On Thursday, 09/24/2015 at 03:53 EDT, Christian Borntraeger
<borntrae...@de.ibm.com> wrote:
> z/VM does call SIE on behalf of the guest hypervisor. So for CPU bound
> workload, which causes almost no SIE exits things are fine. It is the
sweet spot for 2nd level. As
> soon as the KVM guest will have many exits (e.g. some I/O, memory
fault-in, reschedule) or more
> than one CPU > in the KVM host things can get really slow as z/VM then
has to interpret lots
> of things.
>
> In addition z/VM 2nd level support was in no way optimized to speed up a
KVM
> hypervisor, so
> I assume that some of the optimizations for z/VM under z/VM have to fall
back
> to the slow path.
To the best of my knowledge, there are no optimizations for for 2nd level
z/VM, since "SIE is SIE". (SIE is the name of the instruction that
implements the Interpretive Execution Facility.)
I call this the "SIE Instruction Pancake Effect". Each SIE that issued
in a vertical software stack will "pancake" down until there is hardware
to run it. But as it pancakes, the "sphere" of memory and CPU resources
around the SIE instruction gets smaller and smaller. If the host reaches
outside the sphere, the instruction ends ("SIE exit").
If the sphere was small to begin with, you will see the least effect. If
the sphere is large, then the Pancake Effect is very noticeable as SIE
repeatedly exits, taking many more virtual SIE instructions to accomplish
the same thing as would a single real SIE instruction.
Alan Altmark
Senior Managing z/VM and Linux Consultant
Lab Services System z Delivery Practice
IBM Systems & Technology Group
ibm.com/systems/services/labservices
office: 607.429.3323
mobile; 607.321.7556
alan_altm...@us.ibm.com
IBM Endicott
----------------------------------------------------------------------
For LINUX-390 subscribe / signoff / archive access instructions,
send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit
http://www.marist.edu/htbin/wlvindex?LINUX-390
----------------------------------------------------------------------
For more information on Linux on System z, visit
http://wiki.linuxvm.org/
