Re: [Python-Dev] Conflict between time docs and timeit

[Antoine Pitrou]

On Fri, 18 Mar 2011 17:06:50 +1100
Steven D'Aprano  wrote:
> 
> In contrast, timeit defaults to using time.time() under all operating 
> systems other than Windows, and says:
> 
>  ...on Windows, clock() has microsecond granularity but
>  time()'s granularity is 1/60th of a second; on Unix,
>  clock() has 1/100th of a second granularity and time()
>  is much more precise.
> 
> 
> Should timeit be changed, or the docs, or have I missed something?

Well, time.clock() is less precise (in terms of timing granularity),
but more accurate if the system if not otherwise idle - since it will
report CPU time for the current process instead of total wall clock
time, removing the direct contribution of other processes.

That said, benchmarks on a loaded system are inaccurate anyway, because
of other factors (such as cost of context switching, TLB and cache
misses, concurrent memory access from several processes).

I think a rule-of-thumb can be made:
- for short-running benchmarks (a couple of seconds at most), use
  time.time() which will give increased precision
- for longer-running benchmarks, use time.clock() (or the system's
  "time" command) which will avoid counting the runtime of other
  processes

timeit is in the former case.

(this is all about Unix, by the way)

Regards

Antoine.


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[Python-Dev] Conflict between time docs and timeit

[Steven D'Aprano]

I see that the documentation for the time module has this to say about 
time.clock:


On Unix, return the current processor time as a floating
point number expressed in seconds. The precision, and in fact
the very definition of the meaning of “processor time”,
depends on that of the C function of the same name, but in
any case, this is the function to use for benchmarking Python
or timing algorithms.


In contrast, timeit defaults to using time.time() under all operating 
systems other than Windows, and says:


...on Windows, clock() has microsecond granularity but
time()'s granularity is 1/60th of a second; on Unix,
clock() has 1/100th of a second granularity and time()
is much more precise.


Should timeit be changed, or the docs, or have I missed something?


--
Steven
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