CPU Archives - Page 2 of 2 - frankdenneman.nl

Contention on lightly utilized hosts

April 27, 2011 by frankdenneman

Often I receive the question why a virtual machine is not receiving resources while the ESXi host is lightly utilized and is accumulating idle time. This behavior is observed while reviewing the DRS distribution chart or the Host summary tab in the vSphere Client.
A common misconception is that low utilization (Low MHz) equals schedule opportunities. Before focusing on the complexities of scheduling and workload behavior, let’s begin by reviewing the CPU distribution chart.
The chart displays the sum of all the active virtual machines and their utilization per host. This means that in order to have 100% CPU utilization of the host, every active vCPU on the host needs to consume 100% of their assigned physical CPU (pCPU). For example, an ESXi host equipped with two Quad core CPUs need to simultaneously run eight vCPUs and each vCPU must consume 100% of “their” physical CPU. Generally this is a very rare condition and is only seen during boot storms or incorrect configured scheduled anti-virus scanning.

But what causes latency (ready time) during low host utilization? Lets take a closer look at some common factors that affect or prohibit the delivery of the entitled resources:

Amount of physical CPUs available in the system
Amount of active virtual CPUs
VSMP
CPU scheduler behavior and vCPU utilization
Load correlation and load synchronicity
CPU scheduler behavior

Amount of physical CPUs: To schedule a virtual CPU (vCPU) a physical (pCPU) needs to be available. It is possible that the CPU scheduler needs to queue virtual machines behind other virtual machines if more vCPUs are active than available pCPUs.
Amount of active virtual CPUs: The keyword is active, an ESXi host only needs to schedule if a virtual machine is actively requesting CPU resources, contrary to memory where memory pages can exist without being used. Many virtual machines can run on host without actively requesting CPU time. Queuing will be caused if the amount of active vCPUs exceeds the number of physical CPUs.
vSMP: (Related to the previous bullit) Virtual machines can contain multiple virtual processors. In the past vSMP virtual machine could experience latency due to the requirement of co-scheduling. Co-scheduling is the process of scheduling a set of processes on different physical CPUs at the same time. In vSphere 4.1 an advanced co-scheduling (relaxed co-scheduling) was introduced which reduced the latency radically. However ESX still needs to co-schedule vCPUs occasionally. This is due to the internal working of the Guest OS. The guest OS expects the CPUs it manages to run at the same pace. In a virtualized environment, a vCPU is an entity that can be scheduled and unscheduled independently from its sibling vCPUs belonging to the same virtual machine. And it might happen that the vCPUs do not make the same progress. If the difference in progress of the VM sibling vCPUs is too large, it can cause problems in the Guest OS. To avoid this, the CPU scheduler will occasionally schedule all sibling vCPUs. This behavior usually occurs if a virtual machine is oversized and does not host multithreaded applications. The “impact of oversized virtual machine series” offer more info on right-sizing virtual machines.
CPU scheduler behavior and vCPU utilization: The local-host CPU scheduler uses a default time slice (quantum) of 50 milliseconds. A quantum is the amount of time a virtual CPU is allowed to run on a physical CPU before a vCPU of the same priority gets scheduled. When a vCPU is scheduled, that particular pCPU is not useable for other vCPUs and can introduce queuing.
A small remark is necessary, a vCPU isn’t necessarily scheduled for the full 50 milliseconds, it can block before using up its quantum, and reducing the effective time slice the vCPU is occupying the physical CPU.
Load correlation and load synchronicity: Load correlation defines the relationship between loads running in different machines. If an event initiates multiple loads, for example, a search query on front-end webserver resulting in commands in the supporting stack and backend. Load synchronicity is often caused by load correlation but can also exist due to user activity. It’s very common to see spikes in workload at specific hours, for example think about log-on activity in the morning. And for every action, there is an equal and opposite re-action, quite often load correlation and load synchronicity will introduce periods of collective non-or low utilization, which reduce the displayed CPU utilization.
Local-host CPU scheduler behavior: The behavior of the CPU scheduler can impact the on scheduling of the virtual CPU. The CPU scheduler prefers to schedule the vCPU on the same pCPU it was scheduled before, to improve the chance of cache-hits. It might choose to ignore an idle CPU and wait a little bit so it can schedule the vCPU on the same pCPU again. If ESXi operates on a “Non-Uniform Memory Access” (NUMA) architecture, the NUMA CPU scheduler is active and will have effect on certain schedule decisions. The local host CPU scheduler will adjust progress and fairness calculations when Intel Hyper Threading is enabled on the system.
Understanding CPU scheduling behavior can help you avoid latency, although understanding workload behavior and right sizing your virtual machines can help to improve performance. Frankdenneman.nl hosts multiple articles about the CPU scheduler and can be found here, however the technical paper “vSphere 4.1 CPU scheduler” is a must read if you want to learn more about the CPU scheduler.

Beating a dead horse – using CPU affinity

January 11, 2011 by frankdenneman

Lately the question about setting CPU affinity is rearing its ugly head again. Will it offer performance advantages for the virtual machine? Yes it can, but only in very specific cases. Additional settings and changes to the virtual infrastructure are required to obtain a performance increase over the default scheduling techniques. Setting CPU affinity by itself will not result in any performance gain, but usually a performance decrease.

What does CPU affinity do?
By setting a CPU affinity on the virtual machine you are limiting the available CPUs on which the virtual machine can run. It does not dedicate that CPU to that virtual machine and therefore does not restrict the CPU scheduler from using that CPU for other virtual machines.

When will CPU-affinity help?
Under a controlled environment some specific workloads can benefit from using CPU affinity. When the virtual machine workload is cache bound and has a larger cache footprint than the available cache of one CPU it can profit from aggregated caches. However, if this workload has high intra-thread communications and is running on specific CPU architectures setting CPU affinity can have the opposite effect and become detrimental to the performance of the application.

CPU-affinity can also be used to isolate a physical CPU to a virtual CPU. But requires a lot of changes and increases management. It will never dedicate the physical CPU to the virtual machine as the VMkernel schedules all its processes across all available CPUs regardless of any custom setting a virtual machine has. Furthermore the scheduling overhead stays the same whether CPU-affinity is set on the virtual machine or not.

To determine if you application fit this description can be a challenge and maintaining such configurations usually result in a nightmare. Generally CPU-affinity is only used for simulations and load testing and it is better left unused for every other cases. Setting CPU-affinity results in less choice for the CPU scheduler to schedule the virtual machine, but there is more to it as well:

Controlled environment
Already mentioned but this cannot be stressed enough, CPU affinity does not equal isolation of a physical CPU. In other words, when a virtual machine is pinned to a physical CPU it does not control or own that CPU. The VMkernel CPU scheduler still considers that physical CPU a valid CPU to schedule other virtual machines on. If isolation of a CPU is the end-goal, than all other residing virtual machines on the host (and virtual machine that will be created in the future) must be configured with CPU affinity as well and the specific CPU(s) assigned to the virtual machine must excluded from all other virtual machines.

Setting CPU affinity results in manual CPU micro management and can be a nightmare to maintain. To make it worse, think of the impact a migration will have, the administrator needs to configure the virtual machines on the destination host to exclude the CPU from all active virtual machines as well.

(Update: Recent vSphere versions offer the “Latency Sensitive” functionality, isolating cores for vCPUs)

Virtual Machine worlds
A virtual machine is made of multiple worlds (threads), besides the vCPU world, worlds are active for the virtual machine MKS subsystem, CD-ROM and VMX file. Although the vCPU world generates the greater part of the CPU load, sometimes a physical CPU is required to run the other worlds. If CPU affinity is set, then all the worlds that constitute the virtual machine can only run on the specified CPUs. If set incorrectly, it can reduce the throughput of the virtual machine as the worlds must compete between each other for CPU time. Therefore it is recommended to add an additional CPU for these worlds. For example; configure a CPU affinity setting that contains 3 physical CPUs for a 2 vCPU virtual machine.

Resource entitlements
As CPU affinity will not automatically isolate the CPU for that specific virtual machine, shares and reservations needs to be set to guarantee a specific performance level. Because the scheduler will attempt to maintain fairness for all virtual machines it is possible that other virtual machines will be scheduled on the set of CPU specified in the affinity set of the virtual machine. Adjust the shares and reservations of the virtual machine accordingly to ensure priority over other active virtual machines. Be aware that CPU reservations are friendly; although the vCPU is guaranteed a specific portion of physical resources, it might happen that an external thread/interloper (other virtual machine) is using the vCPU; this thread will not instantly be de-scheduled. Even when the waiting virtual machine has a 100% CPU reservation configured.

To make it worse, in the case when multiple virtual machines are affinity-bound to the same processor it is possible that the CPU scheduler cannot meet the specified reservation. Be aware that admission control ignores affinity, so multiple virtual machines can have a full reservation equal to a full core but still need to compete with other affinity bound virtual machines. More information about how CPU reservations work can be found in the article: “Reservations and CPU Scheduling”.

CPU reservations and HA admission control
If the virtual machine with the reservation is running in a HA cluster with a “Host failures cluster tolerates” admission control policy, the CPU reservation will influence the Slot size of the Cluster and can therefore impact the consolidation ratio of the cluster. More info about slot-sizes can be found on the HA deepdive.

CPU affinity and DRS clusters.
Because vMotion is not allowed if a virtual machine is configured with CPU affinity, that virtual machine cannot be placed in a DRS cluster with automation mode set to fully automated. If a virtual machine needs to be configured with CPU affinity, the administrator has three choices:

Place the virtual machine on a stand-alone host

Set DRS automation level to manual / partially automated

Set virtual machine automation mode to manual / partially automated

Stand-alone host
If the virtual machine is placed on the stand-alone host the performance of the virtual machine depends on the level of contention and the virtual machine resource entitlement. During resource contention it can only fall back on its resource entitlement and hopefully gain a higher priority than the other residing virtual machines. If the virtual machine was located on an ESXi host in a DRS cluster, the virtual machine could have been migrated to receive its resource entitlement on another host. By choosing CPU-affinity, you are betting only on one horse, the local CPU scheduler of one host instead of leveraging the full suite of resource management vSphere delivers today.

DRS set to Manual or partially automated
If the DRS automation level is set to manual or partially automated, the cluster will not automatically load balance virtual machines and DRS will recommend migrations. These recommendations must be applied manually by the administrator. DRS imbalance calculation will be invoked every 300 seconds but is also triggered if the cluster detects resource demand and supply changes, as well as changes in the resource settings in the cluster. As you can imagine, this behavior will create an incredible load on the administrator to let the cluster operate as efficiently as possible if he wants to ensure that the virtual machines are receiving their resource entitlements.

Set Virtual machine automation mode to manual / partially automated
By changing the automation mode on VM-level, the virtual machine can still be placed inside a fully automated DRS cluster. Although DRS will not automatically migrate this virtual machine, it can migrate other virtual machines to ensure every virtual machine will receive its resource entitlement. However additional measures (shares and reservations) must be taken to guarantee the virtual machine enough physical resources.

CPU architectures
Today new CPU architectures, such as the Intel Nehalem and AMD Opteron’s offer a variety of on-die caches, multiple cores \ logical CPUs and an optimized local\remote memory subsystem. These features can either helpful or be detrimental to the performance of a virtual machine with CPU affinity.

Cache level
If a virtual machine is spanned across two processors (packages) it effectively results in having two L3 caches available to the virtual machine. Today’s CPU architectures offer dedicated L1 and L2 cache per core and a shared last-level L3 cache for all cores inside the CPU package. Because access to Last level cache is faster than (normal) memory, it makes sense to span the virtual machine across two processor packages to increase the amount of available L3 cache.

However the inter-socket communication speed can reduce –or remove- the positive effect of having low-latency cache available and if the workload can fit inside one cache (small cache footprint) and uses intensive intra-thread communication, than placement in one processor packaged is to be preferred over spanning multiple packages.

HyperThreading
If a virtual machine is running on a HyperThreading-enabled system it is best to set the CPU-affinity to logical CPUs not belonging to the same core. The HT threads on a core are translated by the VMkernel as logical CPUs and are consecutively numbers, for example Core 1 contains LCPU0 and LCPU1, Core 2 contains LCPU2 and LCPU3, etc. If CPU-affinity is set to logical CPUs belonging to the same core, both vCPUs of the virtual machine need to compete with each other for physical CPU resources. By scheduling a virtual machine on logical CPUs of different cores, it doesn’t have to compete and can benefit the vCPUs’ throughput because the VMkernel allows the vCPU to use the entire Cores’ resources if only one logical CPU residing on the core is active.

NUMA
If CPU affinity is set on a virtual machine running in a NUMA architecture (Intel Nehalem and AMD Opteron) the virtual machine is treated as a NON-NUMA client and gets excluded from NUMA scheduling. Therefore the NUMA scheduler will not set a memory affinity for the virtual machine to its current NUMA node and the VMkernel can allocate memory from every available NUMA node in the system Therefore the virtual machine may end up running on a different NUMA node than were its memory is residing, resulting in unnecessary memory latency and possibly higher %Ready time as the instruction must wait until the memory is fetched from a remote node.

Bottomline
The bottomline is that almost in every case CPU affinity is better left unused. Scheduling threads is very complex, scheduling threads belonging to multiple virtual machines with different priorities, activity, progress and still considering optimal use of the underlying CPU and memory architecture is mind-blowing complex. The CPU scheduler is aware of all these components and together with the global scheduler (DRS) it can see to it that the virtual machine will receive its resource entitlement. If the virtual machine must have access to physical resources at any time, other mechanisms such as resource allocation settings will have a better effect than using the advanced setting CPU-affinity.

Impact of oversized virtual machines part 3

January 3, 2011 by frankdenneman

In part 1 of the series of post on the impact of oversized virtual machines NUMA architecture, memory overhead reservation and share levels are reviewed, part 2 zooms in on the impact of memory overhead reservation and share levels on HA and DRS. This part looks at CPU scheduling, memory management and what impact oversized virtual machines have on the environment when a bootstorm occurs.
Multiprocessor virtual machine
In most cases, adding more CPUs to a virtual machine does not automatically guarantee increase throughput of the application, because some workloads cannot always take advantage of all the available CPUs. Sharing resources and scheduling these processes will introduce additional overhead.
For example, a four-way virtual machine is not four times as productive as a single-CPU system. If the application is unable to scale than the application will not benefit from these additional available resource.

Progress
Although relaxed co-scheduling reduces the requirement of the VMkernel to simultaneous schedule all vCPUs of the virtual machine, periodically scheduling the unused or idle vCPUs is still necessary to keep the progress of each vCPU in the virtual machine acceptably synchronized.

Esxtop also gives scheduling stats for SMP virtual machines;

%CRUN: All VCPUs want to run at once. CRUN is the amount of time between when a PCPU is told to run a certain VCPU on an SMP VM and when it is actually able to run that VM. This should be almost 0.

%CSTOP: If a VCPU gets ahead of another VCPU of the same SMP VM, then we ask the faster VCPU to stop until the other one can catch up. The time spent in this stopped state is CSTOP.

Single thread application
Only applications with multiple threads and allow them to be scheduled in parallel can benefit from multiprocessor systems. A single-threaded application can only be scheduled on one CPU at the time and will not benefit from the multiple CPUs available. The Guest OS is able to migrate the thread between the available CPUs, introducing unnecessary overhead such as interrupts or context switches and cache misses.

Timer interrupts
In older guest operating systems, the unused virtual CPUs still take timer interrupts, which consumes a small amount of additional CPU. Please refer to KB articles “High CPU Utilization of Inactive Virtual Machines – KB1077”

Configured memory
Oversizing the memory configuration of a virtual machine can impact the performance of the virtual machine itself or even worse, impact the other active virtual machines on the host and in the cluster. Using memory reservations on oversized virtual machines will make it go from bad to worse.

Application memory management
Excess memory is a problem when the application uses this memory opportunistically, in other words the application is hoarding memory. Java, SAP and often Oracle workloads assume it can use all the memory it detects. Because ESX cannot determine which memory is important to the virtual machine, it always backs memory pages of the virtual machine with physical pages. Besides creating a large memory footprint on the physical level, these kinds of applications add a third level of memory management as well.

Due to this additional management level, the Guest OS does not understand which pages are important and which are not. And because the Guest OS isn’t aware, it can not return inactive pages to the balloon driver when requested, therefor impacting the performance of the application during contention even more.

Setting memory reservation at virtual machine level will guarantee the availability of physical memory and will secure a certain level of application performance (if memory bound). However setting memory reservations at virtual machine level will impact the virtual infrastructure and the larger the memory reservation, the larger the impact. Visit “Impact of memory reservation” for more info.

To avoid these effects, it is recommended to monitor the behavior of the application over time and tune the configuration of the virtual machine and its reservation to get proper performance and limit the impact of its configured memory and the memory reservation.

NUMA node
If the virtual machines mentioned in the previous paragraph are configured with more memory than available in their home NUMA node, the system needs to fetch the memory from remote NUMA nodes. Accessing memory from remote nodes introduces latencies and generally reduced throughput of the vCPU. ESX does not communicate any NUMA information to the Guest OS and therefore both the Guest OS as well as the application are unaware of the non-uniform latency characteristics of the underlying platform. The Guest OS and application are therefor unable to prioritize which memory it will use.

If the virtual machine uses all the available memory of a NUMA node, it will lead to a higher degree of remote memory of all the other active virtual machines using the pCPU, leading to higher memory latencies and less throughput of the other virtual machines and eventually an intra-node migration. For more information about NUMA nodes, please read the articles: Sizing VMs and NUMA nodes and ESX 4.1 NUMA Scheduling.

Attempt to configure virtual machine with less memory than available in a NUMA node.

Swap file
During boot a swap file is created that equals the virtual machines configured memory minus the configured memory reservation. If no memory reservation is set, the virtual machine swap file (.vswap) equals the configured memory. Large virtual machines will generate an additional requirement for storing these large swap files reducing the consolidation ratio of virtual machines per VMFS datastore.

Bootstorms

A bootstorm is the occurrence of powering on a multitude of virtual machines simultaneously.

Virtual infrastructures running versions prior to ESX 4.1 can encounter memory contention when a bootstorm occurs of virtual machines running windows. Windows checks how much memory is available to the OS by zeroing out pages it detects. Transparent page sharing will collapse these pages but this will not occur immediately. Transparent Page Sharing is a cycle-driven process that tries to make a pass over the virtual machine memory with a timeframe of 3600 seconds. The level of contention will impact the speed of the TPS process. During a bootstorm, this zero-out behavior and delayed TPS process can introduce contention. Usually this contention is short-lived. Unfortunately during the startup phase of the guest OS the balloon driver will not be loaded and this situation can lead to compressing (10% of configured memory) and swapping useless data straight to disk.
ESXTOP will display swapped out memory but due to the nature of the data will show little to none swap-in.
ESX 4.1 uses a new technique called zero-page sharing. An in-depth post about this cool new technique will follow shortly.
End-note
This post concludes the three-part series about the impact of oversized virtual machines. The reason I wrote these articles is that I know many organizations still size their virtual machines on assumed peak loads happing somewhere in the (late) future of that service or application. Many organizations are using the same policy or method used for physical machines. The beauty of using virtual machines is the flexibility an organization has when it comes to determining the size of a machine during its lifecycle. Leverage these mechanisms and incorporate this in your service catalog and daily operations. Size the virtual machine according to its current or near-future workload.

Reservations and CPU scheduling

June 8, 2010 by frankdenneman

Most of my resource management articles focus more on the behavior of memory management than on CPU management. Mainly because the Memory scheduler within ESX is such an interesting complex system which comprises of memory allocation, swapping and reclamation with algorithms such as Idle Memory Tax and mechanisms like ballooning and swapping. But lately it seems that CPU scheduling seems to attract more and more my attention. The discussion Duncan and I had prior to posting his article about how CPU limits actually sparked the interest how CPU scheduling works when setting reservations, so additional to Duncan excellent article, I want to take a closer look how the ESX CPU scheduler handles CPU reservations and shares and show why CPU scheduling is more fair that memory management.
Similar to memory, the resource allocation settings, reservations, shares and limits can be set on CPU level. Limits and shares have similar behavior on CPU as well as Memory. Reservation act differently, let’s take a quick look at the resource allocation settings:

Shares:Shares indicate the proportional value of the entity on the same hierarchical level. If everything else is equal, reservations, limits and active utilization, the virtual machine that is allocated twice as many shares as another virtual machine is entitled to consume twice as many CPU cycles.

Limit: A limit is a mechanism to restrict physical resource usage of the virtual machine. A limit ensures that the VM will never receive more CPU cycles than specified, even if extra cycles are available on the host.

Reservation: A reservation is a guarantee of the specified amount of physical resources regardless of the total number of shares in his environment.

Now reservations act differently when setting it on a CPU than setting it on memory. When the virtual machine does not use its CPU cycles, these CPU cycles are redistributed to other active virtual machines, so unused reservations are not wasted. Contrary to memory management, when the memory will not be reclaimed by the scheduler once the virtual machine touched the pages.
By redistributing available CPU cycles and not letting the virtual machine hoard CPU resources, the VMkernel tries to properly divide the resources and achieve better fairness among virtual machines and improve utilization of the resources. To achieve both goals and divide the CPU resources among virtual machines the CPU scheduler calculates a MHzPerShare metric. This metric tries to identify which virtual machines are “ahead” of their entitlement and which virtual machines are “behind” and do not fully utilize their entitlement.

MHzPerShare = MHzUsed / Shares

MHzUsed is the current utilization of the virtual machine measured in Megahertz.
Shares is the current configured amount of shares of the virtual machine.
For example, the virtual machine is using 2500 MHZ and has 1000 shares, this means that the MHzPershare value is 2.5.The VMkernel will calculate the MHzPerShare number of each active virtual machine and the virtual machine with the lowest MHzPerShare value will have the highest priority of running on the CPU. If the virtual machine with the lowest MHzPerShare value decides not to use it right to allocate the cycles, the cycles can be used by the virtual machine with the next lower MHzPerShare value.

Although not shown, reservations play a important part in this calculation. As mentioned before, reservations overrule shares and guarantee the amount of physical resources regardless of the amount of shares. This means that the virtual machine always can use the CPU cycles specified in its reservation, even if the virtual machine has a greater MHzPerShare value. So how exactly do reservations and shares interact with each other when it comes to calculating the MHzPerShare value?
For example:
In a 6 GHz system, 1 virtual machine is running and 2 are powered on, VM1 is running a memory intensive app and doesn’t really care much about CPU cycles, the virtual machine is configured with 1000 CPU shares and no reservation. The 2 other virtual machines run CPU intensive apps and are currently competing for resources. VM2 has a reservation of 2250 MHz and has a default share setting of 1000 shares, the other CPU intensive virtual machine, VM3 is equipped with 2vcpu’s and therefore receives 2000 shares, but the administrator didn’t set any reservation.
Now VM1 is running at 500 MHz, with its 1000 shares, the MHzPerShare value equals 0.5. Because VM2 is in need of CPU cycles, it immediately utilizes its reservations and “occupies” all 2250 MHz, its
MHzPerShare value equals 2.25 (2250/1000).

Now because VM3 doesn’t have any reservation and is in need of CPU cycles, the VMkernel looks at its MHzPerShare value to decide how many CPU cycles it can use before distributing excess CPU cycles to other virtual machines. The kernel will distribute cycles to VM3 until it reaches the same MHzPerShare value of VM2, which is 2.25. In theory this means that the VMkernel will allocate 2000 x 2.25 = 4500 MHz before looking at another VM. Due to the fact that CPU scheduler already allocated 500 MHz to VM1 and 2250 MHz to VM2 of the available 6GHz, it can allocate VM3 3250 Mhz.

Because VM2 has a reservation it can allocate up to its reservation even when initially VM3 has a lower MHzPerShare value (0) and the CPU cycle requirements of VM1 are met at 500MHz. However due to the fairness principle VM2’s own MHzPerShare value influences the VMkernel’s decision how much cycles to allocate to VM3 before considering allocating additional cycles to vm2 again.
Now for some reason the application in VM3 is leveling out at 2000 MHz, VM1 is still using 500 MHz and VM2 is in desperate need of extra CPU cycles. No settings are changed so VM1 and VM2 has a 1000 shares each and VM2 has a reservation of 2250MHz, VM3 has 2000 shares and no reservation is set.
The VMkernel will satisfy the request of VM1, resulting in a MHzPerShare value of 0.5. VM2 claims its reservation and utilizes 2250 MHz resulting in a MHzPerShare value of 2.25, VM3 can allocate up to 4500 before reaching the MHzPerShare value of VM3, but stops consuming above 2000Mhz, ending up with a MHzPerShare value of 2000/2000 = 1, this means that inside the 6GHz host 1250 cycles are available.
The CPU scheduler will shop around with these available cycles and see which VM is interested. Now the VMkernel will offer the cycles to the virtual machines in the increasing order of MHzPerShare, so first it will ask VM1 (0.5), because its CPU request is satisfied, it will forfeit its claim, VM2 also forfeits this claim, so VM3 will happily accepts the remaining cycles and its resource usage will increase to 3500 MHz.
So here you have it, both shares and reservation interact or even battle with each other to allocate CPU cycles for the virtual machines. Shares are by many perceived as an inferior resource allocation setting, hopefully this demonstrates the power of shares, it can in combination with utilization become a very important factor in ESX resource management.