Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1619742 > unrolled thread

bfq-mq performance comparison to cfq

Back to article view | Back to linux.kernel

Contents

  bfq-mq performance comparison to cfq Andreas Herrmann <aherrmann@suse.com> - 2017-04-10 11:10 +0200
    Re: bfq-mq performance comparison to cfq Paolo Valente <paolo.valente@linaro.org> - 2017-04-10 12:00 +0200
      Re: bfq-mq performance comparison to cfq Bart Van Assche <Bart.VanAssche@sandisk.com> - 2017-04-10 17:20 +0200
        Re: bfq-mq performance comparison to cfq Paolo Valente <paolo.valente@linaro.org> - 2017-04-11 09:40 +0200
          Re: bfq-mq performance comparison to cfq Bart Van Assche <Bart.VanAssche@sandisk.com> - 2017-04-19 07:10 +0200
            Re: bfq-mq performance comparison to cfq Paolo Valente <paolo.valente@linaro.org> - 2017-04-19 09:10 +0200
              Re: bfq-mq performance comparison to cfq Bart Van Assche <Bart.VanAssche@sandisk.com> - 2017-04-19 17:50 +0200
      Re: bfq-mq performance comparison to cfq Paolo Valente <paolo.valente@linaro.org> - 2017-04-11 09:30 +0200
      Re: bfq-mq performance comparison to cfq Andreas Herrmann <aherrmann@suse.com> - 2017-04-11 18:40 +0200

#1619742 — bfq-mq performance comparison to cfq

Hi Paolo,

I've looked at your WIP branch as of 4.11.0-bfq-mq-rc4-00155-gbce0818
and did some fio tests to compare the behavior to CFQ.

My understanding is that bfq-mq is supposed to be merged sooner or
later and then it will be the only reasonable I/O scheduler with
blk-mq for rotational devices. Hence I think it is interesting to see
what to expect performance-wise in comparison to CFQ which is usually
used for such devices with the legacy block layer.

I've just done simple tests iterating over number of jobs (1-8 as the
test system had 8 CPUs) for all (random/sequential) read/write
patterns. Fixed set of fio parameters used were '-size=5G
--group_reporting --ioengine=libaio --direct=1 --iodepth=1
--runtime=10'.

I've done 10 runs for each such configuration. The device used was an
older SAMSUNG HD103SJ 1TB disk, SATA attached. Results that stick out
the most are those for sequential reads and sequential writes:

 * sequential reads
  [0] - cfq, intel_pstate driver, powersave governor
  [1] - bfq_mq, intel_pstate driver, powersave governor

 jo             [0]               [1]
 bs       mean     stddev    mean       stddev
  1 & 17060.300 &  77.090 & 17657.500 &  69.602
  2 & 15318.200 &  28.817 & 10678.000 & 279.070
  3 & 15403.200 &  42.762 &  9874.600 &  93.436
  4 & 14521.200 & 624.111 &  9918.700 & 226.425
  5 & 13893.900 & 144.354 &  9485.000 & 109.291
  6 & 13065.300 & 180.608 &  9419.800 &  75.043
  7 & 12169.600 &  95.422 &  9863.800 & 227.662
  8 & 12422.200 & 215.535 & 15335.300 & 245.764

 * sequential writes
  [0] - cfq, intel_pstate driver, powersave governor
  [1] - bfq_mq, intel_pstate driver, powersave governor

 jo            [0]               [1]
 bs      mean     stddev    mean       stddev
  1 & 14171.300 & 80.796 & 14392.500 & 182.587
  2 & 13520.000 & 88.967 &  9565.400 & 119.400
  3 & 13396.100 & 44.936 &  9284.000 &  25.122
  4 & 13139.800 & 62.325 &  8846.600 &  45.926
  5 & 12942.400 & 45.729 &  8568.700 &  35.852
  6 & 12650.600 & 41.283 &  8275.500 & 199.273
  7 & 12475.900 & 43.565 &  8252.200 &  33.145
  8 & 12307.200 & 43.594 & 13617.500 & 127.773

With performance instead of powersave governor results were
(expectedly) higher but the pattern was the same -- bfq-mq shows a
"dent" for tests with 2-7 fio jobs. At the moment I have no
explanation for this behavior.


Regards,
Andreas

[toc] | [next] | [standalone]

#1619763

> Il giorno 10 apr 2017, alle ore 11:05, Andreas Herrmann <aherrmann@suse.com> ha scritto:
> 
> Hi Paolo,
> 
> I've looked at your WIP branch as of 4.11.0-bfq-mq-rc4-00155-gbce0818
> and did some fio tests to compare the behavior to CFQ.
> 
> My understanding is that bfq-mq is supposed to be merged sooner or
> later and then it will be the only reasonable I/O scheduler with
> blk-mq for rotational devices. Hence I think it is interesting to see
> what to expect performance-wise in comparison to CFQ which is usually
> used for such devices with the legacy block layer.
> 
> I've just done simple tests iterating over number of jobs (1-8 as the
> test system had 8 CPUs) for all (random/sequential) read/write
> patterns. Fixed set of fio parameters used were '-size=5G
> --group_reporting --ioengine=libaio --direct=1 --iodepth=1
> --runtime=10'.
> 
> I've done 10 runs for each such configuration. The device used was an
> older SAMSUNG HD103SJ 1TB disk, SATA attached. Results that stick out
> the most are those for sequential reads and sequential writes:
> 
> * sequential reads
>  [0] - cfq, intel_pstate driver, powersave governor
>  [1] - bfq_mq, intel_pstate driver, powersave governor
> 
> jo             [0]               [1]
> bs       mean     stddev    mean       stddev
>  1 & 17060.300 &  77.090 & 17657.500 &  69.602
>  2 & 15318.200 &  28.817 & 10678.000 & 279.070
>  3 & 15403.200 &  42.762 &  9874.600 &  93.436
>  4 & 14521.200 & 624.111 &  9918.700 & 226.425
>  5 & 13893.900 & 144.354 &  9485.000 & 109.291
>  6 & 13065.300 & 180.608 &  9419.800 &  75.043
>  7 & 12169.600 &  95.422 &  9863.800 & 227.662
>  8 & 12422.200 & 215.535 & 15335.300 & 245.764
> 
> * sequential writes
>  [0] - cfq, intel_pstate driver, powersave governor
>  [1] - bfq_mq, intel_pstate driver, powersave governor
> 
> jo            [0]               [1]
> bs      mean     stddev    mean       stddev
>  1 & 14171.300 & 80.796 & 14392.500 & 182.587
>  2 & 13520.000 & 88.967 &  9565.400 & 119.400
>  3 & 13396.100 & 44.936 &  9284.000 &  25.122
>  4 & 13139.800 & 62.325 &  8846.600 &  45.926
>  5 & 12942.400 & 45.729 &  8568.700 &  35.852
>  6 & 12650.600 & 41.283 &  8275.500 & 199.273
>  7 & 12475.900 & 43.565 &  8252.200 &  33.145
>  8 & 12307.200 & 43.594 & 13617.500 & 127.773
> 
> With performance instead of powersave governor results were
> (expectedly) higher but the pattern was the same -- bfq-mq shows a
> "dent" for tests with 2-7 fio jobs. At the moment I have no
> explanation for this behavior.
> 

I have :)

BFQ, by default, is configured to privilege latency over throughput.
In this respect, as various people and I happened to discuss a few
times, even on these mailing lists, the only way to provide strong
low-latency guarantees, at the moment, is through device idling.  The
throughput loss you see is very likely to be the consequence of that
idling.

Why does the throughput go back up at eight jobs?  Because, if many
processes are born in a very short time interval, then BFQ understands
that some multi-job task is being started.  And these parallel tasks
usually prefer overall high throughput to single-process low latency.
Then, BFQ does not idle the device for these processes.

That said, if you do always want maximum throughput, even at the
expense of latency, then just switch off low-latency heuristics, i.e.,
set low_latency to 0.  Depending on the device, setting slice_ilde to
0 may help a lot too (as well as with CFQ).  If the throughput is
still low also after forcing BFQ to an only-throughput mode, then you
hit some bug, and I'll have a little more work to do ...

Thanks,
Paolo

> Regards,
> Andreas

[toc] | [prev] | [next] | [standalone]

#1620026

On Mon, 2017-04-10 at 11:55 +0200, Paolo Valente wrote:
> That said, if you do always want maximum throughput, even at the
> expense of latency, then just switch off low-latency heuristics, i.e.,
> set low_latency to 0.  Depending on the device, setting slice_ilde to
> 0 may help a lot too (as well as with CFQ).  If the throughput is
> still low also after forcing BFQ to an only-throughput mode, then you
> hit some bug, and I'll have a little more work to do ...

Hello Paolo,

Has it been considered to make applications tell the I/O scheduler
whether to optimize for latency or for throughput? It shouldn't be that
hard for window managers and shells to figure out whether or not a new
application that is being started is interactive or not. This would
require a mechanism that allows applications to provide such information
to the I/O scheduler. Wouldn't that be a better approach than the I/O
scheduler trying to guess whether or not an application is an interactive
application?

Bart.

[toc] | [prev] | [next] | [standalone]

#1620983

> Il giorno 10 apr 2017, alle ore 17:15, Bart Van Assche <bart.vanassche@sandisk.com> ha scritto:
> 
> On Mon, 2017-04-10 at 11:55 +0200, Paolo Valente wrote:
>> That said, if you do always want maximum throughput, even at the
>> expense of latency, then just switch off low-latency heuristics, i.e.,
>> set low_latency to 0.  Depending on the device, setting slice_ilde to
>> 0 may help a lot too (as well as with CFQ).  If the throughput is
>> still low also after forcing BFQ to an only-throughput mode, then you
>> hit some bug, and I'll have a little more work to do ...
> 
> Hello Paolo,
> 
> Has it been considered to make applications tell the I/O scheduler
> whether to optimize for latency or for throughput? It shouldn't be that
> hard for window managers and shells to figure out whether or not a new
> application that is being started is interactive or not. This would
> require a mechanism that allows applications to provide such information
> to the I/O scheduler. Wouldn't that be a better approach than the I/O
> scheduler trying to guess whether or not an application is an interactive
> application?
> 

IMO that would be an (or maybe the) optimal solution, in terms of both
throughput and latency.  We have even developed a prototype doing what
you propose, for Android.  Unfortunately, I have not yet succeeded in
getting support, to turn it into candidate production code, or to make
a similar solution for lsb-compliant systems.

Thanks,
Paolo


> Bart.

[toc] | [prev] | [next] | [standalone]

#1625817

On 04/11/17 00:29, Paolo Valente wrote:
>
>> Il giorno 10 apr 2017, alle ore 17:15, Bart Van Assche <bart.vanassche@sandisk.com> ha scritto:
>>
>> On Mon, 2017-04-10 at 11:55 +0200, Paolo Valente wrote:
>>> That said, if you do always want maximum throughput, even at the
>>> expense of latency, then just switch off low-latency heuristics, i.e.,
>>> set low_latency to 0.  Depending on the device, setting slice_ilde to
>>> 0 may help a lot too (as well as with CFQ).  If the throughput is
>>> still low also after forcing BFQ to an only-throughput mode, then you
>>> hit some bug, and I'll have a little more work to do ...
>>
>> Has it been considered to make applications tell the I/O scheduler
>> whether to optimize for latency or for throughput? It shouldn't be that
>> hard for window managers and shells to figure out whether or not a new
>> application that is being started is interactive or not. This would
>> require a mechanism that allows applications to provide such information
>> to the I/O scheduler. Wouldn't that be a better approach than the I/O
>> scheduler trying to guess whether or not an application is an interactive
>> application?
>
> IMO that would be an (or maybe the) optimal solution, in terms of both
> throughput and latency.  We have even developed a prototype doing what
> you propose, for Android.  Unfortunately, I have not yet succeeded in
> getting support, to turn it into candidate production code, or to make
> a similar solution for lsb-compliant systems.

Hello Paolo,

What API was used by the Android application to tell the I/O scheduler 
to optimize for latency? Do you think that it would be sufficient if the 
application uses the ioprio_set() system call to set the I/O priority to 
IOPRIO_CLASS_RT?

Thanks,

Bart.

[toc] | [prev] | [next] | [standalone]

#1625869

> Il giorno 19 apr 2017, alle ore 07:01, Bart Van Assche <bart.vanassche@sandisk.com> ha scritto:
> 
> On 04/11/17 00:29, Paolo Valente wrote:
>> 
>>> Il giorno 10 apr 2017, alle ore 17:15, Bart Van Assche <bart.vanassche@sandisk.com> ha scritto:
>>> 
>>> On Mon, 2017-04-10 at 11:55 +0200, Paolo Valente wrote:
>>>> That said, if you do always want maximum throughput, even at the
>>>> expense of latency, then just switch off low-latency heuristics, i.e.,
>>>> set low_latency to 0.  Depending on the device, setting slice_ilde to
>>>> 0 may help a lot too (as well as with CFQ).  If the throughput is
>>>> still low also after forcing BFQ to an only-throughput mode, then you
>>>> hit some bug, and I'll have a little more work to do ...
>>> 
>>> Has it been considered to make applications tell the I/O scheduler
>>> whether to optimize for latency or for throughput? It shouldn't be that
>>> hard for window managers and shells to figure out whether or not a new
>>> application that is being started is interactive or not. This would
>>> require a mechanism that allows applications to provide such information
>>> to the I/O scheduler. Wouldn't that be a better approach than the I/O
>>> scheduler trying to guess whether or not an application is an interactive
>>> application?
>> 
>> IMO that would be an (or maybe the) optimal solution, in terms of both
>> throughput and latency.  We have even developed a prototype doing what
>> you propose, for Android.  Unfortunately, I have not yet succeeded in
>> getting support, to turn it into candidate production code, or to make
>> a similar solution for lsb-compliant systems.
> 
> Hello Paolo,
> 
> What API was used by the Android application to tell the I/O scheduler 
> to optimize for latency? Do you think that it would be sufficient if the 
> application uses the ioprio_set() system call to set the I/O priority to 
> IOPRIO_CLASS_RT?
> 

That's exactly the hack we are using in our prototype.  However, it
can only be a temporary hack, because it mixes two slightly different
concepts: 1) the activation of weight raising and other mechanisms for
reducing latency for the target app, 2) the assignment of a different
priority class, which (cleanly) means just that processes in a lower
priority class will be served only when the processes of the target
app have no pending I/O request.  Finding a clean boosting API would
be one of the main steps to turn our prototype into a usable solution.

Thanks,
Paolo

> Thanks,
> 
> Bart.

[toc] | [prev] | [next] | [standalone]

#1626409

On Wed, 2017-04-19 at 09:02 +0200, Paolo Valente wrote:
> > Il giorno 19 apr 2017, alle ore 07:01, Bart Van Assche <bart.vanassche@sandisk.com> ha scritto:
> > What API was used by the Android application to tell the I/O scheduler 
> > to optimize for latency? Do you think that it would be sufficient if the 
> > application uses the ioprio_set() system call to set the I/O priority to 
> > IOPRIO_CLASS_RT?
> 
> That's exactly the hack we are using in our prototype.  However, it
> can only be a temporary hack, because it mixes two slightly different
> concepts: 1) the activation of weight raising and other mechanisms for
> reducing latency for the target app, 2) the assignment of a different
> priority class, which (cleanly) means just that processes in a lower
> priority class will be served only when the processes of the target
> app have no pending I/O request.  Finding a clean boosting API would
> be one of the main steps to turn our prototype into a usable solution.

Hello Paolo,

Sorry but I do not agree that you call this use of I/O priorities a hack.
I also do not agree that I/O requests submitted by processes in a lower
priority class will only be served by the I/O scheduler when there are no
pending requests in a higher class. It wouldn't be that hard to modify I/O
schedulers that support I/O priorities to avoid the starvation you referred
to. What I expect that will happen is that sooner or later a Linux
distributor will start receiving bug reports about the heuristics for
detecting interactive and streaming applications and that the person who
will work on that bug report will realize that it will be easier to remove
those heuristics from BFQ and to modify streaming applications and the
software that starts interactive applications (e.g. a window manager) to
use a higher I/O priority.

Please also note that what I described above may require to introduce
additional I/O priorities in the Linux kernel next to the existing I/O
priorities RT, BE and NONE and that this may require to map multiple of
these priorities onto the same drive priority.

Bart.

[toc] | [prev] | [next] | [standalone]

#1620974

> Il giorno 10 apr 2017, alle ore 11:55, Paolo Valente <paolo.valente@linaro.org> ha scritto:
> 
>> 
>> Il giorno 10 apr 2017, alle ore 11:05, Andreas Herrmann <aherrmann@suse.com> ha scritto:
>> 
>> Hi Paolo,
>> 
>> I've looked at your WIP branch as of 4.11.0-bfq-mq-rc4-00155-gbce0818
>> and did some fio tests to compare the behavior to CFQ.
>> 
>> My understanding is that bfq-mq is supposed to be merged sooner or
>> later and then it will be the only reasonable I/O scheduler with
>> blk-mq for rotational devices. Hence I think it is interesting to see
>> what to expect performance-wise in comparison to CFQ which is usually
>> used for such devices with the legacy block layer.
>> 
>> I've just done simple tests iterating over number of jobs (1-8 as the
>> test system had 8 CPUs) for all (random/sequential) read/write
>> patterns. Fixed set of fio parameters used were '-size=5G
>> --group_reporting --ioengine=libaio --direct=1 --iodepth=1
>> --runtime=10'.
>> 
>> I've done 10 runs for each such configuration. The device used was an
>> older SAMSUNG HD103SJ 1TB disk, SATA attached. Results that stick out
>> the most are those for sequential reads and sequential writes:
>> 
>> * sequential reads
>> [0] - cfq, intel_pstate driver, powersave governor
>> [1] - bfq_mq, intel_pstate driver, powersave governor
>> 
>> jo             [0]               [1]
>> bs       mean     stddev    mean       stddev
>> 1 & 17060.300 &  77.090 & 17657.500 &  69.602
>> 2 & 15318.200 &  28.817 & 10678.000 & 279.070
>> 3 & 15403.200 &  42.762 &  9874.600 &  93.436
>> 4 & 14521.200 & 624.111 &  9918.700 & 226.425
>> 5 & 13893.900 & 144.354 &  9485.000 & 109.291
>> 6 & 13065.300 & 180.608 &  9419.800 &  75.043
>> 7 & 12169.600 &  95.422 &  9863.800 & 227.662
>> 8 & 12422.200 & 215.535 & 15335.300 & 245.764
>> 
>> * sequential writes
>> [0] - cfq, intel_pstate driver, powersave governor
>> [1] - bfq_mq, intel_pstate driver, powersave governor
>> 
>> jo            [0]               [1]
>> bs      mean     stddev    mean       stddev
>> 1 & 14171.300 & 80.796 & 14392.500 & 182.587
>> 2 & 13520.000 & 88.967 &  9565.400 & 119.400
>> 3 & 13396.100 & 44.936 &  9284.000 &  25.122
>> 4 & 13139.800 & 62.325 &  8846.600 &  45.926
>> 5 & 12942.400 & 45.729 &  8568.700 &  35.852
>> 6 & 12650.600 & 41.283 &  8275.500 & 199.273
>> 7 & 12475.900 & 43.565 &  8252.200 &  33.145
>> 8 & 12307.200 & 43.594 & 13617.500 & 127.773
>> 
>> With performance instead of powersave governor results were
>> (expectedly) higher but the pattern was the same -- bfq-mq shows a
>> "dent" for tests with 2-7 fio jobs. At the moment I have no
>> explanation for this behavior.
>> 
> 
> I have :)
> 
> BFQ, by default, is configured to privilege latency over throughput.
> In this respect, as various people and I happened to discuss a few
> times, even on these mailing lists, the only way to provide strong
> low-latency guarantees, at the moment, is through device idling.  The
> throughput loss you see is very likely to be the consequence of that
> idling.
> 
> Why does the throughput go back up at eight jobs?  Because, if many
> processes are born in a very short time interval, then BFQ understands
> that some multi-job task is being started.  And these parallel tasks
> usually prefer overall high throughput to single-process low latency.
> Then, BFQ does not idle the device for these processes.
> 
> That said, if you do always want maximum throughput, even at the
> expense of latency, then just switch off low-latency heuristics, i.e.,
> set low_latency to 0.  Depending on the device, setting slice_ilde to
> 0 may help a lot too (as well as with CFQ).  If the throughput is
> still low also after forcing BFQ to an only-throughput mode, then you
> hit some bug, and I'll have a little more work to do ...
> 

I forgot two pieces of information:
1) The throughput drop lasts only for a few seconds, after which BFQ
stops caring about the latency of the newborn fio processes, and aims
only at throughput.
2) One of my main goals, if and after BFQ is merged, is to get about
the same low-latency guarantees, without idling, and thus without
losing throughput.

Paolo


> Thanks,
> Paolo
> 
>> Regards,
>> Andreas

[toc] | [prev] | [next] | [standalone]

#1621464

On Mon, Apr 10, 2017 at 11:55:43AM +0200, Paolo Valente wrote:
> 
> > Il giorno 10 apr 2017, alle ore 11:05, Andreas Herrmann <aherrmann@suse.com> ha scritto:
> > 
> > Hi Paolo,
> > 
> > I've looked at your WIP branch as of 4.11.0-bfq-mq-rc4-00155-gbce0818
> > and did some fio tests to compare the behavior to CFQ.
> > 
> > My understanding is that bfq-mq is supposed to be merged sooner or
> > later and then it will be the only reasonable I/O scheduler with
> > blk-mq for rotational devices. Hence I think it is interesting to see
> > what to expect performance-wise in comparison to CFQ which is usually
> > used for such devices with the legacy block layer.
> > 
> > I've just done simple tests iterating over number of jobs (1-8 as the
> > test system had 8 CPUs) for all (random/sequential) read/write
> > patterns. Fixed set of fio parameters used were '-size=5G
> > --group_reporting --ioengine=libaio --direct=1 --iodepth=1
> > --runtime=10'.
> > 
> > I've done 10 runs for each such configuration. The device used was an
> > older SAMSUNG HD103SJ 1TB disk, SATA attached. Results that stick out
> > the most are those for sequential reads and sequential writes:
> > 
> > * sequential reads
> >  [0] - cfq, intel_pstate driver, powersave governor
> >  [1] - bfq_mq, intel_pstate driver, powersave governor
> > 
> > jo             [0]               [1]
> > bs       mean     stddev    mean       stddev
> >  1 & 17060.300 &  77.090 & 17657.500 &  69.602
> >  2 & 15318.200 &  28.817 & 10678.000 & 279.070
> >  3 & 15403.200 &  42.762 &  9874.600 &  93.436
> >  4 & 14521.200 & 624.111 &  9918.700 & 226.425
> >  5 & 13893.900 & 144.354 &  9485.000 & 109.291
> >  6 & 13065.300 & 180.608 &  9419.800 &  75.043
> >  7 & 12169.600 &  95.422 &  9863.800 & 227.662
> >  8 & 12422.200 & 215.535 & 15335.300 & 245.764

For the sake of completeness here the corresponding results when
setting low_latency=0 for sequential reads

 [1] - bfq_mq, intel_pstate driver, powersave governor, low_latency=1 (default)
 [2] - bfq_mq, intel_pstate driver, powersave governor, low_latency=0

jo             [2]               [1]
bs       mean     stddev    mean       stddev
 1 & 17959.500 &  62.376 & 17657.500 &  69.602
 2 & 16137.200 & 696.527 & 10678.000 & 279.070
 3 & 16223.600 &  41.291 &  9874.600 &  93.436
 4 & 16012.200 &  88.924 &  9918.700 & 226.425
 5 & 15937.900 &  51.172 &  9485.000 & 109.291
 6 & 15849.300 &  54.021 &  9419.800 &  75.043
 7 & 15794.300 &  98.857 &  9863.800 & 227.662
 8 & 15494.800 & 895.513 & 15335.300 & 245.764

> > * sequential writes
> >  [0] - cfq, intel_pstate driver, powersave governor
> >  [1] - bfq_mq, intel_pstate driver, powersave governor
> > 
> > jo            [0]               [1]
> > bs      mean     stddev    mean       stddev
> >  1 & 14171.300 & 80.796 & 14392.500 & 182.587
> >  2 & 13520.000 & 88.967 &  9565.400 & 119.400
> >  3 & 13396.100 & 44.936 &  9284.000 &  25.122
> >  4 & 13139.800 & 62.325 &  8846.600 &  45.926
> >  5 & 12942.400 & 45.729 &  8568.700 &  35.852
> >  6 & 12650.600 & 41.283 &  8275.500 & 199.273
> >  7 & 12475.900 & 43.565 &  8252.200 &  33.145
> >  8 & 12307.200 & 43.594 & 13617.500 & 127.773

... and for sequential writes

 [1] - bfq_mq, intel_pstate driver, powersave governor, low_latency=1 (default)
 [2] - bfq_mq, intel_pstate driver, powersave governor, low_latency=0

jo             [2]               [1]
bs       mean     stddev    mean       stddev

 1 & 14444.800 & 248.806 & 14392.500 & 182.587
 2 & 13929.300 &  89.137 &  9565.400 & 119.400
 3 & 13875.400 &  83.084 &  9284.000 &  25.122
 4 & 13845.000 & 106.445 &  8846.600 &  45.926
 5 & 13784.800 &  66.304 &  8568.700 &  35.852
 6 & 13774.900 &  51.845 &  8275.500 & 199.273
 7 & 13741.900 &  92.647 &  8252.200 &  33.145
 8 & 13732.400 &  88.575 & 13617.500 & 127.773

> > With performance instead of powersave governor results were
> > (expectedly) higher but the pattern was the same -- bfq-mq shows a
> > "dent" for tests with 2-7 fio jobs. At the moment I have no
> > explanation for this behavior.
> > 
> 
> I have :)
> 
> BFQ, by default, is configured to privilege latency over throughput.
> In this respect, as various people and I happened to discuss a few
> times, even on these mailing lists, the only way to provide strong
> low-latency guarantees, at the moment, is through device idling.  The
> throughput loss you see is very likely to be the consequence of that
> idling.
> 
> Why does the throughput go back up at eight jobs?  Because, if many
> processes are born in a very short time interval, then BFQ understands
> that some multi-job task is being started.  And these parallel tasks
> usually prefer overall high throughput to single-process low latency.
> Then, BFQ does not idle the device for these processes.

Thanks for the explanation!

> That said, if you do always want maximum throughput, even at the
> expense of latency, then just switch off low-latency heuristics, i.e.,
> set low_latency to 0.

That helped a lot. (See above.)

> Depending on the device, setting slice_ilde to 0 may help a lot too
> (as well as with CFQ).  If the throughput is still low also after
> forcing BFQ to an only-throughput mode, then you hit some bug, and
> I'll have a little more work to do ...


Thanks,

Andreas

[toc] | [prev] | [standalone]

Back to top | Article view | linux.kernel

csiph-web