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

braingateway braingateway@gmail....
Sat Nov 6 17:33:30 CDT 2010


Joe Kington :
>
>
> On Sat, Nov 6, 2010 at 4:46 PM, braingateway <braingateway@gmail.com 
> <mailto:braingateway@gmail.com>> wrote:
>
>     Joe Kington :
>     >
>     > On Sat, Nov 6, 2010 at 12:13 PM, braingateway
>     <braingateway@gmail.com <mailto:braingateway@gmail.com>
>     > <mailto:braingateway@gmail.com <mailto:braingateway@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?
>
>
> Not to take this on too much of a tangent, but since you asked:
>
> "x.base is y.base" usually doesn't work, even when x.base and y.base 
> point to the same memory...
>
> Again, consider the transpose of an array combined with a bit of indexing:
> import numpy as np
> x = np.arange(10)
> y = x[:10].T
> x.base is y.base # <-- yields False
> y.base is x # <-- yields False
> np.may_share_memory(x,y) # <-- correctly yields True
>
> The moral of this story is don't use object identity of any sort to 
> determine if ndarrays share memory.
>
>     I also heard that even may_share_memory is 'True', does not
>     necessarily
>     mean they share any element.
>
>  
> The reason why "may_share_memory" is not a guarantee that the arrays 
> actually do is due to situations like this:
> import numpy as np
> x = np.arange(10)
> a = x[::2]
> b = x[1::2]
> np.may_share_memory(a, b) # <-- yields True
>
> However, we could change every element in "a" without affecting "b", 
> so they don't _actually_ share memory.   The reason why 
> "may_share_memory" yields True is essentially due to the fact that "a" 
> and "b" are both views into overlapping regions of the same array. 
>
> As far as I know, "may_share_memory" is really the only test available 
> without dropping down to C to determine if two ndarray objects share 
> chunks of memory. It just errs on the side of caution, if you will.  
> If it returns True, then the two ndarrays refer to overlapping regions 
> of memory.
>
Oh, this is really clarify all the mystery in my mind. I think I will 
backup this mail :)

Thanks Sincerely,

LittleBigBrain

> Of course, checking to see if the contents of the array are exactly 
> the same is a _completely_ different test, and "(x == 
> y.ravel()).all()" is a fine way to do that.  Just keep in mind that 
> checking if the elements are the same is very different than checking 
> if the arrays share the same memory.  For example:
> x = np.arange(10)
> y = x.copy()
> (x == y).all() # <-- correctly yields True
> np.may_share_memory(x, y) # <-- correctly yields False
> They're two different tests, entirely.  Of course, for what you're 
> doing here, testing to see if the elements are the same is entirely 
> reasonable.
>

>  
>
>
>     Back to the original question: is there anyone actually saw the
>     overwrite_a or overwrite_b really showed its effect?
>
>
> I'm afraid I'm not much help on the original question...
>
>  
>
>     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.
>     >
>     >     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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