[bug#70494] [PATCH 11/23] scripts: substitute: Untangle selecting fast vs small compressions.

["Ludovic Courtès" <ludo@gnu.org>]

Christopher Baines <mail@cbaines.net> skribis:

> Pulling the logic up to the script makes this code more portable and not
> reliant on setting a global variable.
>
> * guix/scripts/substitute.scm (%prefer-fast-decompression?): Rename to…
> (%default-prefer-fast-decompression?): this.
> (call-with-cpu-usage-monitoring): Use multiple values to return the results
> from the thunk as well as the cpu usage.
> (display-narinfo-data): Update accordingly.
> (download-nar): Add prefer-fast-decompression? as a keyword argument, remove
> code to set! it and monitor the cpu-usage.
> (process-substitution, process-substitution/fallback): Accept and pass through
> prefer-fast-decompression? to download-nar.
> (guix-substitute): Move the cpu usage monitoring and prefer fast decompression
> switching logic here.
>
> Change-Id: I4e80b457b55bcda8c0ff4ee224dd94a55e1b24fb

[...]

> +                 ;; Create a hysteresis: depending on CPU usage, favor
> +                 ;; compression methods with faster decompression (like ztsd)
> +                 ;; or methods with better compression ratios (like lzip).
> +                 ;; This stems from the observation that substitution can be
> +                 ;; CPU-bound when high-speed networks are used:
> +                 ;; <https://lists.gnu.org/archive/html/guix-devel/2020-12/msg00177.html>.
> +                 ;; To simulate "slow" networking or changing conditions, run:
> +                 ;; sudo tc qdisc add dev eno1 root tbf rate 512kbit latency
> +                 ;; 50ms burst 1540 and then cancel with: sudo tc qdisc del
> +                 ;; dev eno1 root

This comment’s formatting was broken (by M-q I guess).

Otherwise LGTM!