[Qemu-devel] Performance Profiling 2 VMs

[kalyan tata <manjulakalyan.tech@gmail.com>]

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Hi All,

I am new to qemu development.
Sorry If this is not the correct forum for this question, it would be great
if you could direct me to correct forum.

I am seeing very low virtio network throughput on an older (2.6.18) linux
guest  vs another newer guest (3.10) both running on the same host. (same
config 2 vcpus, no multi Q etc.)  I see very high CPU usage on the 2.6.18
guest at very low network throughput and want to profile to find
bottleneck.

I tried to use "perf kvm" but the analysis  shows overhead as  max .25 %
where as top in VM shows 100% cpu. (I used following as a guide
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html-single/Virtualization_Tuning_and_Optimization_Guide/index.html#sect-Virtualization_Tuning_Optimization_Guide-Monitoring_Tools-perf_kvm
)

     0.25%  :5235    [uhci_hcd]        [g] 0xffffffff80182236
     0.24%  :5235    [uhci_hcd]        [g] 0xffffffff8018226a
     0.23%  :5235    [virtio_ring]     [g] vring_new_virtqueue
     0.20%  :5236    [uhci_hcd]        [g] 0xffffffff80182236
     0.18%  :5236    [uhci_hcd]        [g] 0xffffffff8018226a
     0.18%  :5235    [uhci_hcd]        [g] 0xffffffff8016f385
     0.14%  :5236    [uhci_hcd]        [g] 0xffffffff802fbe0f
     0.14%  :5235    [uhci_hcd]        [g] 0xffffffff8001161a
     0.14%  :5235    [virtio_ring]     [g] virtqueue_is_broken


My basic question is - Is there a way I can profile the older version of
linux guest so i can see the bottleneck (where the guest is spending CPU
cycles) My aim is to see if i can patch the older version in the critical
path with improvements made in newer version

Thanks
Kal

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