Am 24.09.2015 um 03:34 schrieb Grzegorz Powiedziuk:
> That’s very interesting. I wasn’t aware of this “stress” tool. So I’ve
> downloaded it and run a couple tests with it.
> If I run basic —cpu 1 test (-n according to help is a dry run), the KVM
> server spins the CPU 100% in user time. So no stealing at all.
>
> Could you run a couple of tests like this (I am providing my own results):
>
> KVM server (2 CPU but it is one threaded task so doesn’t matter how many)
Does changing the KVM server to 1 CPU makes things better?
Having more than one CPU makes it harder for z/VM to virtualize
KVMs usage of SIE.
> # time for i in {1..500}; do dd if=/dev/zero of=/dev/shm/test bs=1M count=10
> ;echo interation $i done; done
> ….
> 10485760 bytes (10 MB) copied, 0.00273469 s, 3.8 GB/s
> interation 500 done
>
> real 0m2.223s
> user 0m0.171s
> sys 0m2.002s
>
> During the test (I’ve changed 1..500 to 1..5000 to have more time to catch
> top output) top was showing on average:
> %Cpu1 : 7.0 us, 83.7 sy, 0.0 ni, 7.0 id, 0.0 wa, 0.0 hi, 0.0 si, 2.3
> st
>
>
> KVM virtual machine(1 CPU adding CPUs will not make difference in this case):
> # time for i in {1..500}; do dd if=/dev/zero of=/dev/shm/test bs=1M count=10
> ;echo interation $i done; done
> 10485760 bytes (10 MB) copied, 0.0524781 s, 200 MB/s
> interation 500 done
[....]
> So it seems that when it is about only CPU the difference is very small or
> none. I am getting similar results if I do complicated equations with big
> numbers (virtual machines solves it almost in the same time as 1st level
> host).
>
> But when memory is involved, everything slows down drastically. I wonder what
> results you will get from the dd from /dev/zero to /dev/shm.
>
> And no, my system has plenty of memory, paging in z/VM is ZERO. Hardly
> anything runs on this LPAR.
> KVM host has plenty of real memory - 8G and the virtual machine is set to 4G.
> It still has few Gigs left. No swapping, nothing else runs here.
That certainly makes sense. Shared memory does use page protection for change
bit tracking. KVM does use page protection as well for dirty tracking of guests.
And if I remember correctly z/VM 6.3 also uses page table magic for
change/reference
tracking. This is all fine. Unless: z/VM provides shadow page tables for the
3rd
level guest backing of a 2nd level hypervisor. It therefore needs to
emulate/trap
most page table operations and page faults, which can be pretty expensive.
Christian
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