Re: [PATCH v2] bitbake.conf: omit XZ threads and RAM from sstate signatures

[Adrian Bunk <bunk@stusta.de>]

On Mon, Feb 24, 2020 at 12:59:55PM +0000, André Draszik wrote:
> The number of threads used, and the amount of memory allowed
> to be used, should not affect sstate signatures, as they
> don't affect the result.

Unfortunately they can affect the result.

> Otherwise, it becomes impossible to re-use sstate from
> automated builders on developer's machines (as the former
> might execute bitbake with certain constraints different
> compared to developer's machines).
>...
> -XZ_DEFAULTS ?= "--memlimit=50% --threads=${@oe.utils.cpu_count()}"
>...

Threaded compression can result in slightly worse compression
than single-threaded compression.

With memlimit the problem is actually the opposite way,
and worse than what you were trying to fix:

When a developer hits memlimit during compression, the documented
behavour of xz is to scale down the compression level.

I assume 50% wrongly gives the same sstate signature no matter how
much RAM is installed on the local machine?

cu
Adrian