* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > Undwarf vs frame pointers > ------------------------- > > With frame pointers enabled, GCC adds instrumentation code to every > function in the kernel. The kernel's .text size increases by about > 3.2%, resulting in a broad kernel-wide slowdown. Measurements by Mel > Gorman [1] have shown a slowdown of 5-10% for some workloads. > > In contrast, the undwarf unwinder has no effect on text size or runtime > performance, because the debuginfo is out of band. So if you disable > frame pointers and enable undwarf, you get a nice performance > improvement across the board, and still have reliable stack traces. > > Another benefit of undwarf compared to frame pointers is that it can > reliably unwind across interrupts and exceptions. Frame pointer based > unwinds can skip the caller of the interrupted function if it was a leaf > function or if the interrupt hit before the frame pointer was saved. > > The main disadvantage of undwarf compared to frame pointers is that it > needs more memory to store the undwarf table: roughly 3-5MB depending on > the kernel config. Note that it's not just a performance improvement, but also an instruction cache locality improvement: 3.2% .text savings almost directly transform into a similarly sized reduction in cache footprint. That can transform to even higher speedups for workloads whose cache locality is borderline. I _really_ like this feature, and the independence of the debuginfo data format. Logistically it's too bad we are 3 days away from the merge window to be able to pick this up: > 56 files changed, 3466 insertions(+), 1765 deletions(-) OTOH most of the diffstat is in objtool. Any objections to applying the first 3 objtool patches straight away and see whether anything breaks? That would significantly reduce the size of the rest of the patch set. > I'm not tied to the 'undwarf' name, other naming ideas are welcome. Ha, a new bike shed painting job! ;-) I think 'undwarf' isn't a bad name, it's short, catchy and describes the purpose of the effort. But I cannot resist some other suggestions, after 'elf' and 'dwarf' the obvious candidates from the peoples of Middle-earth would be: - 'Hobbit' - 'Eagle' - 'Ent' - 'Dragon' - 'Troll' - 'Ainur' 'struct troll_entry' has a certain charm to it. 'Eagle' is even nicer IMHO: larger than a dwarf but so much faster - and eagles are beautiful! Plus the name is 2 letters shorter than 'unwdwarf', win-win. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
