Question

1. Explain three ways of memory compaction.

2. Demonstrate three types of scheduling by process manager with the help of diagram(s).

3. List differences between hosted and bare metal virtualization. Also, provide one example of each.

Answer 1

Memory compaction is the process of moving allocated objects together and leaving empty space together. Memory management is the functionality of an operating system which handles or manages primary memory and moves processes back and forth between main memory and disk during execution. Memory management keeps track of each and every memory location, regardless of either it is allocated to some process or it is free. It checks how much memory is to be allocated to processes. It decides which process will get memory at what time. It tracks whenever some memory gets freed or unallocated and correspondingly it updates the status.

Three ways of memory compaction are:

Swapping

Swapping is a mechanism in which a process can be swapped temporarily out of main memory (or move) to secondary storage (disk) and make that memory available to other processes. At some later time, the system swaps back the process from the secondary storage to main memory.

Though performance is usually affected by swapping process but it helps in running multiple and big processes in parallel and that's the reason Swapping is also known as a technique for memory compaction.

Fragmentation

As processes are loaded and removed from memory, the free memory space is broken into little pieces. It happens after sometimes that processes cannot be allocated to memory blocks considering their small size and memory blocks remains unused. This problem is known as Fragmentation.

Fragmentation is of two types –

External fragmentation

Total memory space is enough to satisfy a request or to reside a process in it, but it is not contiguous, so it cannot be used.

Internal fragmentation

Memory block assigned to process is bigger. Some portion of memory is left unused, as it cannot be used by another process.

Paging

A computer can address more memory than the amount physically installed on the system. This extra memory is actually called virtual memory and it is a section of a hard that's set up to emulate the computer's RAM. Paging technique plays an important role in implementing virtual memory.

Paging is a memory management technique in which process address space is broken into blocks of the same size called pages (size is power of 2, between 512 bytes and 8192 bytes). The size of the process is measured in the number of pages.

Similarly, main memory is divided into small fixed-sized blocks of (physical) memory called frames and the size of a frame is kept the same as that of a page to have optimum utilization of the main memory and to avoid external fragmentation.

Demonstrate three types of scheduling by process manager with the help of diagram(s).

Schedulers

Schedulers are special system software which handle process scheduling in various ways. Their main task is to select the jobs to be submitted into the system and to decide which process to run. Schedulers are of three types −

• Long-Term Scheduler
• Short-Term Scheduler
• Medium-Term Scheduler

Long Term Scheduler

It is also called a job scheduler. A long-term scheduler determines which programs are admitted to the system for processing. It selects processes from the queue and loads them into memory for execution. Process loads into the memory for CPU scheduling.

The primary objective of the job scheduler is to provide a balanced mix of jobs, such as I/O bound and processor bound. It also controls the degree of multiprogramming. If the degree of multiprogramming is stable, then the average rate of process creation must be equal to the average departure rate of processes leaving the system.

On some systems, the long-term scheduler may not be available or minimal. Time-sharing operating systems have no long term scheduler. When a process changes the state from new to ready, then there is use of long-term scheduler.

Short Term Scheduler

It is also called as CPU scheduler. Its main objective is to increase system performance in accordance with the chosen set of criteria. It is the change of ready state to running state of the process. CPU scheduler selects a process among the processes that are ready to execute and allocates CPU to one of them.

Short-term schedulers, also known as dispatchers, make the decision of which process to execute next. Short-term schedulers are faster than long-term schedulers.

Medium Term Scheduler

Medium-term scheduling is a part of swapping. It removes the processes from the memory. It reduces the degree of multiprogramming. The medium-term scheduler is in-charge of handling the swapped out-processes.

A running process may become suspended if it makes an I/O request. A suspended processes cannot make any progress towards completion. In this condition, to remove the process from memory and make space for other processes, the suspended process is moved to the secondary storage. This process is called swapping, and the process is said to be swapped out or rolled out. Swapping may be necessary to improve the process mix.

List differences between hosted and bare metal virtualization. Also, provide one example of each.

Bare-metal virtualization

Bare-metal virtualization installs a Type 1 hypervisor -- a software layer that handles virtualization tasks -- directly onto the hardware prior to the system installing any other OSes, drivers or applications. Common hypervisors include VMware ESXi and Microsoft Hyper-V. Admins often refer to bare-metal hypervisors as the OSes of virtualization, though hypervisors aren't OSes in the traditional sense.

Once admins install a bare-metal hypervisor, that hypervisor can discover and virtualize the system's available CPU, memory and other resources. The hypervisor creates a virtual image of the system's resources, which it can then provision to create independent VMs. VMs are essentially individual groups of resources that run OSes and applications. The hypervisor manages the connection and translation between physical and virtual resources, so VMs and the software that they run only use virtualized resources.

Since virtualized resources and physical resources are inherently bound to each other, virtual resources are finite. This means the number of VMs a bare-metal hypervisor can create is contingent upon available resources. For example, if a server has 24 CPU cores and the hypervisor translates those physical CPU cores into 24 vCPUs, you can create any mix of VMs that use up to that total amount of vCPUs -- e.g., 24 VMs with one vCPU each, 12 VMs with two vCPUs each and so on. Though a system could potentially share additional resources to create more VMs -- a process known as oversubscription -- this practice can lead to undesirable consequences.

Once the hypervisor creates a VM, it can configure the VM by installing an OS such as Windows Server 2019 and an application such as a database. Consequently, the critical characteristic of a bare-metal hypervisor and its VMs is that every VM remains completely isolated and independent of every other VM. This means that no VM within a system shares resources with or even has awareness of any other VM on that system.

Because a VM runs within a system's memory, admins can save a fully configured and functional VM to a disk where they can then back up and reload the VM onto the same or other servers in the future, or duplicate it to invoke multiple instances of the same VM on other servers in a system.

Hosted virtualization

Hosted virtualization offers many of the same characteristics and behaviors as bare-metal virtualization. The difference comes from how the system installs the hypervisor. In a hosted environment, the system installs the host OS prior to installing a suitable hypervisor -- such as VMware Workstation, KVM or Oracle VirtualBox -- atop that OS.

Once the system installs a hosted hypervisor, the hypervisor operates much like a bare-metal hypervisor. It discovers and virtualizes resources and then provisions those virtualized resources to create VMs. The hosted hypervisor and the host OS manage the connection between physical and virtual resources so that VMs -- and the software that runs within them -- only use those virtualized resources.

However, with hosted virtualization, the system can't virtualize resources for the host OS or any applications installed on it, because those resources are already in use. This means that a hosted hypervisor can only create as many VMs as there are available resources, minus the physical resources the host OS requires.

The VMs the hypervisor creates can each receive guest OSes and applications. In addition, every VM created under a hosted hypervisor is isolated from every other VM. Similar to bare-metal virtualization, VMs in a hosted system run in memory and the system can save or load them as disk files to protect, restore or duplicate the VM as desired.

Hosted hypervisors are most commonly used in endpoint systems, such as laptop and desktop PCs, to run two or more desktop environments, each with potentially different OSes. This can benefit business activities such as software development.

In spite of this, organizations use hosted virtualization less often because the presence of a host OS offers no benefits in terms of virtualization or VM performance. The host OS imposes an unnecessary layer of translation between the VMs and the underlying hardware. Inserting a common OS also poses a SPOF for the entire computer, meaning a fault in the host OS affects the hosted hypervisor and all of its VMs.

Although hosted hypervisors have fallen by the wayside for many enterprise tasks, the technology has found new life in container-based virtualization. Containers are a form of virtualization that relies on a container engine, such as Docker, LXC or Apache Mesos, as a hosted hypervisor. The container engine creates and manages virtual instances -- the containers -- that share the services of a common host OS such as Linux.

The crucial difference between hosted VMs and containers is that the system isolates VMs from each other, while containers directly use the same underlying OS kernel. This enables containers to consume fewer system resources compared to VMs. Additionally, containers can start up much faster and exist in far greater numbers than VMs, enabling for greater dynamic scalability for workloads that rely on microservice-type software architectures, as well as important enterprise services such as network load balancers.

Container engines are a variation on hosted hypervisor technology, but containers and VMs are not directly interchangeable. Still, the fundamental concepts of hosted virtualization apply to containers. You can also mix container and VM technologies in a single environment. For example, you can create a Type 1 VM and then load that Type 1 VM onto a Linux OS, Docker or other container instance. This means admins can create a container deployment within a traditional VM to reap the benefits of both technologies.

Bare-metal vs. hosted virtualization

The verdict isn't about which type of virtualization is necessarily better. Rather, admins should match the virtualization technology with their particular use case. Bare-metal virtualization, hosted virtualization and container virtualization can all find a meaningful place together.

Bare-metal virtualization works best in the data center for admins looking to maximize the use of server resources by running two or more VMs on the same system. Admins who use bare-metal virtualization gain the benefits of VM logical isolation, mobility and performance.

Hosted virtualization has a place on endpoint computers. It enables laptop or desktop computers to run two or more VMs -- in most cases, those VMs represent different OSes and desktop configurations. For example, one VM might run a Windows 10 desktop, while another VM might run a macOS desktop. Software developers creating and testing software on systems that might not otherwise support the native OS might find hosted virtualization particularly useful. The use of virtualization avoids the expense and hassle of purchasing and maintaining multiple physical endpoints.

Container technology is still maturing, but it has found a place in the data center to enable complex, dynamic, highly scalable services and next-generation software architectures. These technologies don't often work efficiently with a traditional monolithic software approach reflected in conventional VMs.

Server Dedication

One of the significant differences between a bare-metal server and a virtual machine server is the level of dedication that exists with clients. A bare-metal server is a physical server that is dedicated to a single customer, meaning that they aren’t shared between tenants. Virtual machine servers, on the other hand, can be shared among multiple tenants through the hypervisor which can create multiple operating system instances and, as a result, numerous isolated virtual server envrionments.

Cost

When comparing pricing, the first thing that should be noted is that the billing differs between bare-metal and virtual machine servers. For bare-metal servers, the customer is billed a consistent amount on a monthly basis for the hosting, while virtual machine servers follow a predetermined billing period and only cost the customer for the resources used. Secondly, bare-metal servers are traditionally more expensive than virtual ones, however, this can be dependent on whether they’re being used for enterprise-grade workloads over a sustained period of time.

Performance & Security

In the realm of performance, there is no argument as to whether or not bare-metal servers can harness more processing power than virtual servers although the margin may not be as noticeable as expected. With virtual machine servers, on the other hand, they have the ability to scale up and down, as needed, almost instantly.

The primary difference between the security of bare-metal servers and virtual servers is their tenancy. As mentioned previously, bare-metal servers are single-tenant while virtual machine servers are multi-tenant. Through this aspect alone, bare-metal servers are considered more secure and private.

Examples of popular bare-metal hypervisors are Microsoft Hyper-V, Citrix XenServer and VMware ESXi.

Examples of this type of hosted virtualization include VMware Fusion, Oracle Virtual Box, Oracle VM for x86, Solaris Zones, Parallels and VMware Workstation.