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

From: Alan D. Brunelle
Date: Wed Sep 26 2007 - 11:28:45 EST

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,


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

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