I wonder how the benchmarks would be affected by lowering the number of virtual cores assigned to the VMs. At least with VMware products, especially hosted ones, it's often recommended to never set the number of virtual cores equal to or more than the number of physical cores (ie. not counting hyper-threading logical cores). Some (many?) workloads supposedly get better performance by doing this.
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VirtualBox Is Still Running Slower Than QEMU-KVM
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>>HPC Challenge wouldn't build under QEMU-KVM since the reported CPU string was unrecognized as "QEMU Virtual 1.7.0"
Just use -cpu host as cpu model. If not, you don't use most cpu features (sse2,ss3,...). It should give you a big difference if benchmark use theses cpu features.
(It could be great to have the full qemu command detail, to see which options are used for cpu, disks,....)
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Originally posted by vick View PostI wonder how the benchmarks would be affected by lowering the number of virtual cores assigned to the VMs. At least with VMware products, especially hosted ones, it's often recommended to never set the number of virtual cores equal to or more than the number of physical cores (ie. not counting hyper-threading logical cores). Some (many?) workloads supposedly get better performance by doing this.
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Originally posted by GreatEmerald View PostLeaving one core unassigned means it can be dedicated to the host, and if the guest decides to eat all your resources, you will still be able to stop the machine instead of doing a hard reboot. I think that lowers the performance of the guest, but gives you more reliability and more performance for the host.
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Originally posted by GreatEmerald View PostLeaving one core unassigned means it can be dedicated to the host, and if the guest decides to eat all your resources, you will still be able to stop the machine instead of doing a hard reboot. I think that lowers the performance of the guest, but gives you more reliability and more performance for the host.
It'd be nice if Phoronix tested this less cores sometimes gives better performance even on an idle host advice (sometimes even given by VMware engineers themselves) commonly given to VMware users in a more formal testing environment.Last edited by vick; 10 January 2014, 12:18 PM.
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I've also read from VMware's official support pages and community forums that it's expected to lose performance when assigning virtual cores to be equal to or more than the number of physical cores on hosted VMs.
Even VirtualBox recommends something similar: https://www.virtualbox.org/manual/ch...ings-processor
You should not, however, configure virtual machines to use more CPU cores than you have available physically (real cores, no hyperthreads).
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Originally posted by TAXI View PostWhy is everybody wondering about the slowdown with equal or more cores? The emulation itself needs CPU power, too, so if the guest tries to eat all up a slowdown is to be expected.
I'm questioning if these tests are accurate comparisons at all. It's like setting up the tests so that VMs specifically look bad.Last edited by vick; 11 January 2014, 01:19 PM.
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Originally posted by TAXI View PostWhy is everybody wondering about the slowdown with equal or more cores? The emulation itself needs CPU power, too, so if the guest tries to eat all up a slowdown is to be expected.
I also would like VM testing changed to add additional test points where they're configured with 1 core, 2 or more but less than the number of physical cores, equal to the number of physical cores and equal to the number of logical cores (ie. include hyper-threading). It could be that single-threaded performance in a 1 core configuration be higher than in a multi-core one. It would be nice to know this if a VM's single-threaded workload is more important than its multi-threaded ones.
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