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Alex Martelli aleax at aleax.it
Fri Oct 3 13:22:31 EDT 2003 
Michael Hudson wrote:

> Alex Martelli <aleax at aleax.it> writes:
> 
>> So -- it's clear to me that I do NOT understand what's going on
>> here.  If just the ags[:] is 35.2 usec, how can the ags[:].pop(0)
>> be 24.5 ... ???
> 
> How quiet was your machine when you ran the tests?  I see behaviour

Very, and the numbers were highly repeatable:

-- running just now:

[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)' 'x=ags[:]'
10000 loops, best of 3: 36.9 usec per loop
[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)' 'x=ags[:]'
10000 loops, best of 3: 35.2 usec per loop
[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)' 'x=ags[:]'
10000 loops, best of 3: 35.8 usec per loop
[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)' 'x=ags[:]'
10000 loops, best of 3: 35.6 usec per loop

-- and from a copy & paste of the screen as of a while ago:

[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)'
'x=ags[:].pop(0)'
10000 loops, best of 3: 24.5 usec per loop
[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)'
'x=ags[:].pop(0)'
10000 loops, best of 3: 24.5 usec per loop

-- _however_ -- retrying the latter now:

[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)'
'x=ags[:].pop(0)'
10000 loops, best of 3: 48.8 usec per loop
[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)'
'x=ags[:].pop(0)'
10000 loops, best of 3: 46.1 usec per loop
[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)'
'x=ags[:].pop(0)'
10000 loops, best of 3: 46.5 usec per loop
[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)'
'x=ags[:].pop(0)'
10000 loops, best of 3: 24.5 usec per loop
[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)'
'x=ags[:].pop(0)'
10000 loops, best of 3: 24.4 usec per loop
[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)'
'x=ags[:].pop(0)'
10000 loops, best of 3: 24.5 usec per loop

-- _SO_ -- immediately going back to the just-copy tests...:

[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)' 'x=ags[:]'
100000 loops, best of 3: 19.3 usec per loop
[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)' 'x=ags[:]'
100000 loops, best of 3: 19.3 usec per loop
[alex at lancelot python2.3]$ python timeit.py -s'ags=range(1000)' 'x=ags[:]'
100000 loops, best of 3: 19.3 usec per loop


SO -- so much for the "quiet"... clearly there _IS_ something periodically
running and distorting the elapsed-time measurements (which are the default
here on Linux) by a hefty factor of 2.  So much for this kind of casual
benchmarking...!-)  I guess I've been doing a little too much of it...


> more in line with what you'd expect:
> 
> [mwh at pc150 build]$ ./python ../Lib/timeit.py -s'ags=range(1000)'
> ['x=ags[:]'
> 10000 loops, best of 3: 31.6 usec per loop
> [mwh at pc150 build]$ ./python ../Lib/timeit.py -s'ags=range(1000)'
> ['x=ags[:].pop(0)'
> 10000 loops, best of 3: 39.8 usec per loop

Yep, makes more sense.  So, moving to the more-stable -c (CPU time,
as given by time.clock):

[alex at lancelot python2.3]$ python timeit.py -c -s'ags=range(1000)'
'x=ags[:]'
100000 loops, best of 3: 19.2 usec per loop
[alex at lancelot python2.3]$ python timeit.py -c -s'ags=range(1000)'
'x=ags[:]'
100000 loops, best of 3: 19.2 usec per loop
[alex at lancelot python2.3]$ python timeit.py -c -s'ags=range(1000)'
'x=ags[:]'
100000 loops, best of 3: 19.2 usec per loop
[alex at lancelot python2.3]$ python timeit.py -c -s'ags=range(1000)'
'x=ags[:].pop(0)'
10000 loops, best of 3: 24 usec per loop
[alex at lancelot python2.3]$ python timeit.py -c -s'ags=range(1000)'
'x=ags[:].pop(0)'
10000 loops, best of 3: 24 usec per loop
[alex at lancelot python2.3]$ python timeit.py -c -s'ags=range(1000)'
'x=ags[:].pop(0)'
10000 loops, best of 3: 24 usec per loop
[alex at lancelot python2.3]$ python timeit.py -c -s'ags=range(1000)'
'x=ags[:].pop(0)'
10000 loops, best of 3: 23 usec per loop
[alex at lancelot python2.3]$ python timeit.py -c -s'ags=range(1000)'
'ags.reverse(); x=ags[:].pop()'
10000 loops, best of 3: 23 usec per loop
[alex at lancelot python2.3]$ python timeit.py -c -s'ags=range(1000)'
'ags.reverse(); x=ags[:].pop()'
10000 loops, best of 3: 22 usec per loop
[alex at lancelot python2.3]$ python timeit.py -c -s'ags=range(1000)'
'ags.reverse(); x=ags[:].pop()'
10000 loops, best of 3: 23 usec per loop
[alex at lancelot python2.3]$ python timeit.py -c -s'ags=range(1000)'
'ags.reverse(); x=ags[:].pop()'
10000 loops, best of 3: 23 usec per loop

it WOULD seem to be confirmed that reverse-then-pop() is VERY
slightly faster than pop(0) -- 22/23 instead of 23/24 usec for
a 1000-long list... of which 19.2 are the apparently-repeatable
overhead of copying that list.  I still wouldn't say that I
"KNOW" this, though -- the margin is SO tiny and uncertain...!!!

It seems to scale up linearly going from 1000 to 5000: just
the copy, 105-108; ags[:].pop(0), 120-123; reverse then pop,
116-117.  This is always with the -c (once burned...).


Alex
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