Re: [Libunwind-devel] unw_backtrace(): Performance slowdown for ARM?

[Milian Wolff]

On Tuesday, January 16, 2018 4:33:12 AM CET Luke Diamand wrote:
> Hi!
> 
> I could be wrong about this, but I'm finding a roughly 10x slowdown on
> ARM between 1.2.1 and current HEAD (e1ca8748).
> 
> I have a small test program here:
> 
> https://gist.github.com/luked99/7c67d94da5c33cac5a1b41a500013355
> 
> On my Raspberry Pi 2, this is getting ~180us to do a self-backtrace in
> 1.2.1 and around 1.8ms in HEAD.
> 
> I tried running perf, and that seems to be saying that it is opening
> and closing files a lot.

How did you run perf? For starters, can you use `perf stat` on the two 
versions and your benchmark so that we can see whether it's CPU bound or not?

Then, please record a profile with

perf record --call-graph dwarf 

and then inspect that with "perf report" on your ARM machine or hotspot [1] on 
your devel machine, assuming you have a sysroot with the debug symbols 
available there.

This should make it much easier to find out where the performance is going to.

[1]: https://github.com/KDAB/hotspot

> I also had a go at git bisecting, but there's a separate bug which was
> introduced and then fixed which caused it to segfault, which overlaps
> with this problem, along with some other performance regression fixes.
> 
> 90d0d15c4ec3bb812dcb44c081757ebf6756cc6b didn't crash, but was slow.
> 2a5d1a629634b3225fccb1ed8a0a765bccc77f2e crashed.
> v1.2.1 is fast.
> 
> I'm configuring with:
> 
> CC=arm-linux-gnueabihf-gcc CFLAGS="-g" configure --disable-debug
> --host=arm-linux \
>     --enable-shared --disable-static --with-sysroot=/home/lgd/arm
> --enable-maintainer-mode \
>     --disable-debug-frame --disable-block-signals
> 
> Any ideas as to what's going on?
> 
> Thanks
> Luke
> 
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-- 
Milian Wolff
address@hidden
http://milianw.de