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Date: Mon, 24 Feb 2020 15:40:22 +0200
From: Adrian Bunk <bunk@stusta.de>
To: =?utf-8?B?QW5kcsOp?= Draszik <git@andred.net>
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Cc: openembedded-core@lists.openembedded.org
Subject: Re: [PATCH v2] bitbake.conf: omit XZ threads and RAM from sstate signatures
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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