[LLVMdev] On LLD performance

[Bruce Hoult]

With a 4 second difference between best and worst runs in both cases, and
only 0.2 second difference between the best for the two different cases
(user time), I don't think you can make any conclusion that one is better
than the other! They are very very similar.

On Wed, Mar 18, 2015 at 5:36 PM, Davide Italiano <davide at freebsd.org> wrote:

> On Mon, Mar 16, 2015 at 11:00 PM, David Blaikie <dblaikie at gmail.com>
> wrote:
> >
> >
> > On Mon, Mar 16, 2015 at 10:52 PM, Davide Italiano <davide at freebsd.org>
> > wrote:
> >>
> >> 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().
> >> 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).
> >
> >
> > Anyone tried a DenseMap instead of an unordered_map? If you need pointer
> > validity to the elements, a DenseMap with unique_ptrs rather than direct
> > values could be an option. Chandler's usual argument here is that walking
> > the map is cheap with high locality (as in a DenseMap) even if the nodes
> > themselves involve indirection. Could be worth an experiment.
> >
>
> I did now. It actually makes things slower for the aforementioned
> workload (linking clang). It was worth trying though.
>
> Patch, in case somebody wants to try at home:
> https://people.freebsd.org/~davide/llvm/densemap_membermap.diff
>
> Patched:
> real    1m27.849s  user    2m47.373s   sys     0m16.370s
> real    1m29.583s  user    2m47.771s   sys     0m16.816s
> real    1m25.956s  user    2m43.397s   sys     0m15.254s
> real    1m29.380s  user    2m47.618s   sys     0m15.386s
> real    1m25.426s  user    2m43.388s   sys     0m16.961s
>
> Unpatched:
> real    1m26.872s  user    2m46.999s sys     0m16.540s
> real    1m28.187s  user    2m47.084s sys     0m17.149s
> real    1m24.814s  user    2m43.311s  sys     0m16.979s
> real    1m25.011s  user    2m43.184s  sys     0m16.975s
> real    1m25.536s  user    2m44.577s  sys     0m16.784s
>
> --
> Davide
>
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare
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