[SciPy-User] scipy.linalg.solve()'s overwrite option does not work

braingateway braingateway at gmail.com
Sat Nov 6 18:28:50 EDT 2010 

josef.pktd at gmail.com :
> On Sat, Nov 6, 2010 at 6:18 PM, braingateway <braingateway at gmail.com> wrote:
>   
>> josef.pktd at gmail.com :
>>     
>>> On Sat, Nov 6, 2010 at 5:46 PM, braingateway <braingateway at gmail.com> wrote:
>>>
>>>       
>>>> Joe Kington :
>>>>
>>>>         
>>>>> On Sat, Nov 6, 2010 at 12:13 PM, braingateway <braingateway at gmail.com
>>>>> <mailto:braingateway at gmail.com>> wrote:
>>>>>
>>>>>     David Warde-Farley:
>>>>>     > On 2010-11-05, at 9:21 PM, braingateway wrote:
>>>>>     >
>>>>>     >
>>>>>     >> Hi everyone,
>>>>>     >> I believe the overwrite option is used for reduce memory usage.
>>>>>     But I
>>>>>     >> did following test, and find out it does not work at all. Maybe I
>>>>>     >> misunderstood the purpose of overwrite option. If anybody could
>>>>>     explain
>>>>>     >> this, I shall highly appreciate your help.
>>>>>     >>
>>>>>     >
>>>>>     > First of all, this is a SciPy issue, so please don't crosspost
>>>>>     to NumPy-discussion.
>>>>>     >
>>>>>     >
>>>>>     >>>>> a=npy.random.randn(20,20)
>>>>>     >>>>> x=npy.random.randn(20,4)
>>>>>     >>>>> a=npy.matrix(a)
>>>>>     >>>>> x=npy.matrix(x)
>>>>>     >>>>> b=a*x
>>>>>     >>>>> import scipy.linalg as sla
>>>>>     >>>>> a0=npy.matrix(a)
>>>>>     >>>>> a is a0
>>>>>     >>>>>
>>>>>     >> False
>>>>>     >>
>>>>>     >>>>> b0=npy.matrix(b)
>>>>>     >>>>> b is b0
>>>>>     >>>>>
>>>>>     >> False
>>>>>     >>
>>>>>     >
>>>>>     > You shouldn't use 'is' to compare arrays unless you mean to
>>>>>     compare them by object identity. Use all(b == b0) to compare by value.
>>>>>     >
>>>>>     > David
>>>>>     >
>>>>>     >
>>>>>     Thanks for reply, but I have to say u did not understand my post
>>>>>     at all.
>>>>>     I did this 'is' comparison on purpose, because I wanna know if the
>>>>>     overwrite flag is work or not.
>>>>>     See following example:
>>>>>      >>> a=numpy.matrix([0,0,1])
>>>>>      >>> a
>>>>>     matrix([[0, 0, 1]])
>>>>>      >>> a0=a
>>>>>      >>> a0 is a
>>>>>     True
>>>>>
>>>>>
>>>>> Just because two ndarray objects aren't the same doesn't mean that
>>>>> they don't share the same memory...
>>>>>
>>>>> Consider this:
>>>>> import numpy as np
>>>>> x = np.arange(10)
>>>>> y = x.T
>>>>> x is y # --> Yields False
>>>>> Nonetheless, x and y share the same data, and storing y doesn't double
>>>>> the amount of memory used, as it's effectively just a pointer to the
>>>>> same memory as x
>>>>>
>>>>> Instead of using "is", you should use "numpy.may_share_memory(x, y)"
>>>>>
>>>>>           
>>>> Thanks a lot for pointing this out! I were struggling to figure out
>>>> whether the different objects share memory or not. And good to know
>>>> a0=numpy.matrix(a) actually did not share the memory.
>>>>  >>> print 'a0 shares memory with a?', npy.may_share_memory(a,a0)
>>>> a0 shares memory with a? False
>>>>  >>> print 'b0 shares memory with b?', npy.may_share_memory(b,b0)
>>>> b0 shares memory with b? False
>>>> I also heard that even may_share_memory is 'True', does not necessarily
>>>> mean they share any element. Maybe, is 'a0.base is a' usually more
>>>> suitable for this purpose?
>>>>
>>>> Back to the original question: is there anyone actually saw the
>>>> overwrite_a or overwrite_b really showed its effect?
>>>> If you could show me a repeatable example, not only for
>>>> scipy.linalg.solve(), it can also be other functions, who provide this
>>>> option, such as eig(). If it does not show any advantage in memory
>>>> usage, I might still using numpy.linalg.
>>>>
>>>>         
>>> import numpy as np
>>>
>>> a=np.random.randn(20,20)
>>> abak = a.copy()
>>> x=np.random.randn(20,4)
>>> xbak = x.copy()
>>> a=np.matrix(a)
>>> x=np.matrix(x)
>>> b=a*x
>>> b = np.array(b)
>>> bbak = b.copy()
>>>
>>> import scipy.linalg as sla
>>> a0=np.matrix(a)
>>> print a is a0
>>> #False
>>> b0=np.matrix(b)
>>> print b is b0
>>> #False
>>> X=sla.solve(a,b,overwrite_a=True,debug=True)
>>> print X is b
>>> #False
>>> print (X==b).all()
>>> #False
>>> print 'a:', (a0==a).all(), (abak==a).all()
>>> print 'b:', (b0==b).all(), (bbak==b).all()
>>> #
>>> Y = sla.solve(a,b,overwrite_a=True,overwrite_b=True,debug=True)
>>> print 'a:', (a0==a).all(), (abak==a).all()
>>> print 'b:', (b0==b).all(), (bbak==b).all()
>>>
>>> print (X==Y).all()
>>>
>>> printout
>>> -----------
>>> False
>>> False
>>> solve:overwrite_a= True
>>> solve:overwrite_b= False
>>> False
>>> False
>>> a: False False
>>> b: True True
>>> solve:overwrite_a= True
>>> solve:overwrite_b= True
>>> a: False False
>>> b: True True
>>> False
>>>
>>>
>>>       
>> Thanks a lot! So right now I see it might be some bugs in my Scipy
>> version!, After running your code, I got following result:
>>  >>> sla.__version__
>> '0.4.9'
>>     
>
> mine:
>   
>>>> sla.__version__
>>>>         
> '0.4.9'
>
> I have no idea if the overwrite option depends on which Lapack/Blas
> implementation is used. I have a generic oldish ATLAS.
>
> Josef
>   
Hummm..., Possible, I am using MKL. Hope there is someone using MKL, do 
this test also.

LittleBigBrain
>
>   
>>  >>>
>> False
>> False
>> solve:overwrite_a= True
>> solve:overwrite_b= False
>> False
>> False
>> a: True True
>> b: True True
>> solve:overwrite_a= True
>> solve:overwrite_b= True
>> a: True True
>> b: True True
>> True
>>     
>>> The first solve overwrites a, the second solve solves a different
>>> problem and the solutions X and Y are not the same.
>>> (if the first solve allows overwriting of be instead, then X==Y )
>>>
>>> I never got a case with overwritten b, and as you said there was no
>>> sharing memory with the original array, the copy has always the same
>>> result as your original.
>>>
>>> This was for quick playing with your example, no guarantee on no mistakes.
>>>
>>> Josef
>>>
>>>
>>>
>>>       
>>>>>     This means a0 and a is actually point to a same object. Then a0 act
>>>>>     similar to the C pointer of a.
>>>>>     I compared a0/b0 and a/b by 'is' first to show I did create a new
>>>>>     object
>>>>>     from the original matrix, so the following (a0==a).all()
>>>>>     comparison can
>>>>>     actually prove the values inside the a and b were not overwritten.
>>>>>
>>>>>     Sincerely,
>>>>>     LittleBigBrain
>>>>>     > _______________________________________________
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>>>>>     >
>>>>>
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