[ptxdist] Loss of speedup for subsequent different -but still similar- images build

["Henzer, Kevin" <KevinHenzer@eaton.com>]

Hello !

Thanks for adding me to the mailing list, and thank for this nice tool that PTXdist is !

I'm new to PTXdist, and I have inherited a build setup that creates two different but quite similar system images, following two different global configurations (config_1 and config_2). 
One is the rescue update image of the system, the other the full-featured image. It is using a script to build them both in a row. 

Before a recent update, calling the build process successively on those two configurations gave a big speed-up for the second build. The system was reusing most of the packages.

./ptxdist go --ptxconfig=configs/config_1 --platformconfig=configs/platformconfig 	# takes time only the first time, else quite fast
./ptxdist images --ptxconfig=configs/ config_1  --platformconfig=configs/platformconfig  # quite fast

./ptxdist go --ptxconfig=configs/config_2 --platformconfig=configs/platformconfig 	# quite fast
./ptxdist images --ptxconfig=configs/ config_2  --platformconfig=configs/platformconfig  # quite fast



But after updating PTXdist to newer version (now using PTXdist version 2021.03.0), and updating the config files, it seems something has changed, and the outcome is different. 

./ptxdist go --ptxconfig=configs/config_1 --platformconfig=configs/platformconfig 	# takes time at each build
./ptxdist images --ptxconfig=configs/ config_1  --platformconfig=configs/platformconfig  # quite fast

./ptxdist go --ptxconfig=configs/config_2 --platformconfig=configs/platformconfig 	# takes time at each build, seems the system is rebuilding everything


And I got this as first output when switching from one configuration to the other:

-----------------------------------------------------------------
target: host-system-perl.[some hash].cfghash
-----------------------------------------------------------------

Configuration changed! Reconfiguring...



I checked this package and seems to me it is just ensuring that Perl is installed on the host and that the right features are available.
I checked all config options that I think are linked to this package (I searched in rules/host-system-perl.make) : 

PTXCONF_HOST_SYSTEM_PERL_BUILD
PTXCONF_HOST_SYSTEM_PERL_JSON
PTXCONF_HOST_SYSTEM_PERL_LOCALE_PO
PTXCONF_HOST_SYSTEM_PERL_XMLPARSER
PTXCONF_HOST_SYSTEM_PERL_YAPP

And they are set the same in both config files.


Still, the hash of this package is changing between each call with different config. 

So, I was wondering:
- Can it be that the reconfiguration and "rebuild" of this package is what triggers the full image rebuild?  
- If yes, what could trigger this host-system-perl reconfiguration, in your opinion ? 


And most important for me, how are those .cfghash computed ? If you have a quick link to the relevant part of the doc, I'll be very happy !

In advance, thanks !

Regards,

Kevin Henzer
   



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