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Description of problem:
When running a virtual machine w/ QEMU-KVM which has more cores than one numa cell or is spread over several numa cells, the performance of the machine drops by ~50%.
This leaves us with a maximal VM size of 6 cores and 32GB memory (on an AMD MagnyCours system) which has a total of 48 cores and 256GB memory. Enterprise support should not be limited at these low boundaries.
Some sort of NUMA intelligence within the VM (or the QEMU process) would be required to solve this.
Version-Release number of selected component (if applicable):
RedHat 6.0
RedHat 6.1
qemu-0.12.1.2-2.113
(qemu-0.14.1 also tested)
How reproducible:
a.) Run an >6 core instance on a magny cours AMD system and within this VM run any kind of benchmark
b.) Run an >4 core instance on an Intel Nehalem system and within this VM run any kind of benchmark
c.) Run an unpinned VM on any numa enabled system and within this VM run any kind of benchmark
Steps to Reproduce:
1.
2.
3.
Actual results:
runtime performance of benchmarks (like stream) or heavy computations using the whole memory in the unpinned VM drops by >50%
Expected results:
a.) The VM is numa aware and therefore knows itself how to handle numa affinities
b.) The QEMU KVM process handles the NUMA logic (supposedly inefficient?)
Additional info:
different internal computations were done. parts of the results can be revealed on request.
We support numa pinning in kvm so one can use libvirt and pin the vcpus/memory to the right nodes. The numa topology can be exposed to the guest as well using -numa command line option.
Automatic numa allocation and migration of memory w/o pinning is planned for further releases. We have folks working on userspace daemon that load balance memory and match it against the physical numa topology and for rhel7 we're working on a similar solution on the kernel level. That's why I'm inclined to close this issue since it will be solved by the above options.