[erlang-questions] '--' BIF to be used with caution?
Wed Feb 11 09:58:38 CET 2009
On Wed, Feb 11, 2009 at 1:25 AM, Richard O'Keefe <> wrote:
> I asked why (--) could not be implemented in Erlang instead of C.
> On 10 Feb 2009, at 9:44 pm, Hynek Vychodil wrote:
>> It's easy, constants for serious amount |2|.
> I am not sure how to interpret this.
> Let's face it, *as currently implemented* -- is not
> only inefficient, it can block scheduling for non-
> trivial amounts of time. So as things stand now,
> fully informed programmers are unlikely to make very
> much use of it, so in any program where the constant
> factors are likely to make much difference, it is
> _already_ so slow that something else should be done.
> For example, I just measured some times:
> 681.69 msec, bag difference of unsorted lists using --
> 0.64 msec, bag difference of sorted lists using own
> sorted list difference code in Erlang
> 24.97 msec, bag difference of unsorted lists by
> sorting then using sorted list difference.
> The one relying on a C coded -- is the slowest of them.
> OK, so the lists were 10,000 elements long. Make them
> 10 times smaller, and the times are
> 5.24 msec unsorted then --
> 0.08 msec presorted, use own Erlang
> 1.63 msec unsorted, use sort then own Erlang.
> Still the same conclusion: even coded in C, -- is too
> slow to use with any lists that might be nontrivial.
> So who cares about the constant factors in an operation
> that you shouldn't be using? Every time we move something
> out of C into Erlang we simplify the trusted base.
Nice, and now measure this for 5 element long lists.
--Hynek (Pichi) Vychodil
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