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

[Neil Horman <nhorman@tuxdriver.com>]

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.

Neil