Xen users - Jul 2010 - benchmarks + disappointing cstate results

Hello,

I do a lot of benchmarking on Xen/KVM and recently started a new set of
benchmark runs with
special attention to cstates. Since modern Intel CPUs need C3 state for their
turbo mode I played
with Xen option max_cstate=1 to check the effect. Normally you would expect a
drop of performance
with max_cstate=1 since it blocks the necessary precondition of Intel Nehalem to
enable turbo mode.
My tests show the opposite instead! Using max_cstate=1 has no effect or causes
increases in
performance instead. The conclusion: Xen cannot use turbo mode and even worse:
without max_cstate=1
you are penalized even more.

Regards Andreas

date: 22.07.2010
Supermicro X8SIL-F, BIOS 1.1, Intel Xeon X3450, 8 GB RAM reg ECC
benchmark: time emerge apache (apache 2.2.15 using Gentoo on x86_64)
xen with parameters "dom0_mem=1024m dom0_max_vcpus=2"
pvops0 kernel from 14.07.10 (Jeremy''s xen/stable-2.6.32.x)
xenified kernel 2.6.31-xen0-r14 (from Andrew Lyon)
xenified kernel 2.6.32-xen0-r3 (from Suse backported minimal)

native linux 2.6.31.12 with maxcpus=2
	real    0m41.193s
	user    0m41.479s
	sys     0m9.609s

2.6.32-xen0-r3 + xen 3.4.3, max_cstate=1
2.6.32-xen0-r3 + xen 3.4.2
2.6.32-xen0-r3 + xen-4.0.1_rc4, max_cstate=1
	real    0m53.122s
	user    0m41.795s
	sys     0m16.233s

2.6.31-xen0-r14 + xen 3.4.2
2.6.31-xen0-r14 + xen 3.4.2, max_cstate=1
	real    0m54.670s
	user    0m41.695s
	sys     0m18.757s

2.6.31-xen0-r14 + xen 3.4.3, max_cstate=1
	real    0m56.983s
	user    0m40.767s
	sys     0m20.597s

2.6.32.16-pvops0 + xen-4.0.1_rc4, max_cstate=1
2.6.32.16-pvops0 + xen-4.0.1_rc4
2.6.32.16-pvops0 + xen 3.4.3
	real    0m58.298s
	user    0m45.319s
	sys     0m30.126s

2.6.32-xen0-r3 + xen-4.0.1-rc4
	real    0m59.352s
	user    0m39.258s
	sys     0m20.405s

2.6.32-xen0-r3 + xen 3.4.3
	real    1m1.352s
	user    0m39.682s
	sys     0m21.645s

2.6.31-xen0-r14 + xen-4.0.1-rc4
2.6.31-xen0-r14 + xen 3.4.3
	real    1m9.633s
	user    0m41.735s
	sys     0m26.326s


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