Oh. Or I'm just out of date. Apparently the way you choose compilers
options (like flambda) in a switch changed awhile ago:

https://discuss.ocaml.org/t/experimental-new-layout-for-the-ocaml-variants-packages-in-opam-repository/6779

Sorry about the noise!

On Fri, Oct 8, 2021 at 6:11 PM Michael Bacarella <
michael.bacarella@gmail.com> wrote:

> Sorry, you actually said as much "On the same examples, same options *(flambda
> everywhere)*,"
>
> Though, the expectation that was thwarted for me is that there's no
> specific multicore flambda switch. Just multicore. And that 20-25% speedup
> felt like a familiar we-switched-to-flambda speedup to me.
>
> On Fri, Oct 8, 2021 at 5:58 PM Michael Bacarella <
> michael.bacarella@gmail.com> wrote:
>
>> 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
>>>
>>