<![CDATA[CPU C-states]]>Hello,

One of the homelab platforms I'm running XCP-ng on, is a AMD 7950X.
I recently noticed, that in Ubuntu Linux 24.04, PowerTop reports C-states C1-C3 (see screenshot).

Screenshot 2025-01-16 213724.png
However, in XCP-ng 8.3, the command « xenpm get-cpuidle-states » only reports C0 and C1.

I've tried adding « cpuidle » to the Xen image boot parameters in Grub-efi.cfg which doesn't seem to make any difference to the output of the get-cpuidle-states command. Looking at the output of « xe host-dmesg » I am not sure what to look for; the lines referenced in an Xen documentation page do not seem to be present in the output on XCP-ng.

As this is a homelab project, I wish to reduce power consumption as much as is technically feasible and leverage the supported C-states (unless latency from switching to/out of C3 would be killing performance).

]]>https://xcp-ng.org/forum/topic/10276/cpu-c-statesRSS for NodeSun, 17 May 2026 00:02:26 GMTFri, 17 Jan 2025 12:07:10 GMT60<![CDATA[Reply to CPU C-states on Fri, 17 Jan 2025 17:34:57 GMT]]>It's very likely in Xen itself since a part of the power management is done there. Another topic to bring during the Xen Winter Meetup @TeddyAstie 😉

]]>https://xcp-ng.org/forum/post/88368https://xcp-ng.org/forum/post/88368Fri, 17 Jan 2025 17:34:57 GMT<![CDATA[Reply to CPU C-states on Fri, 17 Jan 2025 16:29:00 GMT]]>@redakula said in CPU C-states:

@FrankAtHome
I looked in to this as well a while back and the conclusion i came to, is that the newer AMD power management functions are not supported properly in the old kernel used by XCP-NG (And maybe xen?).
A lot of work seems to have been done in the 6.x kernel series on this.

I get the same on my 7900 - only C0 and C1 😬
I tried booting it on a Ubuntu 24.04 live cd and power consumption was easily 15% lower at idle.

that sounds plausible as explanation.

I guess I will have to patiently await XCP-ng 6 branch and any improvements this may bring on AMD systems.

]]>https://xcp-ng.org/forum/post/88361https://xcp-ng.org/forum/post/88361Fri, 17 Jan 2025 16:29:00 GMT<![CDATA[Reply to CPU C-states on Fri, 17 Jan 2025 15:41:49 GMT]]>@FrankAtHome
I looked in to this as well a while back and the conclusion i came to, is that the newer AMD power management functions are not supported properly in the old kernel used by XCP-NG (And maybe xen?).
A lot of work seems to have been done in the 6.x kernel series on this.

I get the same on my 7900 - only C0 and C1 😬
I tried booting it on a Ubuntu 24.04 live cd and power consumption was easily 15% lower at idle.

]]>https://xcp-ng.org/forum/post/88357https://xcp-ng.org/forum/post/88357Fri, 17 Jan 2025 15:41:49 GMT<![CDATA[Reply to CPU C-states on Fri, 17 Jan 2025 14:08:44 GMT]]>@FrankAtHome The server is a HP DL360 G8 Intel Xeon CPU E5-2680v2.

Same hardware/bios config as the other servers running XCP 8.2, so, nothing special setup. I'm not sure if it even has C5/C6 states available. It's an enterprise server, you don't want it to sleep.

Here's the output from a desktop HP Elitedesk 800 G3 Intel Core i7-7700 running XCP 8.3, again, nothing special setup. Now with more C-states!

# xenpm get-cpuidle-states 0
All C-states allowed

cpu id               : 0
total C-states       : 7
idle time(ms)        : 276484
C0                   : transition [              277556]
                       residency  [               16869 ms]
C1                   : transition [              197012]
                       residency  [                9993 ms]
C2                   : transition [               23581]
                       residency  [               11105 ms]
C3                   : transition [                2943]
                       residency  [                2094 ms]
C4                   : transition [                8525]
                       residency  [                8576 ms]
C5                   : transition [                5493]
                       residency  [               17883 ms]
C6                   : transition [               40001]
                       residency  [              224993 ms]
cc3                  : [                1946 ms]
cc6                  : [                7945 ms]
cc7                  : [              238808 ms]
]]>https://xcp-ng.org/forum/post/88339https://xcp-ng.org/forum/post/88339Fri, 17 Jan 2025 14:08:44 GMT<![CDATA[Reply to CPU C-states on Fri, 17 Jan 2025 13:21:05 GMT]]>@Andrew

ok that's interesting.

Which CPU is this on? AMD ? Intel ?
Did you specify any boot parameters in grub to unlock these C-States ?

]]>https://xcp-ng.org/forum/post/88333https://xcp-ng.org/forum/post/88333Fri, 17 Jan 2025 13:21:05 GMT<![CDATA[Reply to CPU C-states on Fri, 17 Jan 2025 13:15:17 GMT]]>@FrankAtHome Here's output on one of my XCP 8.3 servers that shows C0 to C4:

# xenpm get-cpuidle-states 0
All C-states allowed

cpu id               : 0
total C-states       : 5
idle time(ms)        : 1854395327
C0                   : transition [           827366196]
                       residency  [            26328742 ms]
C1                   : transition [           464493378]
                       residency  [            40172860 ms]
C2                   : transition [            97685021]
                       residency  [            43104018 ms]
C3                   : transition [            13728657]
                       residency  [            12613910 ms]
C4                   : transition [           251459140]
                       residency  [          1755416981 ms]
pc2                  : [           462155589 ms]
pc3                  : [            17985256 ms]
pc6                  : [           781427622 ms]
cc3                  : [            10502718 ms]
cc6                  : [          1685502286 ms]
]]>https://xcp-ng.org/forum/post/88332https://xcp-ng.org/forum/post/88332Fri, 17 Jan 2025 13:15:17 GMT<![CDATA[Reply to CPU C-states on Fri, 17 Jan 2025 12:44:46 GMT]]>Power management is a fairly complex thing in Xen. Adding @TeddyAstie in the loop if you have ideas.

]]>https://xcp-ng.org/forum/post/88330https://xcp-ng.org/forum/post/88330Fri, 17 Jan 2025 12:44:46 GMT