[RFC][gdbsupport] Improve thread scheduling in parallel_for_each

[Tom de Vries]

On 7/15/22 21:05, Tom Tromey wrote:
>>>>>> "Tom" == Tom de Vries via Gdb-patches <gdb-patches@sourceware.org> writes:
> 
> Tom> This introduces a performance regression on a particular test-case I happened
> Tom> to use:
> Tom> ...
> Tom> $ for n in $(seq 1 10); do \
> Tom>     time gdb -q -batch libxul.so.debug 2>&1 | grep real:; \
> Tom>   done
> Tom> ...
> Tom> so revert to the original schedule by reducing the worker threads:
> Tom> ...
> 
> This seems like making a change and then undoing it somewhere else?
> 
> Tom> Still, the performance experiment yields a slight performance loss.
> 
> Sounds bad.
> 
> Tom>    if (n_threads < 0)
> Tom> -    n_threads = std::thread::hardware_concurrency ();
> Tom> +    {
> Tom> +      n_threads = std::thread::hardware_concurrency ();
> Tom> +      if (n_threads > 0)
> Tom> +	/* Account for main thread.  */
> Tom> +	n_threads--;
> Tom> +    }
> 
> I think it's better if the setting just directly controls how many
> threads there are.  Then elsewhere we can decide what that means -- like
> if it performs better with the defaults to not do any work in the main
> thread, then parallel_for_each can be modified to just send tasks to the
> workers and do nothing in the main thread except wait for results.
> 
> Tom>    size_t elts_per_thread = 0;
> [...]
> Tom> +  elts_per_thread = n_elements / n_threads;
> 
> The initial declaration can be removed and then this latter line can
> declare the variable as well.
> 
> Tom>    for (int i = 0; i < count; ++i)
> Tom>      {
> Tom>        RandomIt end = first + elts_per_thread;
> Tom> +      if (i < left_over)
> Tom> +	end++;
> 
> It may be nice to mention the distribution of leftovers in a comment
> somewhere.

I've ended up committing this patch, which just does the leftovers 
distribution part, with some comments added.

Thanks,
- Tom
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