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Tried the qemu-kvm-1.5.3-69.el7.x86_64 also hit this issue, so clone it to qemu-kvm component to fix it.
host info:
# uname -r && rpm -q qemu-kvm
3.10.0-152.el7.x86_64
qemu-kvm-1.5.3-69.el7.x86_64
guest info:
rhel6.5 64bit GA
Results:
1.virtio_console module installed:
64K-write-sequence: 516 MBPS, 8261 IOPS
2.virtio_console module uninstalled(modprobe -r virtio_console):
64K-write-sequence: 565 MBPS, 9040 IOPS
Best Regards,
sluo
Close this one to focus on qemu-kvm-rhev (Bug 1140521) for below reasons:
1) dataplane in qemu-kvm-rhev should already solve this. The root cause is slow "ppoll" implementation of host kernel, which is O(n) to # of internal iohandlers in QEMU. The # of iohandlers increases greatly when virtio-serial is enabled.
2) To fix this, we'd need to introduce Linux "epoll" as a new event loop backend of QEMU. This is relatively an invasive change to RHEL7 qemu-kvm, also as a strict performance enhancement instead of a bug fix.
3) "epoll", as the long term solution for upstream and qemu-kvm-rhev, will firstly be applied on dataplane (iothread), or even just skip the main loop in the end.
If we want to fix this on RHEL 7, feel free to justify and reopen the bug - in which case we can consider the other fix: modify virtio-serial so it doesn't use all 64 virtqueues unconditionally.
Fam