> Regressions of such magnitude can veto such changes, especially when > they hit everyone, not just those who are highly dependent on the > profiling tools the proposal is concerned about.
The kernel benchmarks were added as an example of openly available data we could find on the potential impact of frame pointers. Note that the email from Mel Gorman is all we have to go on. Unfortunately the original data from the benchmarks is gone so I can't try to reproduce them. I've emailed Mel to see if he still has the benchmarks stored somewhere so we can perhaps try to reproduce the results. I've added a clarification to the change proposal that we don't intend to actually compile the kernel with frame pointers, since the kernel is already built with ORC support and this works well so there's nothing to really be gained by building the kernel with frame pointers. That means we won't see the kernel regressions that were reported by the Suse benchmarks. Unfortunately, there's no readily available benchmarks that I've been able to find that would show the exact impact of frame pointers on common Fedora workflows. The Phoronix benchmark suite could be used but that would imply doing a mass rebuild with frame pointers before we could actually run it and measure the impact. Also, as mentioned in the proposal, all our internal services at Meta are built with frame pointers enabled. We did canaries a few years ago on some of our most CPU intensive services to see if it would make sense to build them without frame pointers, and found that there were no significant enough wins to be had to justify the loss in continuous profiling data caused by building without frame pointers > (Are you referring to a novel kernel-resident tool?) Unfortunately, no, there's no in-kernel DWARF unwinder due to the complexity involved. Instead, the kernel uses ORC and has an unwinder for that. Adding ORC support to all of Linux userspace so that we can unwind it in the kernel isn't likely to happen, since all tooling would have to be changed to support ORC. > The proposal doesn't characterize the "reasonably low overhead" that > this operation targets. That makes it hard to judge the tradeoffs. Characterizing the impact would mean rebuilding most of the distro with frame pointers and running a comprehensive benchmark suite on it. Doing this will be a rather involved process. If you know of any other representative benchmark suites that we could run that wouldn't require rebuilding most of the distro, we could look into running these with and without frame pointers to measure the impact. > If typing that option were a hardship, it could be made default on > Fedora. With broad debuginfod auto-downloading capability, maybe it's > worth considering. The issue with DWARF isn't that we have to add an extra option to perf, it's that without an in kernel DWARF unwinder (which is very unlikely to ever happen as discussed above), it's expensive to use DWARF for stacktrace unwinding, as we have to copy the entire stack and unwind it in user space, which adds substantial overhead. This means we can't use it for continuous profiling. _______________________________________________ devel mailing list -- devel@lists.fedoraproject.org To unsubscribe send an email to devel-le...@lists.fedoraproject.org Fedora Code of Conduct: https://docs.fedoraproject.org/en-US/project/code-of-conduct/ List Guidelines: https://fedoraproject.org/wiki/Mailing_list_guidelines List Archives: https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org Do not reply to spam on the list, report it: https://pagure.io/fedora-infrastructure
