[Numpy-discussion] Different results from repeated calculation, part 2

Bruce Southey bsouthey@gmail....
Fri Aug 15 08:59:50 CDT 2008


Keith Goodman wrote:
> On Thu, Aug 14, 2008 at 11:29 AM, Bruce Southey <bsouthey@gmail.com> wrote:
>   
>> Keith Goodman wrote:
>>     
>>> I get slightly different results when I repeat a calculation.
>>>
>>> I've seen this problem before (it went away but has returned):
>>>
>>> http://projects.scipy.org/pipermail/numpy-discussion/2007-January/025724.html
>>>
>>> A unit test is attached. It contains three tests:
>>>
>>> In test1, I construct matrices x and y and then repeatedly calculate z
>>> = calc(x,y). The result z is the same every time. So this test passes.
>>>
>>> In test2, I construct matrices x and y each time before calculating z
>>> = calc(x,y). Sometimes z is slightly different. But the x's test to be
>>> equal and so do the y's. This test fails (on Debian Lenny, Core 2 Duo,
>>> with libatlas3gf-sse2 but not with libatlas3gf-sse).
>>>
>>> test3 is the same as test2 but I calculate z like this: z =
>>> calc(100*x,y) / (100 * 100). This test passes.
>>>
>>> I get:
>>>
>>> ======================================================================
>>> FAIL: repeatability #2
>>> ----------------------------------------------------------------------
>>> Traceback (most recent call last):
>>>   File "/home/[snip]/test/repeat_test.py", line 73, in test_repeat_2
>>>     self.assert_(result, msg)
>>> AssertionError: Max difference = 2.04946e-16
>>>
>>> ----------------------------------------------------------------------
>>>
>>> Should a unit test like this be added to numpy?
>>>
>>> ------------------------------------------------------------------------
>>>
>>> _______________________________________________
>>> Numpy-discussion mailing list
>>> Numpy-discussion@scipy.org
>>> http://projects.scipy.org/mailman/listinfo/numpy-discussion
>>>       
>> Hi,
>> In the function 'test_repeat_2' you are redefining variables 'x and y'
>> that were first defined using the setup function. (Also, you are not
>> using the __init__ function.) I vaguely recall there are some quirks to
>> Python classes with this, so does the problem go away with if you use
>> 'a,b' instead of 'x, y'? (I suspect the answer is yes given test_repeat_3).
>>
>> Note that you should also test that 'x' and 'y' are same here as well
>> (but these have been redefined...).
>>
>> Otherwise, can you please provide your OS (version), computer processor,
>> Python version, numpy version, version of atlas (or similar) and
>> compiler used?
>>
>>  I went back and reread the thread but I could not see this information.
>>     
>
> Here's a test that doesn't use classes and checks that x and y do not change:
>
> http://projects.scipy.org/pipermail/numpy-discussion/attachments/20070127/52b3a51c/attachment.py
>
> I'm using binaries from Debian Lenny:
>
> $ uname -a
> Linux jan 2.6.25-2-686 #1 SMP Fri Jul 18 17:46:56 UTC 2008 i686 GNU/Linux
>
> $ python -V
> Python 2.5.2
>
>   
>>> numpy.__version__
>>>       
>    '1.1.0'
>
> $ cat /proc/cpuinfo
> processor	: 0
> vendor_id	: GenuineIntel
> cpu family	: 6
> model		: 15
> model name	: Intel(R) Core(TM)2 CPU          6600  @ 2.40GHz
> stepping	: 6
> cpu MHz		: 2402.004
> cache size	: 4096 KB
> physical id	: 0
> siblings	: 2
> core id		: 0
> cpu cores	: 2
> fdiv_bug	: no
> hlt_bug		: no
> f00f_bug	: no
> coma_bug	: no
> fpu		: yes
> fpu_exception	: yes
> cpuid level	: 10
> wp		: yes
> flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov
> pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm
> constant_tsc arch_perfmon pebs bts pni monitor ds_cpl vmx est tm2
> ssse3 cx16 xtpr lahf_lm
> bogomips	: 4807.45
> clflush size	: 64
>
> processor	: 1
> vendor_id	: GenuineIntel
> cpu family	: 6
> model		: 15
> model name	: Intel(R) Core(TM)2 CPU          6600  @ 2.40GHz
> stepping	: 6
> cpu MHz		: 2402.004
> cache size	: 4096 KB
> physical id	: 0
> siblings	: 2
> core id		: 1
> cpu cores	: 2
> fdiv_bug	: no
> hlt_bug		: no
> f00f_bug	: no
> coma_bug	: no
> fpu		: yes
> fpu_exception	: yes
> cpuid level	: 10
> wp		: yes
> flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov
> pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm
> constant_tsc arch_perfmon pebs bts pni monitor ds_cpl vmx est tm2
> ssse3 cx16 xtpr lahf_lm
> bogomips	: 4750.69
> clflush size	: 64
> _______________________________________________
> Numpy-discussion mailing list
> Numpy-discussion@scipy.org
> http://projects.scipy.org/mailman/listinfo/numpy-discussion
>
>   
I do not get this on my Intel Quad core2 Linux x64 system with a x86_64 
running Fedora 10 supplied Python. I do compile my own versions of NumPy 
and currently don't use or really plan to use altas. But I know that you 
previously indicated that this was atlas related 
(http://projects.scipy.org/pipermail/numpy-discussion/2007-January/025750.html) 
.

 From Intel's website, the Intel Core2 Duo E6600 
(http://processorfinder.intel.com/details.aspx?sSpec=SL9S8) supports EM64T
so it is x86 64-bit processor. I do not know Debian but i686 generally 
refers to 32-bit kernel as x86_64 refers to 64-bit. If so, then you are 
running a 32-bits on 64-bit processor. So I would suggest you start by 
compiling your own NumPy without any extras and see if it goes away. If 
not, then it is NumPy otherwise add the extras until you get the same 
system back.

Bruce


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