Database Reference
In-Depth Information
An SMP VM
with width = 4
An UP VM
with width = 1
vCPU0
vCPU1
vCPU0
vCPU2
vCPU3
Linux Red Hat
Solaris 10
Hypervisor
Hardware
FIGURE 16.16
SMP and UP VMs. A VM is defined whether it is SMP or UP by its width.
Width is the number of vCPUs in a VM. If the width of a VM = 1, the VM is UP, otherwise,
it is SMP.
Similar to a process on a general-purpose OS, a vCPU can be in different states
such as running, ready, and wait states. At a certain point in time, a vCPU can be
scheduled by the hypervisor at only a single core (akin to scheduling an OS process
at a core). For instance, a UP VM running on a host machine equipped with 2 × Xeon
5405 (i.e., total of 8 pCPUs) will run on only 1 of the 8 available cores at a time.
Inherently parallel workloads, such as MapReduce applications, prefer SMP VMs.
We next discuss how the hypervisor schedules vCPUs on pCPUs.
16.6.5 v
irtual
CPu s
CheDuling
anD
X
en
'
s
s
CheDulers
General-purpose OSs support two levels of scheduling, process and thread schedul-
ing. With a hypervisor, one extra level of scheduling is added, that is, vCPU schedul-
ing. The hypervisor schedules vCPUs on the underlying pCPU(s), thereby providing
each guest VM with a portion of the underlying physical processing capacity.
We
briefly
discuss two popular schedulers from Xen,
Simple Earliest Deadline
First
(SEDF) and
Credit Scheduler
(CS) [16,18]. As its name suggests, SEDF is sim-
ple, whereby only two parameters,
n
(the slice) and
m
(the period), are involved.
AVM(or domain
U
i
in Xen's parlance) can request
n
every
m
. SEDF specifies a dead-
line for each vCPU computed in terms of
n
and
m
. The deadline is defined as the
latest time
a vCPU should be scheduled within the period
m
. For instance, a domain
U
i
can request
n
= 10 ms and
m
= 100 ms. Accordingly, a vCPU at this domain can
be scheduled by SEDF as late as 90 ms into the 100-ms period, yet still meet its
deadline. SEDF operates by searching across the set of all runnable vCPUs, held in
a queue, and selecting the one with the earliest deadline.
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