CPU C-states
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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).
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).
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Power management is a fairly complex thing in Xen. Adding @TeddyAstie in the loop if you have ideas.
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@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] -
ok that's interesting.
Which CPU is this on? AMD ? Intel ?
Did you specify any boot parameters in grub to unlock these C-States ? -
@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] -
@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. -
@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.
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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
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