Re: OpenMP for both clang and GCC, part II

On 2019-06-06 16:12, Stuart Henderson wrote:
> On 2019/06/06 11:13, j@bitminer.ca wrote:
...
>>
>> Improvement would be measured how? By number of supporting ports, or
>> number of frequently used numerically intensive ports (such as audio,
>> video,
>> or math tools)? I'd rather not judge and just try to give users
>> options.
>
> I'm thinking more at a very basic level for now: e.g. whether these
> programs actually run any faster on OpenBSD when OpenMP is used.

For typical interactive use, web browsing, editing, building, nobody
uses code that uses OpenMP.

For photo editing, e.g. darktable, possibly it will be noticeably
quicker.

For long calculations, I have seen 2 cores with OpenMP take half the
time
as one core with/without. So yes, it works. But Amdahl's Law applies.
If a calculation is long, and 20% is serial, and 80% parallelizable,
then the runtimes will be 100%, 60%, 46% and 40% at 1, 2, 3 and 4 cores.
The difference between 3 and 4 cores is not much. If 100% takes 4
hours,
then the difference between 1 and 2 cores is significant.

...
>
> Other ongoing work would be remembering that this is a thing and making
> sure it's disabled where needed in new ports. (The worst case is if it
> gets picked up and used at build time if the runtime is present and
> produces a package that needs the runtime to work, but is otherwise
> not very identifiable until someone tries to run the package on a
> machine without the runtime.)

Under my scheme, only clang needs its runtime to be installed; the
gcc-libs package has its runtime always (see patches published
yesterday).

>
>> The list of ports explicitly disabling openmp, and consumers of their
>> shared
>> libraries is a little surprising (emacs anyone?). Here is the list,
>> approximately, to one level deep:
>
> It's disabled in a few things where it was noticed (usually by
> reviewing
> lists of options in configure --help or in build log), but I don't
> think
> there has been much concerted effort to disable it throughout the tree.
> bcallah made a start at this but IIRC only did a few categories.

I'll see how much I can cover in the next while.