[yocto] Why can diffsigs take sometimes really looooong?

[Richard Purdie]

On Fri, 2018-07-13 at 19:18 +0300, Uwe Geuder wrote:
> At times I find the diffsigs command useful/educational to understand
> what is going on in my build.
> 
> $ bitbake-diffsigs -t myimage do_image
> 
> Often the result is shown in no time. However, recently I got some
> cases were it takes 150 (!) minutes to show a simple difference (1
> line
> changed in do_install of systemd).
> 
> In comparision building after that change (with sstate) takes some 10
> minutes. And building everything from scratch (no sstate) takes just
> a
> bit over 50 minutes on the same machine.
> 
> Of course the build can make good use of my 8 cores / 16 threads,
> whereas diffsigs seems to run in only 1 core. Still, shouldn't every
> build calculate all the dependencies, so running diffsigs should only
> be
> a small fraction of that work?
> 
> Is there a natural explanation why diffsigs can sometimes be so slow?
> Just curious to understand what is going on there.
> 
> I am on Rocko 2.4.3 if that makes a difference.

Is your sstate directory on something slow like NFS?

Cheers,

Richard