View Only

VMkernel Scheduler 

May 27, 2008 10:56 PM


Details on the ESX Server scheduler are commonly requested when I engage  customers and partners.  People want to know more about how the  scheduler works, when SMP should be used, and what the deal is with SMP  co-scheduling.  This page will answer these questions and others as they  arise in the forum or the discussion portion of this page.

Terminology and Architecture

In VMware parlance, the monitor is the part of our products that  provides a virtual interface to the guest operating systems.  The  VMkernel is the part of our products that manages interactions with the  devices, handles memory allocation, and schedules access to the CPU  resources, among other things.  This is shown in the following figure.

This document will provide information on one part of the VMkernel: the scheduler.

Performance Scaling and the Scheduler

It is a critical requirement for enterprise deployments that an  operating system provide fast and fair access to the underlying  resources.  As a critical part of this design, the scheduler has  undergone countless engineer-years of development to guarantee that this  requirement is met.  We've now released dozens of papers showing linear  scaling of workloads as vCPU count is scaled up within a single VM and  VM count is scaled up within a single host.  Here are a few such papers  that contain supporting data.

  1. VM scaling as demonstrated by Oracle databases.
  2. VM and vCPU scaling under IBM DB2 load.
  3. VM and vCPU scaling with SQL Server running in the VM.

The scheduler's ability to fairly scale up to and beyond totally  committed CPU resources is no accident.  In fact, in a conversation I  had with a QA manager I was assured that the VMkernel's scheduler would  fairly distribute CPU resources to all VMs at least up to 4x CPU  overcommitment.  Of course, on a system with the CPU over-committed by  4x each VM will only run at 1/4 native speed but the scheduler keeps the  VMs running at that performance.  Not one at 1/8 speed, one at 1/10  speed, and another at 1/4 speed.

SMP and the Scheduler

As ESX Server supports uniprocessor (UP) and symmetric multiprocessor  (SMP) VMs, the fair-and-fast requirement for the scheduler must be  upheld in the presence of concurrently executing UP and SMP VMs.   Internal testing of this requirement shows fair scheduling even in the  presence of concurrently executing 1-way, 2-way, and 4-way VMs.

In fact, the ability to fairly execute under such environments is a very  tricky problem for a scheduler.  We've run analysis on competitors'  products and found that the ability to fairly balance differently-sized  VMs is something of which ESX Server alone is capable.  Stay tuned in  the coming months as we back this claim up with performance data.

Cell Size

One construct that assists the scheduler in optimally placing VMs on a  heavily utilized system is a cell.  A cell is a logical grouping of a  subset of CPU cores in the system.  In ESX 3 versions the cell size is  equal to four.  Since the cell is statically assigned to physical cores,  this means that each four-core processor is in exactly one cell.  When  only dual-core processors are present, a cell is comprised of two  sockets.  The most important thing to know about cells is the following:

A VM cannot span more than one cell.

This means that four-way VMs run on only one socket at a time in systems  with quad-core CPUs.  For this case, the number of options presented to  the scheduler is equal to the number of sockets.  In future versions of  ESX we plan to increase the cell size to eight.  In some cases (such as  systems with hexa-core CPUs) a modification of the cell size can  improve performance.  See KB article 1007361 for more information.

UP or SMP?

When and if to use SMP is a common question from VMware users.  The  simple answer to this is to only use SMP when needed.  Why only use SMP  when needed?  There are two reasons:

  1. SMP schedulers are less efficient than UP schedulers.  This is a  simple experiment that can be confirmed with trivial benchmarks like  Netperf or Passmark.  On UP systems (either virtual or native) the UP  hardware abstraction layer (HAL) will provide marginally better results  than the SMP HAL.
  2. Even when unused, virtualization of idle vCPUs requires resources by  the kernel.  Memory is needed to maintain data structures and CPU  resources are needed to virtualize the idle system.  The amount of work  needed to support an idle CPU varies greatly but usually is in the realm  of 1-2% of a single CPU core.
  3. The work required to deliver timer interrupts increases  quadratically with the number of vCPUs, like RHEL5, the number of timing  interrupts delivered by the VMkernel can be quite high.  See Red Hat Enterprise Linux for more information on this issue.

What About Co-scheduling?

Back in the days of ESX Server 2.5, SMP VMs had to have their vCPUs  co-scheduled at the same instant to begin running.  Because only 2-way  VMs were supported at this time, that meant that two CPU cores had to be  available simultaneously to launch a 2-way VM.  On a server with a  total of only two cores, this meant that the VM could not be launched  concurrently with any other process on the server.  This would include  the service console, the web interface, or any other process.

This requirement was reduced in ESX Server 3.0 through a process called  relaxed co-scheduling.  Effectively SMP VMs can have their vCPUs  scheduled at slightly different times and idle vCPUs didn't necessarily  have to be scheduled concurrently with running vCPUs.  More details on  this are available in the Co-scheduling SMP VMs in VMware ESX Server page. 

NUMA Considerations

Support for non-uniform memory access (NUMA) architectures was  introduced in ESX Server 2.  This meant that the scheduler became aware  that memory was not uniform across each CPU.  Each CPU node had access  to its own local memory and a larger pool of remote memory (which was  divided as local memory for the other CPU nodes.)  Memory access to  local memory is much faster than remote memory so the scheduler should  favor the placement of processes on nodes that held the processes'  memory.

Subsequent generations of ESX Server continued to optimize for the use  of NUMA memory.  This included placement of vCPUs next to needed memory  and startup of VMs at NUMA nodes with resources available for execution.   All of this is transparently handled by the scheduler but it should be  noted that the newer your version of ESX Server, the better its NUMA  scheduling is.

0 Favorited
0 Files

Tags and Keywords


Jul 31, 2009 07:06 PM

So it is not possible for virtual machine have more cpu resources than physical ESX server on which VM is residing has?

Jul 31, 2009 04:08 PM

A virtual machine can only use 100% of the number of vCPUs provided to it. Increasing limits above this numberif this is even possiblewould not provide a performance gain.


More information on my communities blog and on Twitter:

Jul 31, 2009 11:04 AM

Where to get information of how VMKernel works if VM are assigned of more resources than one physical ESX has? For example cluster is 2 ESX each 2ghz quad core and you reserve 10 ghz for VM in a cluster.

Jul 07, 2009 04:33 PM

Two weeks.

More information on my communities blog and on Twitter:

Jul 07, 2009 09:01 AM

Hi Scott,

Do you have an ETA on the vSphere scheduler document? Something definitive that shows the benefits of vSphere would be very much appreciated.

Many thanks,


Jun 25, 2009 10:39 AM

The count of 1820 possibilities considers individual vCPU mappings to specific cores. This means that a system with only two cores has two ways to place a 2-way VM. But there is no performance difference in these two positions so I eliminate the second choice, which flips the vCPU to CPU mapping. I think that both documents are correct.

I'll update this document for ESX 4 once we release our vSphere scheduler document. In short, there is no longer a cell with VMware vSphere.


More information on my communities blog and on Twitter:

Jun 23, 2009 10:42 AM

Hi Scott,

Can you please update this docnote to explain how the cell system works in ESX4?

Also this OFFICIAL VMWARE document,, on Page 4, states that there are 1820 ways of scheduling a 4 vCPU workload on a 16 core ESX3 system, can you please confirm that this is patently incorrect.



Oct 30, 2008 04:36 AM


Thanks for this document. I think it will be very helpful over time. Keep up the good work!


Ken Cline

Technical Director, Virtualization

Wells Landers

TVAR Solutions, A Wells Landers Group Company

VMware Communities User Moderator

Related Entries and Links

No Related Resource entered.