It seems like when we increased the number of tests per shard, we
started overcommitting the Renoir runner, leading to load averages
higher than the 16 CPU threads could handle, while also running at
75-96% memory usage.
By dropping the concurrency to 16, we should be able to reduce this
memory usage while also reducing the execution time.
Signed-off-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26501>
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