Re: [PATCH] x86: Run checksumming in parallel accross multiple alu's

[Ingo Molnar <mingo@kernel.org>]

* Neil Horman <nhorman@tuxdriver.com> wrote:

> On Mon, Oct 21, 2013 at 12:44:05PM -0700, Eric Dumazet wrote:
> > On Mon, 2013-10-21 at 15:21 -0400, Neil Horman wrote:
> > 
> > > 
> > > Ok, so I ran the above code on a single cpu using taskset, and set irq affinity
> > > such that no interrupts (save for local ones), would occur on that cpu.  Note
> > > that I had to convert csum_partial_opt to csum_partial, as the _opt variant
> > > doesn't exist in my tree, nor do I see it in any upstream tree or in the history
> > > anywhere.
> > 
> > This csum_partial_opt() was a private implementation of csum_partial()
> > so that I could load the module without rebooting the kernel ;)
> > 
> > > 
> > > base results:
> > > 53569916
> > > 43506025
> > > 43476542
> > > 44048436
> > > 45048042
> > > 48550429
> > > 53925556
> > > 53927374
> > > 53489708
> > > 53003915
> > > 
> > > AVG = 492 ns
> > > 
> > > prefetching only:
> > > 53279213
> > > 45518140
> > > 49585388
> > > 53176179
> > > 44071822
> > > 43588822
> > > 44086546
> > > 47507065
> > > 53646812
> > > 54469118
> > > 
> > > AVG = 488 ns
> > > 
> > > 
> > > parallel alu's only:
> > > 46226844
> > > 44458101
> > > 46803498
> > > 45060002
> > > 46187624
> > > 37542946
> > > 45632866
> > > 46275249
> > > 45031141
> > > 46281204
> > > 
> > > AVG = 449 ns
> > > 
> > > 
> > > both optimizations:
> > > 45708837
> > > 45631124
> > > 45697135
> > > 45647011
> > > 45036679
> > > 39418544
> > > 44481577
> > > 46820868
> > > 44496471
> > > 35523928
> > > 
> > > AVG = 438 ns
> > > 
> > > 
> > > We continue to see a small savings in execution time with prefetching (4 ns, or
> > > about 0.8%), a better savings with parallel alu execution (43 ns, or 8.7%), and
> > > the best savings with both optimizations (54 ns, or 10.9%).  
> > > 
> > > These results, while they've changed as we've modified the test case slightly
> > > have remained consistent in their sppedup ordinality.  Prefetching helps, but
> > > not as much as using multiple alu's, and neither is as good as doing both
> > > together.
> > > 
> > > Unless you see something else that I'm doing wrong here.  It seems like a win to
> > > do both.
> > > 
> > 
> > Well, I only said (or maybe I forgot), that on my machines, I got no
> > improvements at all with the multiple alu or the prefetch. (I tried
> > different strides)
> > 
> > Only noises in the results.
> > 
> I thought you previously said that running netperf gave you a stastically
> significant performance boost when you added prefetching:
> http://marc.info/?l=linux-kernel&m=138178914124863&w=2
> 
> But perhaps I missed a note somewhere.
> 
> > It seems it depends on cpus and/or multiple factors.
> > 
> > Last machine I used for the tests had :
> > 
> > processor	: 23
> > vendor_id	: GenuineIntel
> > cpu family	: 6
> > model		: 44
> > model name	: Intel(R) Xeon(R) CPU           X5660  @ 2.80GHz
> > stepping	: 2
> > microcode	: 0x13
> > cpu MHz		: 2800.256
> > cache size	: 12288 KB
> > physical id	: 1
> > siblings	: 12
> > core id		: 10
> > cpu cores	: 6
> > 
> > 
> > 
> > 
> 
> Thats about what I'm running with:
> processor       : 0
> vendor_id       : GenuineIntel
> cpu family      : 6
> model           : 44
> model name      : Intel(R) Xeon(R) CPU           E5620  @ 2.40GHz
> stepping        : 2
> microcode       : 0x13
> cpu MHz         : 1600.000
> cache size      : 12288 KB
> physical id     : 0
> siblings        : 8
> core id         : 0
> cpu cores       : 4
> 
> 
> I can't imagine what would cause the discrepancy in our results (a 
> 10% savings in execution time seems significant to me). My only 
> thought would be that possibly the alu's on your cpu are faster 
> than mine, and reduce the speedup obtained by preforming operation 
> in parallel, though I can't imagine thats the case with these 
> processors being so closely matched.

You keep ignoring my request to calculate and account for noise of 
the measurement.

For example you are talking about a 0.8% prefetch effect while the 
noise in the results is obviously much larger than that, with a 
min/max distance of around 5%:

> > > 43476542
> > > 53927374

so the noise of 10 measurements would be around 5-10%. (back of the 
envelope calculation)

So you might be right in the end, but the posted data does not 
support your claims, statistically.

It's your responsibility to come up with convincing measurements and 
results, not of those who review your work.

Thanks,

	Ingo