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Re: Interesting timing issue I noticed

P: n/a
On 4/15/08, Daniel Fetchinson <fe********@googlemail.comwrote:
Can I then simply ignore the time data then? I do see better performance
obviously the smaller the box is, but I guess my issues is how seriously
to
take all this data. Because I can't claim "performance improvement" if
there
isn't really much of an improvement.

On Tue, Apr 15, 2008 at 11:04 PM, Daniel Fetchinson <
fe********@googlemail.comwrote:
I've written up a stripped down version of the code. I apologize for
the bad
coding; I am in a bit of a hurry.

import random
import sys
import time

sizeX = 320
sizeY = 240
borderX = 20
borderY = 20

# generates a zero matrix
def generate_zero():
matrix = [[0 for y in range(sizeY)] for x in range(sizeX)]
return matrix
# fills zero matrix
def fill_matrix(in_mat):
mat = in_mat
for x in range(sizeX):
for y in range(sizeY):
mat[x][y] = random.randint(1, 100)
return mat
################################################## ####################
# COMPUTES ONLY A PART OF THE ARRAY
def back_diff_one(back_array, fore_array, box):
diff_array = generate_zero()

start = time.time()
for x in range(sizeX):
for y in range(borderY):
diff_array[x][y] = back_array[x][y] - fore_array[x][y]
for y in range((sizeY - borderY), sizeY):
diff_array[x][y] = back_array[x][y] - fore_array[x][y]
for y in range(borderY, (sizeY - borderY)):
for x in range(borderX):
diff_array[x][y] = back_array[x][y] - fore_array[x][y]
for x in range((sizeX - borderX), sizeX):
diff_array[x][y] = back_array[x][y] - fore_array[x][y]

# tracks object
if (len(box) != 0):
for x in range(box[0], box[2]):
for y in range(box[1], box[3]):
diff_array[x][y] = back_array[x][y] - fore_array[x][y]
print "time one inside = " + str(time.time() - start)
return diff_array
################################################## ####################
# COMPUTES EVERY ELEMENT IN THE ARRAY
def back_diff_two(back_array, fore_array):
diff_array = generate_zero()
start = time.time()
for y in range(sizeY):
for x in range(sizeX):
diff_array[x][y] = back_array[x][y] - fore_array[x][y]
end = time.time()
print "time two inside = " + str(end - start)
return diff_array
################################################## ####################
# CODE TO TEST BOTH FUNCTIONS
back = fill_matrix(generate_zero())
fore = fill_matrix(generate_zero())
box = [20, 20, 268, 240]
start1 = time.time()
diff1 = back_diff_one(back, fore, box)
print "time one outside = " + str(time.time() - start1)
start2 = time.time()
diff2 = back_diff_two(back, fore)
print "time one outside = " + str(time.time() - start2)

Here are some results from several test runs:

time one inside = 0.0780000686646
time one outside = 0.125
time two inside = 0.0780000686646
time two outside = 0.141000032425
>>================================ RESTART
================================
>>>
time one inside = 0.0629999637604
time one outside = 0.125
time two inside = 0.0789999961853
time two outside = 0.125
>>================================ RESTART
================================
>>>
time one inside = 0.0620000362396
time one outside = 0.139999866486
time two inside = 0.0780000686646
time two outside = 0.125
>>================================ RESTART
================================
>>>
time one inside = 0.0780000686646
time one outside = 0.172000169754
time two inside = 0.0789999961853
time two outside = 0.125
>>================================ RESTART
================================
>>>
time one inside = 0.0780000686646
time one outside = 0.125
time two inside = 0.0780000686646
time two outside = 0.125
>>================================ RESTART
================================
>>>
time one inside = 0.0620000362396
time one outside = 0.155999898911
time two inside = 0.0780000686646
time two outside = 0.125
>>================================ RESTART
================================
>>>
time one inside = 0.077999830246
time one outside = 0.125
time two inside = 0.077999830246
time two outside = 0.125
>>================================ RESTART
================================
>>>
time one inside = 0.0780000686646
time one outside = 0.171000003815
time two inside = 0.077999830246
time two outside = 0.125
>>================================ RESTART
================================
>>>
time one inside = 0.0629999637604
time one outside = 0.18799996376
time two inside = 0.0620000362396
time two outside = 0.125

Why is a large percentage of the time, the execution time for the
(ostensibly smaller) first loop is actually equal to or LARGER than
the
second?
>
>
First of all, your method of timing is not the best. Use the timeit
module instead: http://docs.python.org/lib/module-timeit.html
>
Second of all the number of subtractions is not that different between
the two variants of your functions. back_diff_one does 75360
subtractions per call while back_diff_two does 76800, these two
numbers are almost the same. It's true that back_diff_one first only
calculates a part of the arrays but after "# tracks object" you do a
bunch of more substractions that will make up the total count.
>
HTH,
Daniel
>

Please keep the discussion on the list.

Yes, if I were you I would discard your original timing data and redo
it using the timeit module. Whatever that gives should be reliable and
you can start from there. In any case your two functions are doing
roughly the same number of operations.

HTH,
Daniel

BTW, using the following

################################################## ####################
# CODE TO TEST BOTH FUNCTIONS
back = fill_matrix(generate_zero())
fore = fill_matrix(generate_zero())
box = [20, 20, 268, 240]

def test1( ):
diff1 = back_diff_one(back, fore, box)

def test2( ):
diff2 = back_diff_two(back, fore)

if __name__=='__main__':
from timeit import Timer
t = Timer("test1( )", "from __main__ import test1")
print t.timeit( 50 )
t = Timer("test2( )", "from __main__ import test2")
print t.timeit( 50 )
and removing all your timing code from the two functions gives

1.63772082329
1.82889485359

which is consistent with your expectation that the version that
computes only a part of the image runs faster. But only marginally,
which is again consistent with the fact that the number of operations
is only marginally different.

HTH,
Daniel
Jun 27 '08 #1
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