My gut here says you're unwittingly comparing regular ocaml to flambda
ocaml.

https://ocaml.org/manual/flambda.html

Perhaps multicore only comes in flambda flavor now (I notice it's not
available as a switch).

On Fri, Oct 8, 2021 at 5:07 PM Christophe Raffalli <christophe@raffalli.eu>
wrote:

>
> Hello,
>
> I managed to install ocaml 4.12.0 with multicore. I could not parallelise
> my
> code in 5mn ;-) but I check just the sequential speed and got a bit
> surprised. On the same examples, same options (flambda everywhere), etc ...
>
>                       Ex 1      Ex 2          Ex 3
> 4.13.1 normal         45s       12s           49s
> 4.12.0 normal         36s       11s           45s
> 4.12.0 multicore      31s       10s           40s
>
> These are not small differences and it is rather surprising that
>
> 4.13.1 is significantly slower than 4.12.0 (20 to 25%)
>
> 4.12.0 + multicore is faster on sequential code.
>
> Other people observe the same ?
> Any idea ? Should I report an issue for the speed degradation of 4.13.1 ?
>
> Christophe
>