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

[Luke Diamand]

On 16 January 2018 at 03:33, Luke Diamand <address@hidden> 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.
>
> 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?

If do:
  export UNW_ARM_UNWIND_METHOD=4

(so only use the exidx tables) then it drops from ~1ms to 95us.

My image only has the exidx tables; it's built with gcc using
bog-standard compiler and linker options.

$ objdump -S bt_perf
Idx Name          Size      VMA       LMA       File off  Algn
....
 15 .ARM.extab    00000264  0000a7e8  0000a7e8  0000a7e8  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
 16 .ARM.exidx    000002a0  0000aa4c  0000aa4c  0000aa4c  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
 17 .eh_frame     00000004  0000acec  0000acec  0000acec  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA