[llvm-dev] LLD: time to enable --threads by default

[Davide Italiano via llvm-dev]

On Thu, Nov 17, 2016 at 8:04 PM, Davide Italiano <davide at freebsd.org> wrote:
> On Thu, Nov 17, 2016 at 7:34 PM, Rui Ueyama <ruiu at google.com> wrote:
>> On Thu, Nov 17, 2016 at 6:30 PM, Davide Italiano <davide at freebsd.org> wrote:
>>>
>>> On Thu, Nov 17, 2016 at 1:20 PM, Rafael Espíndola via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>> >>
>>> >> Thank you for the explanation! That makes sense.
>>> >>
>>> >> Unlike ThinLTO, each thread in LLD consumes very small amount of memory
>>> >> (probably just a few megabytes), so that's not a problem for me. At the
>>> >> final stage of linking, we spawn threads to copy section contents and
>>> >> apply
>>> >> relocations, and I guess that causes a lot of memory traffic because
>>> >> that's
>>> >> basically memcpy'ing input files to an output file, so the memory
>>> >> bandwidth
>>> >> could be a limiting factor there. But I do not see a reason to limit
>>> >> the
>>> >> number of threads to the number of physical core. For LLD, it seems
>>> >> like we
>>> >> can just spawn as many threads as HT provides.
>>> >
>>> >
>>> > It is quite common for SMT to *not* be profitable. I did notice some
>>> > small wins by not using it. On an intel machine you can do a quick
>>> > check by running with half the threads since they always have 2x SMT.
>>> >
>>>
>>> I had the same experience. Ideally I would like to have a way to
>>> override the number of threads used by the linker.
>>> gold has a plethora of options for doing that, i.e.
>>>
>>>   --thread-count COUNT        Number of threads to use
>>>   --thread-count-initial COUNT
>>>                               Number of threads to use in initial pass
>>>   --thread-count-middle COUNT Number of threads to use in middle pass
>>>   --thread-count-final COUNT  Number of threads to use in final pass
>>>
>>> I don't think we need the full generality/flexibility of
>>> initial/middle/final, but --thread-count could be useful (at least for
>>> experimenting). The current interface of `parallel_for_each` doesn't
>>> allow to specify the number of threads to be run, so, assuming lld
>>> goes that route (it may not), that should be extended accordingly.
>>
>>
>> I agree that these options would be useful for testing, but I'm reluctant to
>> expose them as user options because I wish LLD would just work out of the
>> box without turning lots of knobs.
>>
>
> I share your view that lld should work fine out-the-box. I think an alternative
> is having the option as hidden, maybe. I consider the set of users
> tinkering with linker options not large, although there are some
> people who like to override/"tune" the linker anyway, so IMHO we
> should expose a sane default and let users decide if they care or not
> (a similar example is what we do for --thinlto-threads or
> --lto-partitions, even if in the last case we still have that set to 1
> because it's not entirely clear what's a reasonable number).
>

I've seen a case where the linker was pinned to a specific subset of the CPUs
and many linker invocations were launched in parallel.
(actually, this is the only time when I've seen --threads for gold used).
I personally don't expect this to be the common use-case, but it's not hard
to imagine complex build systems adopting a similar strategy.

-- 
Davide

"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare