Il giorno 15 ott 2018, alle ore 20:34, Paolo Valente =

[off-list ref] ha scritto:

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quoted

Il giorno 15 ott 2018, alle ore 17:02, Bart Van Assche =

[off-list ref] ha scritto:

quoted

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On Mon, 2018-10-15 at 16:10 +0200, Linus Walleij wrote:

quoted

+ * 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

quoted

quoted

+ * - "none" for single queue devices as well as last resort

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For SATA SSDs nr_hw_queues =3D=3D 1 so this patch will also affect =

these SSDs.

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Since this patch is an attempt to improve performance, I'd like to =

see

quoted

measurement data for one or more recent SATA SSDs before a decision =

is

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taken about what to do with this patch.=20
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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.
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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
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Two things:

1) By mistake, I put 'none' in the last command line above, but it =
should be mq-deadline:

sudo ./run_main_benchmarks.sh "throughput replayed-startup" "bfq =
mq-deadline"

2) If you are worried about wearing your device with writes, then just =
append 'raw' to the last command line. So:

sudo ./run_main_benchmarks.sh "throughput replayed-startup" "bfq =
mq-deadline" raw

'raw' means: "don't even create files for the background traffic, but =
just read raw sectors".

Thanks,
Paolo

Of course, do not do it for multi-queue devices or single-queues
devices, on steroids, that do 400-500 KIOPS.
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I'll see if I can convince someone to repeat these tests with a recent
SSD.
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And here is again my reply to Jens, which I think holds for your =

repeated

objection too.
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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.
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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.
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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.
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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.
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Thanks,
Paolo
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quoted

Thanks,
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Bart.
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