[LLVMdev] On LLD performance

Sean Silva chisophugis at gmail.com
Mon Mar 16 23:17:45 PDT 2015

On Mon, Mar 16, 2015 at 10:52 PM, Davide Italiano <davide at freebsd.org>

> On Mon, Mar 16, 2015 at 1:54 AM, Davide Italiano <davide at freebsd.org>
> wrote:
> >
> > Shankar's parallel for per-se didn't introduce any performance benefit
> > (or regression).
> > If the change I propose is safe, I would like to see Shankar's change
> > in (and this on top of it).
> > I have other related changes coming next, but I would like to tackle
> > them one at a time.
> >
> Here's an update.
> After http://reviews.llvm.org/D8372 , I updated the profiling data.
> https://people.freebsd.org/~davide/llvm/lld-03162015.svg
> It seems now 85% of CPU time is spent inside
> FileArchive::buildTableOfContents().

I'm rather amazed that that patch changed the total CPU time. Just doing
the work in parallel shouldn't reduce the total CPU time spent on the task.
A reduction in CPU time would happen though if parallelizing it increased
the single-threaded performance of the tasks being done in parallel.
Perhaps using multiple cores means we are using multiple caches, so each
thread is getting much better single-threaded performance due to reduced
memory bottlenecking?

-- Sean Silva

> In particular, 35% of the samples are spent inserting into
> unordered_map, so there's maybe something we can do differently there
> (e.g. , Rui's proposal of a concurrent map doesn't seem that bad).
> Thanks,
> --
> Davide
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare
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