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

[Dave Watson]

On 01/16/18 09:34 AM, Luke Diamand wrote:
> 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

I think there is a similar perf test program under tests/, 'make perf'

> >
> > 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

I didn't notice anything in particular looking at the commit logs.
I'll try and repro when I get a chance