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   systemd.resource-control    ( 5 )

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UNIFIED AND LEGACY CONTROL GROUP HIERARCHIES

The unified control group hierarchy is the new version of kernel
       control group interface, see Control Groups v2[2]. Depending on
       the resource type, there are differences in resource control
       capabilities. Also, because of interface changes, some resource
       types have separate set of options on the unified hierarchy.

       CPU
           CPUWeight= and StartupCPUWeight= replace CPUShares= and
           StartupCPUShares=, respectively.

           The "cpuacct" controller does not exist separately on the
           unified hierarchy.

       Memory
           MemoryMax= replaces MemoryLimit=.  MemoryLow= and MemoryHigh=
           are effective only on unified hierarchy.

       IO
           "IO"-prefixed settings are a superset of and replace
           "BlockIO"-prefixed ones. On unified hierarchy, IO resource
           control also applies to buffered writes.

       To ease the transition, there is best-effort translation between
       the two versions of settings. For each controller, if any of the
       settings for the unified hierarchy are present, all settings for
       the legacy hierarchy are ignored. If the resulting settings are
       for the other type of hierarchy, the configurations are
       translated before application.

       Legacy control group hierarchy (see Control Groups version 1[3]),
       also called cgroup-v1, doesn't allow safe delegation of
       controllers to unprivileged processes. If the system uses the
       legacy control group hierarchy, resource control is disabled for
       the systemd user instance, see systemd(1).