pthreads don't scale on windows xp, but does scale on linux, cygwin 1.5.19

[Tim Prince]

jdeifik wrote:
> I have a dual xeon 2.4ghz machine with hypertreading enabled.
> This gives me 4 logical processors.
> The machine dual boots to windows xp sp2, and linux.
> I have a highly parallelizable program I wrote, and I tested it running 
> 1 to 8 threads,
> running with no source changes on windows and linux.
> 
> Here is the performance on linux using gcc-3.4.3
> threads
> 1    1436.41user 0.10system 7:16.37elapsed 100%CPU (0avgtext+0avgdata 
> 0maxresident)k
> 2    436.00user 0.02system 3:38.15elapsed 199%CPU (0avgtext+0avgdata 
> 0maxresident)k
> 3    369.15user 0.05system 2:03.48elapsed 298%CPU (0avgtext+0avgdata 
> 0maxresident)k
> 4    359.77user 0.08system 1:42.95elapsed 349%CPU (0avgtext+0avgdata 
> 0maxresident)k
> 6    357.83user 0.09system 1:40.94elapsed 354%CPU (0avgtext+0avgdata 
> 0maxresident)k
> 8    358.79user 0.06system 1:41.80elapsed 352%CPU (0avgtext+0avgdata 
> 0maxresident)k
> 
> To compute efficiency, take the single thread elapsed time/(# threads * 
> threaded elapsed time)
> 
> There is virtually perfect scaling. 4 processors scale with an 
> efficiency of about 103%.
> For 6 and 8 threads, efficiency goes up a small amount.
> 
> 
> Here is the performance on windows xp using cygwin pthreads and gcc-3.4.4
> 1    434.60user 0.20system 7:16.47elapsed 99%CPU (0avgtext+0avgdata 
> 509696maxresident)k
> 2    441.78user 0.24system 3:42.06elapsed 199%CPU (0avgtext+0avgdata 
> 510208maxresident)k
> 3    579.68user 0.15system 3:14.50elapsed 298%CPU (0avgtext+0avgdata 
> 511232maxresident)k
> 4    675.39user 0.15system 2:51.50elapsed 393%CPU (0avgtext+0avgdata 
> 512000maxresident)k
> 6    711.70user 0.18system 3:01.20elapsed 392%CPU (0avgtext+0avgdata 
> 511488maxresident)k
> 8    683.35user 0.21system 2:56.05elapsed 388%CPU (0avgtext+0avgdata 
> 512000maxresident)k
> 
> Things are fine for 2 threads, scaling with an efficiency of 96%
> For 3 threads, scaling efficiency is 73%
> For 4 threads, scaling efficiency is 62%
> For 6 threads, scaling efficiency is 39%
> For 8 threads, scaling efficiency is 30%
> 

Windows doesn't have HT aware scheduling, such as recent linux 
schedulers incorporate.  Cygwin doesn't attempt to improve on the 
Windows scheduler.  I won't ask for relevant details about your linux, 
or how you managed to write a program which doesn't deliver close to 
full performance at 2 threads, as that would take this even further Off 
Topic. However, if you are getting good scaling to 2 threads, that 
should enable you to get all the dual processor performance you can 
expect in Windows for practical purposes.  You might try repeating your 
tests with HT disabled in BIOS.

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