Mark Dickinson wrote:
> I have a simple 192-line Python script that begins with the line:
>
> dummy0 = 47
>
> The script runs in less than 2.5 seconds. The variable dummy0 is never
> referenced again, directly or indirectly, by the rest of the script.
>
> Here's the surprise: if I remove or comment out this first line, the
> script takes more than 15 seconds to run. So it appears that adding a
> redundant line produces a spectacular six-fold increase in speed!
Thanks for your post. It is cute, brilliant, and interesting.
I haven't had time to investigate but can point at the likely cause.
All of the global names are stored in a single hash table. Search time
is dictated by two factors, the sparseness of the hash table and the
distribution of hash values. With respect to sparseness, whenever that
table becomes 2/3 full, it grows by a factor of four and becomes only
1/6 full (leading to many fewer collisions). With respect to
distribution, it should be noted that string hash values are decidely
non-random and your variable names likely congested consecutive spaces
in a nearly full table (resulting in seven times as many search probes
to find a global value).
When the extra value was added, it likely resized the table four-fold
and redistributed the hash values into fewer consecutive positions.
Raymond
P.S. To analyze it further, start with something like this:
>>> len(set(hash('dummy%d' %i) & 31 for i in xrange(29)))
26
>>> len(set(hash('dummy%d' %i) & 127 for i in xrange(29)))
29
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