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

[Will Deacon]

On Mon, May 08, 2017 at 08:08:55AM -0600, Jens Axboe wrote:
> 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:
> >> 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.

On my setup, doing this restored a bunch of the performance, but the numbers
are still ~5% worse than 4.10 (as opposed to ~20% worse with mq-deadline).
Disabling NUMA as well cuts this down to ~2%.

Scott -- do you see the same sort of thing?

Will