VMware vSphere Internals Q/A

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Welcome Readers !! This is a second series of VMware vSphere Q/A which is emphasized on ESXi internals and related feature Q/A. I hope it will help you with interview preparation:

Que:1>. What is hyperthreading in terms of VMware terminology?

Ans: Hyperthreading technology allows a single physical processor core to behave like two logical processors. The processor can run two independent applications at the same time. To avoid confusion between logical and physical processors, Intel refers to a physical processor as a socket, and the discussion in this chapter uses that terminology as well.
On processors with Intel Hyper-Threading technology, each core can have two logical processors which share most of the core’s resources, such as memory caches and functional units. Such logical processors
are usually called threads.

Que:2> Does Hyperthreading doubles the performance?
Ans: While hyperthreading does not double the performance of a system, it can increase performance by better utilizing idle resources leading to greater throughput for certain important workload types. An
application running on one logical processor of a busy core can expect slightly more than half of the throughput that it obtains while running alone on a non-hyperthreaded processor. Hyperthreading performance improvements are highly application-dependent, and some applications might see performance degradation with hyperthreading because many processor resources (such as the cache) are shared between logical processors.

Que:3> What processors support Hyperthreading?
Ans: Many processors do not support hyperthreading and as a result have only one thread per core. For such processors, the number of cores also matches the number of logical processors. The following
processors support hyperthreading and have two threads per core.
– Processors based on the Intel Xeon 5500 processor microarchitecture.
– Intel Pentium 4 (HT-enabled)
– Intel Pentium EE 840 (HT-enabled)

Que:4> What does CPU affinity feature do?
Ans: Using CPU affinity, you can assign a virtual machine to a specific processor. This allows you to restrict the assignment of virtual machines to a specific available processor in multiprocessor systems.

How to configure it?
1.In the vSphere Client inventory panel, select a virtual machine and select Edit Settings.
2.Select the Resources tab and select Advanced CPU.
3.Click the Run on processor(s) button.
4.Select the processors where you want the virtual machine to run and click OK.

Que:5> What is VMware Memory Ballooning?
Virtual memory ballooning allows a physical host to recapture unused memory on its guest virtual machines and allocate the resource where needed.
Virtual memory ballooning is a computer memory reclamation technique used by a hypervisor to allow the physical host system to retrieve unused memory from certain guest virtual machines (VMs) and share it
with others. Memory ballooning allows the total amount of RAM required by guest VMs to exceed the amount of physical RAM available on the host. When the host system runs low on physical RAM resources, memory ballooning allocates it selectively to VMs.

If a VM only uses a portion of the memory that it was allocated, the ballooning technique makes it available for the host to use. For example, if all the VMs on a host are allocated 8 GB of memory, some
of the VMs will only use half the allotted share. Meanwhile, one VM might need 12 GB of memory for an intensive process. Memory ballooning allows the host to borrow that unused memory and allocate it to the VMs with higher memory demand.

The guest operating system runs inside the VM, which is allocated a portion of memory. Therefore, the guest OS is unaware of the total memory available. Memory ballooning makes the guest operating system
aware of the host’s memory shortage.

Virtualization providers such as VMware enable memory ballooning. VMware memory ballooning, Microsoft Hyper-V dynamic memory, and the open source KVM balloon process are similar in concept. The host uses
balloon drivers running on the VMs to determine how much memory it can take back from an under-utilizing VM. Balloon drivers must be installed on any VM that participates in the memory ballooning
technique.

Balloon drivers get the target balloon size from the hypervisor and then inflate by allocating the proper number of guest physical pages within the VM. This process is known as inflating the balloon; the process of releasing the available pages is known as deflating the balloon.

VM memory ballooning can create performance problems. When a balloon driver inflates to the point where the VM no longer has enough memory to run its processes within itself, it starts using another VM memory technique known as memory swapping. This will slow down the VM, depending upon the amount of memory to recoup and/or the quality of the storage IOPS delivered to it.

Que:6> How to check VMware ballooning?
To check for ballooning you can either open ESXTOP or the vCenter Performance Graphs

Que 7> What is the limitation of Vmware ballooning?

The balloon driver can inflate up to a maximum of 65%. For instance a VM with 1000MB memory the balloon can inflate to 650MB. The way to avoid ballooning is not to uninstall the balloon driver but to create
a “Memory Reservation” for the virtual machine. In case of full inflation for this particular VM the result is the hypervisor gets 650MB memory reclaimed. The downfall of this is that you risk your VM
to do Guest OS Swapping to its page file! Just remember page file swapping is better than hypervisor swapping. Hypervisor swapping happens without the guest operating system is aware of it. Page file
swapping it is the OS that decides what pages to swap to disk!

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