On 7/8/22 15:29, Christian Hergert wrote: >> Frank Ch. Eigler mentions that elfutils has a more modern unwinding library. >> Could that perhaps solve your performance issues with libunwind? > > I don't think so. The problem is two-fold. > > First, we have to capture enough of the stack to do offline unwinding. I > think the default many people do here is about 8kb of stack. While the > instruction pointer array might fit in a couple cachelines, you now have an > additional few pages to copy as well. And you probably want those pages > aligned in your capture format. So no you need to interleave multiple types > of data frames while padding for alignment. > > Now do that a few thousand times a second. > > The overhead here can be so great that it obscures what you're trying to > find. Furthermore, it's a good chance that you'll cause CPU packages to spin > up to a higher frequency, thusly hiding the exact performance issues you want > to find or reduce to avoid that. > > Now, say you've done the work and captured stacks (what has now turned from a > few MB recording to a few GB recording) you need to decode them. We keep many > lookaside-maps/interval-trees in Sysprof to keep this overhead low, but now > you have to reference .eh/DWARF data. This is the slowest part of the whole > process. What currently takes a second or two could take you easily 10 > minutes.
That is the problem right here: .eh_frame-based unwinding is too slow, so it has to be done offline in userspace. What about instead adding ORC information to userspace? That would be much faster to use. -- Sincerely, Demi Marie Obenour (she/her/hers) _______________________________________________ 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
