[LLVMdev] On LLD performance

Davide Italiano davide at freebsd.org
Tue Mar 17 21:36:30 PDT 2015

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:

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

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


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

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