cgroup: Use and explain cpu.weight.

content/posts/cgroup v2 by hand.adoc

@@ -91,7 +91,7 @@ the power of 2 CPU cores.
 echo "+cpu" > cgroup.subtree_control
 mkdir testgroup
 cd testgroup
-echo "10" > cpu.weight.nice
+echo "50" > cpu.weight
 echo "200000 100000" > cpu.max
 --------------------------------------------------------------------------------
 
@@ -99,9 +99,14 @@ Try adding your current shell to the group with `echo "$$" > cgroup.procs`. All
 processes you start from this shell are now in the same cgroup. But what does
 the example do, exactly?
 
-- *cpu.weight.nice* works like the link:{url-nice}[nice] command (but it is not
-  the same) and has a range from -20 to 19. It is an alternate interface to
-  *cpu.weight* which has a range from 1 to 10,000.
+- *cpu.weight* is the relative amount of CPU cycles the cgroup is getting under
+  load. The CPU cycles are distributed by adding up the weights of all _active_
+  children and giving each the fraction matching the ratio of its weight against
+  the sum.footnote:[link:{url-kernel-doc}#weights[Kernel documentation on
+  Control Group v2, section “Resource Distribution Models” → “Weights”]] It has
+  a range from 1 to 10,000. If one process has a weight of 3,000 and the only
+  other active process has a weight of 7,000, the former will get 30% and the
+  latter 70% of CPU cycles. The default is 100.
 - *cpu.max* sets the “maximum bandwidth limit”. We told the kernel that the
   processes should use at most 200,000 µs every 100,000 µs, meaning they can use
   the power of up to 2 cores.
@@ -115,6 +120,10 @@ You can add a cgroup column to link:{url-htop}[htop] by pressing kbd:[F2] and
 then navigating to “Columns”. Select “CGROUP” in “Available Columns” and press
 kbd:[Enter].
 
+[TIP]
+*cpu.weight.nice* is an alternate interface to *cpu.weight* that uses the same
+values used by link:{url-nice}[nice] and has a range from -20 to 19.
+
 === Controlling CPU core usage
 
 This control group will use the cpuset controller to restrict the processes to