On Tue, 2003-09-16 at 22:08, Andrew Morton wrote:
> But I would like to see some evidence that prefetch ever provides any
> performance gain in-kernel.  I spent some time fiddling a while back and
> was unable to demonstrate any difference.

For SDET on the NUMA-Q, it's pretty small.  Looks like well less than
1%.  Mostly lost in the noise, though.

DISCLAIMER: SPEC(tm) and the benchmark name SDET(tm) are registered 
trademarks of the Standard Performance Evaluation Corporation. This 
benchmarking was performed for research purposes only, and the run
results are non-compliant and not-comparable with any published results.

Prefetch on: 
 Scripts  |  Average Throughput |  Standard Deviation
----------+---------------------+---------------------
       16 |      15795.9100     |     711.3221
       32 |      16450.5800     |     292.6028
       64 |      15800.2400     |     126.2358

Prefetch off:
 Scripts  |  Average Throughput |  Standard Deviation
----------+---------------------+---------------------
       16 |      15672.8600     |     438.5506
       32 |      16376.4100     |     364.5610
       64 |      15744.8100     |     208.9217


For those of you who care, I generated this with a neat little tool that
some interns cooked up for us this summer.  You hand it a little file
like this and it throws back a wealth of information back at you. 
There's a central machine to which has a database of other machines and
figures out the best fit for the job (can be more than 1 machine
requested).  In this case it's easy because there are very few 16x
machines in the database.  Oh, and like good interns, they stole a lot
of code from Steve Pratt's "autobench".  

class host num_cpus=16
+$mytest sdet "1 4 16 32 64" -l /tmp/sdet

disuseprofiler sar

# build the kernel with prefaulting off
build stock 2.6.0-test5 \
    -p http://elm3b114.beaverton.ibm.com/patches/prefetch-off.patch \
    -m -j8
boot
# sdet runs fast on ramfs
fs -mountramfs /tmp/sdet
run "sdet" $mytest
setup umount /tmp/sdet

build stock 2.6.0-test5 -m -j8
boot
fs -mountramfs /tmp/sdet
run "sdet" $mytest
setup umount /tmp/sdet

-- 
Dave Hansen
haveblue@us.ibm.com