[LLVMdev] On LLD performance
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>
>> On Mon, Mar 16, 2015 at 1:54 AM, Davide Italiano <davide at freebsd.org>
>> > 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.
>> It seems now 85% of CPU time is spent inside
>> 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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