log output below using null_blk to remove
>>> as many hardware specific driver dependencies as possible.
>>>
>>> modprobe null_blk queue_mode=2 irqmode=0 completion_nsec=0
>>> submit_queues=1 bs=4096
>>>
>>> taskset 0x1 fio --randrepeat=1 --ioengine=libaio --direct=1 --numjobs=1
>>> --gtod_reduce=1 --name=readtest --filename=/dev/nullb0 --bs=4k
>>> --iodepth=128 --time_based --runtime=15 --readwrite=read
>>
>> I can confirm that I also see a ~20% drop in results from 4.10 to 4.11 on
>> my AMD Seattle board w/ defconfig, but I can't see anything obvious in the
>> log.
>>
>> Things you could try:
>>
>>   1. Try disabling CONFIG_NUMA in the 4.11 kernel (this was enabled in
>>      defconfig between the releases).
>>
>>   2. Try to reproduce on an x86 box
>>
>>   3. Have a go at bisecting the issue, so we can revert the offender if
>>      necessary.
> 
> One more thing to try early: As 4.11 gained support for blk-mq I/O
> schedulers compared to 4.10, null_blk will now also need some extra
> cycles for each I/O request. Try loading the driver with "queue_mode=0"
> or "queue_mode=1" instead of "queue_mode=2".

Since you have 1 submit queues set, you are being loaded with deadline
attached. To compare 4.10 and 4.11, with queue_mode=2 and submit_queues=1,
after loading null_blk in 4.11, do:

# echo none > /sys/block/nullb0/queue/scheduler

and re-test.

-- 
Jens Axboe