[Xen-devel] Impact of P/C-states on server power with xen-unstable

[Niraj Tolia <ntolia@xxxxxxxxx>]

Hi,

So, I was playing with C-states (with and without cpuidle on the xen command line) on Intel Xeons and I discovered that turning C-state support on/off makes no difference in terms of the power used by an idle server.

The processor in question is a Xeon X5355 and, when cpuidle is enabled, xenpm reports a C1 state and will show that the server is ~99.96% resident in C1 when idle. However, the power consumed by a idle server in C1 is virtually identical to that consumed by the server when C-state support is disabled and the idle server is in either P1 or even P0 (both set manually using the userspace governor and xenpm set-scaling-speed). Is the processor doing something that is not exposed to Xen/xenpm? For what it's worth, the power used by the server does show a significant difference between 100% busy CPUs at P0 and P1.

The relevant 'xm dmesg' output for the first core is included below. I am running xen-unstable changeset 19104/31983c30c460.

Cheers,
Niraj


(XEN) cpu0 cx acpi info:
(XEN)     count = 1

(XEN)     flags: bm_cntl[0], bm_chk[0], has_cst[0],
(XEN)            pwr_setup_done[1], bm_rld_set[0]
(XEN)     states[0]:
(XEN)         reg.space_id = 0x0
(XEN)         reg.bit_width = 0x0
(XEN)         reg.bit_offset = 0x0

(XEN)         reg.access_size = 0x0
(XEN)         reg.address = 0x0
(XEN)         type    = 1
(XEN)         latency = 0
(XEN)         power   = 0
(XEN)         dp(@0x00000000)
(XEN) ==cpu0==
(XEN) active state:        C-1

(XEN) max_cstate:        C7
(XEN) states:
(XEN)     C1:    type[C1] latency[000] usage[00000000] duration[0]
(XEN)     C0:    usage[00000000] duration[5708043748]
(XEN) xen_pminfo: @acpi_cpufreq_cpu_init,SYSTEM IO addr space

(XEN) CPU 0 initialization completed

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