Re: [PATCH v2] block: BFQ default for single queue devices

Paolo Valente
2018-10-15T18:34:56Z

> Il giorno 15 ott 2018, alle ore 17:02, Bart Van Assche =
<bvanassche@acm.org> ha scritto:
>=20
> On Mon, 2018-10-15 at 16:10 +0200, Linus Walleij wrote:
>> + * For blk-mq devices, we default to using:
>> + * - "none" for multiqueue devices (nr_hw_queues !=3D 1)
>> + * - "bfq", if available, for single queue devices
>> + * - "mq-deadline" if "bfq" is not available for single queue =
devices
>> + * - "none" for single queue devices as well as last resort
>=20
> For SATA SSDs nr_hw_queues =3D=3D 1 so this patch will also affect =
these SSDs.
> Since this patch is an attempt to improve performance, I'd like to see
> measurement data for one or more recent SATA SSDs before a decision is
> taken about what to do with this patch.=20
>=20

Hi Bart,
as I just wrote to Jens I don't think we need this test any longer.
To save you one hope, I'll paste my reply to Jens below.

Anyway, it is very easy to do the tests you ask:
- take a kernel containing the last bfq commits, such as for-next
- do, e.g.,
git clone https://github.com/Algodev-github/S.git
cd S/run_multiple_benchmarks
sudo ./run_main_benchmarks.sh "throughput replayed-startup" "bfq none"
- compare results

Of course, do not do it for multi-queue devices or single-queues
devices, on steroids, that do 400-500 KIOPS.

I'll see if I can convince someone to repeat these tests with a recent
SSD.

And here is again my reply to Jens, which I think holds for your =
repeated
objection too.

I tested bfq on virtually every device in the range from few hundred
of IOPS to 50-100KIOPS. Then, through the public script I already
mentioned, I found the maximum number of IOPS that bfq can handle:
about 400K with a commodity CPU.

In particular, in all my tests with real hardware, bfq performance
- is not even comparable to that of any of the other scheduler, in
terms of responsiveness, latency for real-time applications, ability
to provide strong bandwidth guarantees, ability to boost throughput
while guaranteeing bandwidths;
- is a little worse than the other schedulers for only one test, on
only some hardware: total throughput with random reads, were it may
lose up to 10-15% of throughput. Of course, the schedulers that reach
a higher throughput leave the machine unusable during the test.

So I really cannot see a reason why bfq could do worse than any of
these other schedulers for some single-queue device (conservatively)
below 300KIOPS.

Finally, since, AFAICT, single-queue devices doing 400+ KIOPS are
probably less than 1% of all the single-queue storage around (USB
drives, HDDs, eMMC, standard SSDs, ...), by sticking to mq-deadline we
are sacrificing 99% of the hardware, to help 1% of the hardware for
one kind of test cases.

Thanks,
Paolo

> Thanks,
>=20
> Bart.
>=20