[EMAIL PROTECTED] wrote:
> i = 0
> while (i < 20):
> i = i + 1
for i in xrange(20):
> (shellIn, shellOut) = os.popen4("/bin/sh -c ':'") # for testing, the
> spawned shell does nothing
> print 'next'
> # for line in shellOut:
> # print line
>
> On my system (AIX 5.1 if it matters, with Python 2.4.3), this simple
> loop spawning 20 subshells takes .75 sec. Ok, that's reasonable. Now,
> if I uncomment the two commented lines, which loop over the empty
> shellOut array, the progam now takes 11 secs. That slowdown seems
> very hard to believe. Why should it slow down so much?
The key fact here is that shellOut isn't an array; it's a living,
breathing file object. If you don't iterate over it, you can run all 20
shell processes in parallel if necessary; but if you do iterate over it,
you're waiting for sh's stdout pipe to reach EOF, which effectively
means you can only run one process at a time.
On my system (OS X 10.4 with Python 2.5 installed), your code runs in
.187 secs with the loop commented out, and in .268 secs otherwise. But I
guess AIX's sh is slower than OS X's.
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