Re: Linux Kernel Markers - performance characterization with large

Re: Linux Kernel Markers - performance characterization with large

[Alan D. Brunelle]

Taking Linux 2.6.23-rc6 + 2.6.23-rc6-mm1 as a basis, I took some sample 
runs of the following on both it and after applying Mathieu Desnoyers 
11-patch sequence (19 September 2007).

    * 32-way IA64 + 132GiB + 10 FC adapters + 10 HP MSA 1000s (one 72GiB
      volume per MSA used)

    * 10 runs with each configuration, averages shown below
          o 2.6.23-rc6 + 2.6.23-rc6-mm1 without blktrace running
          o 2.6.23-rc6 + 2.6.23-rc6-mm1 with blktrace running
          o 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers without blktrace running
          o 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers with blktrace running

    * A run consists of doing the following in parallel:
          o Make an ext3 FS on each of the 10 volumes
          o Mount & unmount each volume
                + The unmounting generates a tremendous amount of writes
                  to the disks - thus stressing the intended storage
                  devices (10 volumes) plus the separate volume for all
                  the blktrace data (when blk tracing is enabled).
                + Note the times reported below only cover the
                  make/mount/unmount time - the actual blktrace runs
                  extended beyond the times measured (took quite a while
                  for the blk trace data to be output). We're only
                  concerned with the impact on the "application"
                  performance in this instance.

Results are:

Kernel                                 w/out BT   STDDEV     w/ BT    STDDEV
-------------------------------------  ---------  ------   ---------  ------
2.6.23-rc6 + 2.6.23-rc6-mm1            14.679982    0.34   27.754796    2.09
2.6.23-rc6 + 2.6.23-rc6-mm1 + markers  14.993041    0.59   26.694993    3.23

It looks to be about 2.1% increase in time to do the make/mount/unmount 
operations with the marker patches in place and no blktrace operations. 
With the blktrace operations in place we see about a 3.8% decrease in 
time to do the same ops.

When our Oracle benchmarking machine frees up, and when the 
marker/blktrace patches are more stable, we'll try to get some "real" 
Oracle benchmark runs done to gage the impact of the markers changes to 
performance...

Alan D. Brunelle
Hewlett-Packard / Open Source and Linux Organization / Scalability and 
Performance Group

Re: Linux Kernel Markers - performance characterization with large IO load on large-ish system

[Mathieu Desnoyers]

* Alan D. Brunelle (Alan.Brunelle@hp.com) wrote:
> Taking Linux 2.6.23-rc6 + 2.6.23-rc6-mm1 as a basis, I took some sample 
> runs of the following on both it and after applying Mathieu Desnoyers 
> 11-patch sequence (19 September 2007).
> 
>    * 32-way IA64 + 132GiB + 10 FC adapters + 10 HP MSA 1000s (one 72GiB
>      volume per MSA used)
> 
>    * 10 runs with each configuration, averages shown below
>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 without blktrace running
>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 with blktrace running
>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers without blktrace running
>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers with blktrace running
> 
>    * A run consists of doing the following in parallel:
>          o Make an ext3 FS on each of the 10 volumes
>          o Mount & unmount each volume
>                + The unmounting generates a tremendous amount of writes
>                  to the disks - thus stressing the intended storage
>                  devices (10 volumes) plus the separate volume for all
>                  the blktrace data (when blk tracing is enabled).
>                + Note the times reported below only cover the
>                  make/mount/unmount time - the actual blktrace runs
>                  extended beyond the times measured (took quite a while
>                  for the blk trace data to be output). We're only
>                  concerned with the impact on the "application"
>                  performance in this instance.
> 
> Results are:
> 
> Kernel                                 w/out BT   STDDEV     w/ BT    STDDEV
> -------------------------------------  ---------  ------   ---------  ------
> 2.6.23-rc6 + 2.6.23-rc6-mm1            14.679982    0.34   27.754796    2.09
> 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers  14.993041    0.59   26.694993    3.23
> 

Interesting results, although we cannot say any of the solutions has much
impact due to the std dev.

Also, it could be interesting to add the "blktrace compiled out" as a
base line.

Thanks for running those tests,

Mathieu

> It looks to be about 2.1% increase in time to do the make/mount/unmount 
> operations with the marker patches in place and no blktrace operations. 
> With the blktrace operations in place we see about a 3.8% decrease in 
> time to do the same ops.
> 
> When our Oracle benchmarking machine frees up, and when the 
> marker/blktrace patches are more stable, we'll try to get some "real" 
> Oracle benchmark runs done to gage the impact of the markers changes to 
> performance...
> 
> Alan D. Brunelle
> Hewlett-Packard / Open Source and Linux Organization / Scalability and 
> Performance Group
> 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68

Re: Linux Kernel Markers - performance characterization with large

[Alan D. Brunelle]

Mathieu Desnoyers wrote:
> * Alan D. Brunelle (Alan.Brunelle@hp.com) wrote:
>> Taking Linux 2.6.23-rc6 + 2.6.23-rc6-mm1 as a basis, I took some sample 
>> runs of the following on both it and after applying Mathieu Desnoyers 
>> 11-patch sequence (19 September 2007).
>>
>>    * 32-way IA64 + 132GiB + 10 FC adapters + 10 HP MSA 1000s (one 72GiB
>>      volume per MSA used)
>>
>>    * 10 runs with each configuration, averages shown below
>>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 without blktrace running
>>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 with blktrace running
>>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers without blktrace running
>>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers with blktrace running
>>
>>    * A run consists of doing the following in parallel:
>>          o Make an ext3 FS on each of the 10 volumes
>>          o Mount & unmount each volume
>>                + The unmounting generates a tremendous amount of writes
>>                  to the disks - thus stressing the intended storage
>>                  devices (10 volumes) plus the separate volume for all
>>                  the blktrace data (when blk tracing is enabled).
>>                + Note the times reported below only cover the
>>                  make/mount/unmount time - the actual blktrace runs
>>                  extended beyond the times measured (took quite a while
>>                  for the blk trace data to be output). We're only
>>                  concerned with the impact on the "application"
>>                  performance in this instance.
>>
>> Results are:
>>
>> Kernel                                 w/out BT   STDDEV     w/ BT    STDDEV
>> -------------------------------------  ---------  ------   ---------  ------
>> 2.6.23-rc6 + 2.6.23-rc6-mm1            14.679982    0.34   27.754796    2.09
>> 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers  14.993041    0.59   26.694993    3.23
>>
>
> Interesting results, although we cannot say any of the solutions has much
> impact due to the std dev.
>
> Also, it could be interesting to add the "blktrace compiled out" as a
> base line.
>
> Thanks for running those tests,
>
> Mathieu
Mathieu:

Here are the results from 6 different kernels (including ones with 
blktrace not configured in), with now performing 40 runs per kernel.

  o  All kernels start off with Linux 2.6.23-rc6 + 2.6.23-rc6-mm1

  o  '- bt cfg' or '+ bt cfg' means a kernel without or with blktrace 
configured respectively.

  o  '- markers' or '+ markers' means a kernel without or with the 
11-patch marker series respectively.

38 runs without blk traces being captured (dropped hi/lo value from 40 runs)

Kernel Options       Min val    Avg val    Max val    Std Dev
------------------  ---------  ---------  ---------  ---------
- markers - bt cfg  15.349127  16.169459  16.372980   0.184417
+ markers - bt cfg  15.280382  16.202398  16.409257   0.191861

- markers + bt cfg  14.464366  14.754347  16.052306   0.463665
+ markers + bt cfg  14.421765  14.644406  15.690871   0.233885

38 runs with blk traces being captured (dropped hi/lo value from 40 runs)

Kernel Options       Min val    Avg val    Max val    Std Dev
------------------  ---------  ---------  ---------  ---------
- markers + bt cfg  24.675859  28.480446  32.571484   1.713603
+ markers + bt cfg  18.713280  27.054927  31.684325   2.857186

  o  It is not at all clear why running without blk trace configured 
into the kernel runs slower than with blk trace configured in. (9.6 to 
10.6% reduction)
    
  o  The data is still not conclusive with respect to whether the marker 
patches change performance characteristics when we're not gathering 
traces. It appears
that any change in performance is minimal at worst for this test.
    
  o  The data so far still doesn't conclusively show a win in this case 
even when we are capturing traces, although, the average certainly seems 
to be in its favor.
    
One concern that I should be able to deal easily with is the choice of 
the IO scheduler being used for both the volume being used to perform 
the test on, as well as the one used for storing blk traces (when 
enabled). Right now I was using the default CFQ, when perhaps NOOP or 
DEADLINE would be a better choice. If there is enough interest in seeing 
how that changes things I could try to get some runs in later this week.

Alan D. Brunelle
Hewlett-Packard / Open Source and Linux Organization / Scalability and 
Performance Group

Re: Linux Kernel Markers - performance characterization with large

[Jens Axboe]

On Wed, Sep 26 2007, Alan D. Brunelle wrote:
> Mathieu Desnoyers wrote:
>> * Alan D. Brunelle (Alan.Brunelle@hp.com) wrote:
>>> Taking Linux 2.6.23-rc6 + 2.6.23-rc6-mm1 as a basis, I took some sample 
>>> runs of the following on both it and after applying Mathieu Desnoyers 
>>> 11-patch sequence (19 September 2007).
>>>
>>>    * 32-way IA64 + 132GiB + 10 FC adapters + 10 HP MSA 1000s (one 72GiB
>>>      volume per MSA used)
>>>
>>>    * 10 runs with each configuration, averages shown below
>>>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 without blktrace running
>>>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 with blktrace running
>>>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers without blktrace running
>>>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers with blktrace running
>>>
>>>    * A run consists of doing the following in parallel:
>>>          o Make an ext3 FS on each of the 10 volumes
>>>          o Mount & unmount each volume
>>>                + The unmounting generates a tremendous amount of writes
>>>                  to the disks - thus stressing the intended storage
>>>                  devices (10 volumes) plus the separate volume for all
>>>                  the blktrace data (when blk tracing is enabled).
>>>                + Note the times reported below only cover the
>>>                  make/mount/unmount time - the actual blktrace runs
>>>                  extended beyond the times measured (took quite a while
>>>                  for the blk trace data to be output). We're only
>>>                  concerned with the impact on the "application"
>>>                  performance in this instance.
>>>
>>> Results are:
>>>
>>> Kernel                                 w/out BT   STDDEV     w/ BT    
>>> STDDEV
>>> -------------------------------------  ---------  ------   ---------  
>>> ------
>>> 2.6.23-rc6 + 2.6.23-rc6-mm1            14.679982    0.34   27.754796    
>>> 2.09
>>> 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers  14.993041    0.59   26.694993    
>>> 3.23
>>>
>>
>> Interesting results, although we cannot say any of the solutions has much
>> impact due to the std dev.
>>
>> Also, it could be interesting to add the "blktrace compiled out" as a
>> base line.
>>
>> Thanks for running those tests,
>>
>> Mathieu
> Mathieu:
>
> Here are the results from 6 different kernels (including ones with blktrace 
> not configured in), with now performing 40 runs per kernel.
>
>  o  All kernels start off with Linux 2.6.23-rc6 + 2.6.23-rc6-mm1
>
>  o  '- bt cfg' or '+ bt cfg' means a kernel without or with blktrace 
> configured respectively.
>
>  o  '- markers' or '+ markers' means a kernel without or with the 11-patch 
> marker series respectively.
>
> 38 runs without blk traces being captured (dropped hi/lo value from 40 
> runs)
>
> Kernel Options       Min val    Avg val    Max val    Std Dev
> ------------------  ---------  ---------  ---------  ---------
> - markers - bt cfg  15.349127  16.169459  16.372980   0.184417
> + markers - bt cfg  15.280382  16.202398  16.409257   0.191861
>
> - markers + bt cfg  14.464366  14.754347  16.052306   0.463665
> + markers + bt cfg  14.421765  14.644406  15.690871   0.233885
>
> 38 runs with blk traces being captured (dropped hi/lo value from 40 runs)
>
> Kernel Options       Min val    Avg val    Max val    Std Dev
> ------------------  ---------  ---------  ---------  ---------
> - markers + bt cfg  24.675859  28.480446  32.571484   1.713603
> + markers + bt cfg  18.713280  27.054927  31.684325   2.857186
>
>  o  It is not at all clear why running without blk trace configured into 
> the kernel runs slower than with blk trace configured in. (9.6 to 10.6% 
> reduction)
>     o  The data is still not conclusive with respect to whether the marker 
> patches change performance characteristics when we're not gathering traces. 
> It appears
> that any change in performance is minimal at worst for this test.
>     o  The data so far still doesn't conclusively show a win in this case 
> even when we are capturing traces, although, the average certainly seems to 
> be in its favor.
>    One concern that I should be able to deal easily with is the choice of 
> the IO scheduler being used for both the volume being used to perform the 
> test on, as well as the one used for storing blk traces (when enabled). 
> Right now I was using the default CFQ, when perhaps NOOP or DEADLINE would 
> be a better choice. If there is enough interest in seeing how that changes 
> things I could try to get some runs in later this week.

Alan,

Thanks for running these numbers as well. I don't think you have to
bother with it more. My main concern was a performance regression,
increasing the overhead of running blktrace. So while we (well, you :-))
could run more tests, I'd say the above is Good Enough for me. Mathieu,
you can add my Acked-by: Jens Axboe <jens.axboe@oracle.com> to the
blktrace part of your marker series.

I do wonder about that performance _increase_ with blktrace enabled. I
remember that we have seen and discussed something like this before,
it's still a puzzle to me...

-- 
Jens Axboe

Re: Linux Kernel Markers - performance characterization with large IO load on large-ish system

[Mathieu Desnoyers]

* Jens Axboe (jens.axboe@oracle.com) wrote:
> On Wed, Sep 26 2007, Alan D. Brunelle wrote:
> > Mathieu Desnoyers wrote:
> >> * Alan D. Brunelle (Alan.Brunelle@hp.com) wrote:
> >>> Taking Linux 2.6.23-rc6 + 2.6.23-rc6-mm1 as a basis, I took some sample 
> >>> runs of the following on both it and after applying Mathieu Desnoyers 
> >>> 11-patch sequence (19 September 2007).
> >>>
> >>>    * 32-way IA64 + 132GiB + 10 FC adapters + 10 HP MSA 1000s (one 72GiB
> >>>      volume per MSA used)
> >>>
> >>>    * 10 runs with each configuration, averages shown below
> >>>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 without blktrace running
> >>>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 with blktrace running
> >>>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers without blktrace running
> >>>          o 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers with blktrace running
> >>>
> >>>    * A run consists of doing the following in parallel:
> >>>          o Make an ext3 FS on each of the 10 volumes
> >>>          o Mount & unmount each volume
> >>>                + The unmounting generates a tremendous amount of writes
> >>>                  to the disks - thus stressing the intended storage
> >>>                  devices (10 volumes) plus the separate volume for all
> >>>                  the blktrace data (when blk tracing is enabled).
> >>>                + Note the times reported below only cover the
> >>>                  make/mount/unmount time - the actual blktrace runs
> >>>                  extended beyond the times measured (took quite a while
> >>>                  for the blk trace data to be output). We're only
> >>>                  concerned with the impact on the "application"
> >>>                  performance in this instance.
> >>>
> >>> Results are:
> >>>
> >>> Kernel                                 w/out BT   STDDEV     w/ BT    
> >>> STDDEV
> >>> -------------------------------------  ---------  ------   ---------  
> >>> ------
> >>> 2.6.23-rc6 + 2.6.23-rc6-mm1            14.679982    0.34   27.754796    
> >>> 2.09
> >>> 2.6.23-rc6 + 2.6.23-rc6-mm1 + markers  14.993041    0.59   26.694993    
> >>> 3.23
> >>>
> >>
> >> Interesting results, although we cannot say any of the solutions has much
> >> impact due to the std dev.
> >>
> >> Also, it could be interesting to add the "blktrace compiled out" as a
> >> base line.
> >>
> >> Thanks for running those tests,
> >>
> >> Mathieu
> > Mathieu:
> >
> > Here are the results from 6 different kernels (including ones with blktrace 
> > not configured in), with now performing 40 runs per kernel.
> >
> >  o  All kernels start off with Linux 2.6.23-rc6 + 2.6.23-rc6-mm1
> >
> >  o  '- bt cfg' or '+ bt cfg' means a kernel without or with blktrace 
> > configured respectively.
> >
> >  o  '- markers' or '+ markers' means a kernel without or with the 11-patch 
> > marker series respectively.
> >
> > 38 runs without blk traces being captured (dropped hi/lo value from 40 
> > runs)
> >
> > Kernel Options       Min val    Avg val    Max val    Std Dev
> > ------------------  ---------  ---------  ---------  ---------
> > - markers - bt cfg  15.349127  16.169459  16.372980   0.184417
> > + markers - bt cfg  15.280382  16.202398  16.409257   0.191861
> >
> > - markers + bt cfg  14.464366  14.754347  16.052306   0.463665
> > + markers + bt cfg  14.421765  14.644406  15.690871   0.233885
> >
> > 38 runs with blk traces being captured (dropped hi/lo value from 40 runs)
> >
> > Kernel Options       Min val    Avg val    Max val    Std Dev
> > ------------------  ---------  ---------  ---------  ---------
> > - markers + bt cfg  24.675859  28.480446  32.571484   1.713603
> > + markers + bt cfg  18.713280  27.054927  31.684325   2.857186
> >
> >  o  It is not at all clear why running without blk trace configured into 
> > the kernel runs slower than with blk trace configured in. (9.6 to 10.6% 
> > reduction)
> >     o  The data is still not conclusive with respect to whether the marker 
> > patches change performance characteristics when we're not gathering traces. 
> > It appears
> > that any change in performance is minimal at worst for this test.
> >     o  The data so far still doesn't conclusively show a win in this case 
> > even when we are capturing traces, although, the average certainly seems to 
> > be in its favor.
> >    One concern that I should be able to deal easily with is the choice of 
> > the IO scheduler being used for both the volume being used to perform the 
> > test on, as well as the one used for storing blk traces (when enabled). 
> > Right now I was using the default CFQ, when perhaps NOOP or DEADLINE would 
> > be a better choice. If there is enough interest in seeing how that changes 
> > things I could try to get some runs in later this week.
> 
> Alan,
> 
> Thanks for running these numbers as well. I don't think you have to
> bother with it more. My main concern was a performance regression,
> increasing the overhead of running blktrace. So while we (well, you :-))
> could run more tests, I'd say the above is Good Enough for me. Mathieu,
> you can add my Acked-by: Jens Axboe <jens.axboe@oracle.com> to the
> blktrace part of your marker series.
> 
thanks!

> I do wonder about that performance _increase_ with blktrace enabled. I
> remember that we have seen and discussed something like this before,
> it's still a puzzle to me...
> 

Interesting question indeed.

In those tests, when blktrace is running, are the relay buffers only
written to or they are also read ?

Running the tests without consuming the buffers (in overwrite mode)
would tell us more about the nature of the disturbance causing the
performance increase.

Also, a kernel trace could help us understand more thoroughly what is
happening there.. is it caused by the scheduler ? memory allocation ?
data cache alignment ?

I would suggest that you try aligning the block layer data structures
accessed by blktrace on L2 cacheline size and compare the results (when
blktrace is disabled).

Mathieu

> -- 
> Jens Axboe
> 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68

Re: Linux Kernel Markers - performance characterization with large

[Alan D. Brunelle]

Mathieu Desnoyers wrote:
>> remember that we have seen and discussed something like this before,
>> it's still a puzzle to me...
>>
>>     
> I do wonder about that performance _increase_ with blktrace enabled. I
>
> Interesting question indeed.
>
> In those tests, when blktrace is running, are the relay buffers only
> written to or they are also read ?
>   

blktrace (the utility) was running too - so the relay buffere /were/
being read and stored out to disk elsewhere.

> Running the tests without consuming the buffers (in overwrite mode)
> would tell us more about the nature of the disturbance causing the
> performance increase.
>   

I'd have to write a utility to enable the traces, but then not read. Let
me think about that.

> Also, a kernel trace could help us understand more thoroughly what is
> happening there.. is it caused by the scheduler ? memory allocation ?
> data cache alignment ?
>   

Yep - when I get some time, I'll look into that. [Clearly not a gating
issue for marker support...]

> I would suggest that you try aligning the block layer data structures
> accessed by blktrace on L2 cacheline size and compare the results (when
> blktrace is disabled).
>   

Again, when I get some time! :-)

Alan

Re: Linux Kernel Markers - performance characterization with large

[Ingo Molnar]

* Alan D. Brunelle <Alan.Brunelle@hp.com> wrote:

>  o  All kernels start off with Linux 2.6.23-rc6 + 2.6.23-rc6-mm1
> 
>  o  '- bt cfg' or '+ bt cfg' means a kernel without or with blktrace 
> configured respectively.
> 
>  o  '- markers' or '+ markers' means a kernel without or with the 
> 11-patch marker series respectively.
> 
> 38 runs without blk traces being captured (dropped hi/lo value from 40 runs)
> 
> Kernel Options       Min val    Avg val    Max val    Std Dev
> ------------------  ---------  ---------  ---------  ---------
> - markers - bt cfg  15.349127  16.169459  16.372980   0.184417
> + markers - bt cfg  15.280382  16.202398  16.409257   0.191861
> 
> - markers + bt cfg  14.464366  14.754347  16.052306   0.463665
> + markers + bt cfg  14.421765  14.644406  15.690871   0.233885

actually, the pure marker overhead seems to be a regression:

> - markers - bt cfg  15.349127  16.169459  16.372980   0.184417
> + markers - bt cfg  15.280382  16.202398  16.409257   0.191861

why isnt the marker near zero-cost as it should be? (as long as they are 
enabled but are not in actual use) 2% increase is _ALOT_. That's the 
whole point of good probes: they do not slow down the normal kernel.

_Worst case_ it should be at most a few instructions overhead but that 
does not explain the ~2% wall-clock time regression you measured here.

So there's something wrong going on - either markers have unacceptably 
high cost, or the measurement is not valid.

	Ingo

Re: Linux Kernel Markers - performance characterization with large

[Joshua Root]

Ingo Molnar wrote:
> actually, the pure marker overhead seems to be a regression:
> 
> Kernel Options       Min val    Avg val    Max val    Std Dev
>> - markers - bt cfg  15.349127  16.169459  16.372980   0.184417
>> + markers - bt cfg  15.280382  16.202398  16.409257   0.191861
> 
> why isnt the marker near zero-cost as it should be? (as long as they are 
> enabled but are not in actual use) 2% increase is _ALOT_.

The increase in the mean is actually 0.033, or 0.2%.

> So there's something wrong going on - either markers have unacceptably 
> high cost, or the measurement is not valid.

The third option is that the measurement just needs to be done more 
times. The standard error in the mean for the + markers case is
0.191861 / sqrt(10) = 0.061, which is twice the size of the difference 
being measured.
-- 
Joshua Root,  jmr AT gelato.unsw.edu.au
http://www.gelato.unsw.edu.au

Re: Linux Kernel Markers - performance characterization with large IO load on large-ish system

[Mathieu Desnoyers]

* Ingo Molnar (mingo@elte.hu) wrote:
> 
> * Alan D. Brunelle <Alan.Brunelle@hp.com> wrote:
> 
> >  o  All kernels start off with Linux 2.6.23-rc6 + 2.6.23-rc6-mm1
> > 
> >  o  '- bt cfg' or '+ bt cfg' means a kernel without or with blktrace 
> > configured respectively.
> > 
> >  o  '- markers' or '+ markers' means a kernel without or with the 
> > 11-patch marker series respectively.
> > 
> > 38 runs without blk traces being captured (dropped hi/lo value from 40 runs)
> > 
> > Kernel Options       Min val    Avg val    Max val    Std Dev
> > ------------------  ---------  ---------  ---------  ---------
> > - markers - bt cfg  15.349127  16.169459  16.372980   0.184417
> > + markers - bt cfg  15.280382  16.202398  16.409257   0.191861
> > 
> > - markers + bt cfg  14.464366  14.754347  16.052306   0.463665
> > + markers + bt cfg  14.421765  14.644406  15.690871   0.233885
> 
> actually, the pure marker overhead seems to be a regression:
> 
> > - markers - bt cfg  15.349127  16.169459  16.372980   0.184417
> > + markers - bt cfg  15.280382  16.202398  16.409257   0.191861
> 
> why isnt the marker near zero-cost as it should be? (as long as they are 
> enabled but are not in actual use) 2% increase is _ALOT_. That's the 
> whole point of good probes: they do not slow down the normal kernel.
> 
> _Worst case_ it should be at most a few instructions overhead but that 
> does not explain the ~2% wall-clock time regression you measured here.
> 
> So there's something wrong going on - either markers have unacceptably 
> high cost, or the measurement is not valid.
> 
> 	Ingo

Hi Ingo,

Tests were executed in the following conditions:

"Taking Linux 2.6.23-rc6 + 2.6.23-rc6-mm1 as a basis, I took some sample 
runs of the following on both it and after applying Mathieu Desnoyers 
11-patch sequence (19 September 2007).

   * 32-way IA64 + 132GiB + 10 FC adapters + 10 HP MSA 1000s (one 72GiB
     volume per MSA used)"

Even though the 19 Sept. 2007 markers were released with dependency on
immediate values, there are no optimized immediate values currently
available on ia64. Therefore, we add a d-cache hit for every marker
until we merge immediate values and implement the ia64 optimization.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68