offload: Improve load normalization and configurability.

Fixes <https://issues.guix.gnu.org/43773>.

The computed normalized load was previously obtained by dividing the load
average as found in /proc/loadavg by the number of parallel builds defined for
a build machine.

This normalized load didn't allow to compare machines with different number of
cores, as the load average reported by /proc/loadavg can be as high as the
number of cores; thus comparing that value to a fixed threshold of 2.0 would
mean machines with multiple cores were more likely to be flagged as overloaded
compared to single core machines.

This can be fixed by normalizing using the available number of cores instead
of the number of parallel jobs.

* guix/scripts/offload.scm (<build-machine>)[overload-threshold]: New field.
(node-load): Modify to return a normalized load value between 0 and 1, taking
into account the number of cores available.
(normalized-load): Remove procedure.
(report-load): New procedure.
(choose-build-machine): Adjust to use the modified 'node-load' and the new
'report-load' and 'build-machine-overload-threshold' procedures.
(check-machine-status): Adjust.
* doc/guix.texi (Daemon Offload Setup): Document the offload scheduler and the
new 'overload-threshold' field.

1 files changed, 29 insertions, 1 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 3fc76c8670..553a3b8ae9 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -1081,7 +1081,28 @@ architecture natively supports it, via emulation (@pxref{Transparent
 Emulation with QEMU}), or both.  Missing prerequisites for the build are
 copied over SSH to the target machine, which then proceeds with the
 build; upon success the output(s) of the build are copied back to the
-initial machine.
+initial machine.  The offload facility comes with a basic scheduler that
+attempts to select the best machine.  The best machine is chosen among
+the available machines based on criteria such as:
+
+@enumerate
+@item
+The availability of a build slot.  A build machine can have as many
+build slots (connections) as the value of the @code{parallel-builds}
+field of its @code{build-machine} object.
+
+@item
+Its relative speed, as defined via the @code{speed} field of its
+@code{build-machine} object.
+
+@item
+Its load.  The normalized machine load must be lower than a threshold
+value, configurable via the @code{overload-threshold} field of its
+@code{build-machine} object.
+
+@item
+Disk space availability.  More than a 100 MiB must be available.
+@end enumerate
 
 The @file{/etc/guix/machines.scm} file typically looks like this:
 
@@ -1185,6 +1206,13 @@ when transferring files to and from build machines.
 File name of the Unix-domain socket @command{guix-daemon} is listening
 to on that machine.
 
+@item @code{overload-threshold} (default: @code{0.6})
+The load threshold above which a potential offload machine is
+disregarded by the offload scheduler.  The value roughly translates to
+the total processor usage of the build machine, ranging from 0.0 (0%) to
+1.0 (100%).  It can also be disabled by setting
+@code{overload-threshold} to @code{#f}.
+
 @item @code{parallel-builds} (default: @code{1})
 The number of builds that may run in parallel on the machine.