Am 12.02.2013 14:37, schrieb Gordan Bobic: > On 12/02/2013 13:24, Reindl Harald wrote: >> that just tells that you can disable a lot of services >> and overhead in a VM you would never do on bare metal >> and it tells that the hypervisor can schedule IO much >> more efficient as a generic kernel without less overhead > > Utter nonsense. On bare metal, the Linux kernel scheduling even with full awareness of the underlying CPU cores is > pretty poor. C2Q is particularly good example of this because latency and cache misses between the two sets of 2 > cores causes a 20%+ drop in throughput compared to pinning heavy processes to a specific core. Systems with > multiple sockets also suffer from this issue particularly badly. and that is why you use "elevator=noop" inside a VM and disable as most scheduling in the guest as possible > Now consider that you are effectively hiding the physical CPU layout behind a hypervisor that applies it's > smoke-and-mirrors and makes it even harder for the guest kernel to do something reasonably sensible - so you get > another 20%+ overhead on top from the extra cache misses, extra context switching overheads what the hell - the guest does not need to do "something reasonably sensible" if you are doing things right because only the hypervisor can schedule hardware ressources and with enough RAM on the host there is a lot to optimize > So much experience, so little understanding... oh yeah that is why i disable most subsystems in the guest-kernel since years which produce unneeded overheads in a guest and not implemented in a dedicated hypervisor running on bare-metal or if present optimized for virtualization and nothing else because the VMkernel has only one job instead implement a beast adressing any sort of workloads with overheads
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