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When I run iperf as a client on a CentOS 5 VM running under vmware, why do I see such high %si values in top? We have more experience running Xen and XenServer, and a almost identical centos install under Xen shows only ~0.5%si while running iperf. All tests run with 1Gb network, and the numbers we are getting from iperf are good/fine (900+ Mb).

top - 22:34:59 up 7 min,  2 users,  load average: 0.11, 0.34, 0.21
Tasks: 115 total,   1 running, 114 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.1%us,  2.6%sy,  0.0%ni, 77.4%id,  0.0%wa,  8.9%hi, 11.1%si,  0.0%st
Mem:   8174464k total,  2038784k used,  6135680k free,  1754068k buffers
Swap:  2096472k total,        0k used,  2096472k free,   132820k cached

Details on config:

  • Minimal Centos 5 x86_64 with new-ish kernel
  • OpenVM tools on vmware VM, kernel-xen on Xen vm
  • 1Gb network on the back end in all instances, performance is good
  • VMware ESXi 4.1.x, Xen 3.4.0
  • VMware host tried with both vmxnet2 and vmxnet3 3 adapters, similar results (vmxnet3 was actually higher %si)
  • Little to no other load on the VM's involved
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