DKF: Sometimes you come across something that makes you think “wow!” Here's one such thing: comparing the flat out single threaded performance of Tcl and Python. The problem I was looking at was to compute the sum of all prime numbers less than ten million (there are quite a few of them!) and the limiting factor is an efficient method for generating all the primes in the range. I present here two implementations for doing this in the languages under consideration, based on code originally from http://code.activestate.com/recipes/117119/ by way of StackOverflow

Tcl

proc sum_primes_to {n {i 1}} {
    set total 0
    incr n 0
    for {set q [expr {$i + $i}]} {$q < $n} {incr q} {
        if {![info exists d($q)]} {
            incr total $q
            lappend d([expr {$q*$q}]) $q
        } else {
            foreach p $d($q) {
                lappend d([expr {$p + $q}]) $p
            }
            unset -nocomplain d($q)
        }
    }
    return $total
}

puts [sum_primes_to 10000000]

Python

def sum_primes_to(n):
    total = 0
    d = {}
    q = 2
    while q < n:
        if q not in d:
            total += q
            d[q * q] = [q]
        else:
            for p in d[q]:
                d.setdefault(p + q, []).append(p)
            del d[q]
        q += 1
    return total

print(sum_primes_to(10000000))

So… comparing the performance (overall for the script, with time) on a single system with production-grade builds of both languages, I get this:

Tcl 8.6: 13.250s
Python 2.7: 20.369s
Python 3.5: 22.204s
Python 3.6: 13.874s

These are all production builds that I've built locally to be as fast as possible on my hardware. (Also, they all produce the correct result, 3203324994356.)

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gonwalf 2018.01.31: I tried the same code on python 3.5 and tcl 8.6.6 and 8.6.8 with different results:

Tcl 8.6.6: 10.07s
Python 3.5: 10,80s
on Intel(R) Core(TM) i5-3470 CPU @ 3.20GHz

and

Tcl 8.6.8: 8,53s
Python 3.5: 8,08s
on Intel(R) Xeon(R) CPU E5-1650 v3 @ 3.50GHz

Which compiler flags did you use for the Tcl build?

DKF: 04-Feb-2018: I used the builds of both Tcl and Python built by macports, all running on my laptop (on mains power). All should be in release as-much-optimisation-as-usually-reasonable mode, and with chunks of computation as large as this, the CPU should be scaled up pretty equally. (NB: The system Tcl build on OSX is actually very slow; they enable an option that adds close tracking of low-level metrics but at great performance overhead.) I've got additional experimental builds where I make the Tcl code quite a lot faster, but they're definitely not used by anyone else yet (and aren't yet correct, semantically).

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