Few questions

Josiah Carlson jcarlson at uci.edu
Mon Nov 1 18:36:30 CET 2004


bearophileHUGS at lycos.com (bearophile) wrote:
> 
> Hello, I have few more things to say/ask (left from a discussion in
> another Python Newsgroup).
> 
> Is it possibile (and useful) to write few small sub-sections of the
> Python interpreter in Assembly for Pentium (III/IV)/AMD, to speed up
> the interpreter for Win/Linux boxes running on those CPUs? (Such parts
> don't replace the C versions, kept for compatibilty).
> I think the HLA (High Level Assembly) language can be fit for this
> purpose, it's a cute language:
> http://webster.cs.ucr.edu/AsmTools/HLA/index.html

It may or may not be useful or faster to implement portions of the
Python interpreter in assembly.  Generally assembly has performance
benefits and penalties per processor that are hard to understand.

I would be willing to wager that time would be better spent checking out
the different compile-time options for the interpreter, as C optimizes
fairly well.



> I've done a little comparison of the speed of Python lists and arrays:
[snip code]
> On a old Win2K PC it gives:
> C:\py>speed_test 1
> List test, n = 3000000:
> Timing: 2.804 s
> 
> C:\py>speed_test 2
> Array test, n = 3000000:
> Timing: 3.521 s
> 
> 
> Python lists are arrays of pointers to objects, I think (a test shows
> that here they use about 16 bytes for every number).
> And the Python Arrays are packed: every number here uses 4 bytes.
> Why do lists are faster here?

Crucial observation:
Lists are indeed arrays of pointers that point to 'int objects'.
Arrays (of integers) are arrays of actual stored x-bit integers (where x
is 32 in this case).

In order to write to an array the value of a standard Python integer,
one must look into the 16 byte Python integer to copy the proper 4 bytes
into the array, do bounds checking, etc.

In order to write to a list the value of a standard Python integer, a
pointer copy is sufficient.


Lists win because it is a pointer copy as opposed to an struct lookup
and value copy with bounds checking.


> Memory cleaning: in the last script I've added some calls to a Win
> version of the small "pslist" program, and I've put a "del v" command
> after the timings. And I've seen:
> 
> C:\py>speed_test 1
> List test, n = 3000000:
> 1) Process size: 1408 KB.
> 2) Process size: 48928 KB.
> 3) Process size: 37204 KB.
> 
> C:\py>speed_test 2
> Array test, n = 3000000:
> 1) Process size: 1416 KB.
> 2) Process size: 13152 KB.
> 3) Process size: 1416 KB.
> 
> 1 is at the start of the script before v creation, 2 is after its
> inizialization loop, and 3 is after the "del v" command, like this:
> 
> used_mem(1)
> from array import array
> v = array("l", [0] * n)
> for i in xrange(len(v)):
> 		v[i] = i
> used_mem(2)
> del v
> used_mem(3)
> 
> The garbage collector removes at once the array (this is easy, it's
> just a lump of memory with little extra things), but the memory used
> by the list isn't free even a little time later. (I think that to
> understand how/when such such garbage collector works, I have to read
> the Python C sources...)


Python arrays (from the array module) are allocated as a block, and the
values of integers are stored within.  Because everything is all nice
and contained, it can be easily freed (just like C arrays).

With Python lists, certainly the pointers to the integer objects are
easily allocated and freed, and the integer objects themselves sit in
the integer free-list.

Now, obviously a bunch of those entries aren't being used after one
deletes the big list of integer, so why isn't it being freed?  Due to
the semantics of the free list (you can't reorganize the integers on the
free list, etc.), it cannot be reduced in size.

 - Josiah




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