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

[Mathieu Desnoyers]

* Jens Axboe (jens.axboe@xxxxxxxxxx) wrote:
> On Wed, Sep 26 2007, Alan D. Brunelle wrote:
> > Mathieu Desnoyers wrote:
> >> * Alan D. Brunelle (Alan.Brunelle@xxxxxx) 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@xxxxxxxxxx> 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
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