mboehme added a comment. In D128439#3607350 <https://reviews.llvm.org/D128439#3607350>, @nikic wrote:
> In D128439#3607289 <https://reviews.llvm.org/D128439#3607289>, @mboehme wrote: > >> Just a question for my understanding before I abandon the change: Shouldn't >> the "instructions" count be relatively resistant to noise? (I was assuming >> this is based on performance counters? What would cause noise in this >> metric?) > > There are two primary sources of noise. The first is kernel activity, as the > `instructions` metric is a combined userspace and kernel metric. I do collect > userspace-only as well (this is `instructions:u`). Ah -- that makes sense. > On that metric this does look like a small improvement just above the noise > floor: > http://llvm-compile-time-tracker.com/compare.php?from=8b6f69a4da5baaf3748798a84dd16a2481b7ca7f&to=797ba50f5fd88017925fe765427b1f5f136c3310&stat=instructions:u So based on that, would you recommend that I un-abandon this revision after all and submit it for review? > The other is ASLR. If ASLR is disabled, the `instructions:u` measurements > become perfectly noise-free. This is actually bad, because a lot of changes > will then show statistically significant and perfectly reproducible > compile-time changes that are ultimately just down to how exactly the address > space gets laid out. So this is intentional noise :) > > And yes, execution statistics are collected using `perf`. `callgrind` uses a > CPU simulator instead, which is a much more controlled environment... Thanks for the clarification. I learned a lot about performance measurement from this discussion! Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D128439/new/ https://reviews.llvm.org/D128439 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
