[RFC][gdbsupport] Improve thread scheduling in parallel_for_each

[Tom Tromey]

>>>>> "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.

Tom