Richard Biener wrote: >On Fri, Sep 23, 2016 at 3:02 PM, Markus Trippelsdorf <mar...@trippelsdorf.de> >wrote: > > And tramp3d only uses ten partitions (lto-min-partition=10000). > > With lto-min-partition=50000 (current patch) this decrease to only two > > partitions. As a result we loose the possible speedup on many core > > machines (-flto=n).
Only if the size is close to the lto-min-partition. For larger applications there is little difference. > > E.g. on my 4-core machine I get the following tramp3d compile times with > > -flto=4: > > > > lto-min-partition=50000: 20.146 total > > lto-min-partition=10000: 16.299 total > > lto-min-partition=1000 : 16.093 total > > > > So 50000 looks too big to me. That's only 16 seconds? Seems like it's small so ideally it should have used a single partition... > I think the issue is that the default number of partitions is too high > (32) which pessimizes 4-core machines if the units are too small. Yes, 8 might be a better value as 32 core machines are rare. > Maybe we can tune the triplet lto-partitions, lto-min-partition and > lto-max-partition in a way that it roughly scales the number of > partitions produced with program size rather than quickly raising > to 32 and then hovering there until the first unit hits lto-max-partition? Or use a single partition size rather than have the maximum size a hundred times the minimum size (which doesn't make sense at all). > > Also the "increased optimization opportunities" with fewer partitions > > were unmeasurable in the past. If I recall correctly Honza once said > > that there should be no difference between single vs. many partitions. > > Well, it definitely makes a difference for late IPA passes (that's mainly > IPA PTA). Also anchors don't work with multiple partitions. I get around 1% gain from using a single partition. Wilco
