[yocto] RFC: Backwards compatibility checking sstate-cache

[Mike Looijmans]

On 23-09-17 00:51, Joshua Lock wrote:
> On 22/09/17 15:00, Mike Looijmans wrote:
>> I think this remark in the referenced link is the best summary of "what 
>> could be improved":
>>
>> """the biggest weakness of the sstate signature bits, in my opinion, is that 
>> it only tracks inputs, not outputs. If task A depends on B, and the metadata 
>> input to B changes, then A will be rebuilt, even if the *output* of B didn't 
>> change as a result of the change to its metadata."""
>>
>> For example, if someone fixes a bug in gcc that only applies to MIPS, then 
>> builds that only target an ARM machine shouldn't be affected. Much worse 
>> than that, I have a dependency like:
>> gcc -> ... -> python -> bitstream tool -> FPGA image
>>
>> so this means that a change in gcc causes Python to rebuild, which causes 
>> the bitstream tool to be rebuilt and produce idential output, and that 
>> triggers a roughly 31-hour build of lots of FPGA images. These are the ones 
>> we really want to break. A binary output compare would help a lot, even if 
>> 80% of the libraries fail to create reproducible output, I may still be 
>> spared those 31 hours...
>>
>> I think tracking digital output is technically feasible, and won't require 
>> any change to existing recipes.
>>
>> Also think about "feed" setups. When I see my settop reporting "331 packages 
>> need updating"... It'd be great if that could be avoided.
>>
> 
> That's what packagefeed-stability.bbclass is for. Work on binary 
> reproducibility will improve things here too.

Interesting, thanks for the hint.

That might cut down a bit on the over 4TB/month "update my box" traffic.


Kind regards,

Mike Looijmans
System Expert

TOPIC Products
Materiaalweg 4, NL-5681 RJ Best
Postbus 440, NL-5680 AK Best
Telefoon: +31 (0) 499 33 69 79
E-mail: mike.looijmans at topicproducts.com
Website: www.topicproducts.com

Please consider the environment before printing this e-mail


Visit us at the Space Tech Expo Europe (Stand E-71)