Would the JVM (and probably also the CLI) be considered a virtual machine (the equivalent of the x86 in a "normal" program stack) or a virtual OS (the equivalent of Windows)?
Strictly speaking, it is a virtual machine, ie: it executes a special low-level language (similar to x86 ASM. CLI uses MSIL, JVM uses "byte codes") and translates them into the target machine's op-codes (x86, x86_64, ARM .. etc.) for execution on the host CPU.
It also manages marshaling (ie: correct handling and passing through of variables to native memory stack/heap) to allow function calls from inside the managed world to the outside OS on which the VM runs.
Practically though, neither the JVM nor the CLI alone are very helpful except for automated garbage collection and CPU-architecture-independence, but they are complemented by a large base library (the Java classes, or the .NET BCL) which allows you to do many platform-y things without having to call platform specific APIs and use marshaling manually for everything.
That's why there is a different Java Runtime Environment for each OS. Each one's JVM translates to a specific CPU arch, and uses different platform specific-APIs to accomplish what the unified base library exposes to you as a friendly API inside the managed world.
Hope that helps you.
The jvm is considered a real computer, only not realized in hardware. The machine has it's own storage capacity, it's own memory model, it's own specific behaviour of it's central processing unit and it's own internal machine code. This machine is extendable with new possibilities and modules that are represented with classes, API's, etc...
It has it's own stack based architecture, like most virtual machines.
Related
JVM and CLR are virtual machines. Similarly to bare metal computer machines, they provide virtual machine languages.
On real computer machines, we have operating systems, which provide system calls and APIs. For example the famous book Advanced Programming Unix Environment describes the APIs provided by Unix and Linux. Windows, however, provide different APIs.
On top of virtual machines like JVM and CLR, is there something which
plays the role of an operating system, and
provides programming APIs?
If there is nothing playing the role of an OS on virtual machines like JVM and CLR, what provides programming APIs (such as those in Java, C#, ...) similar to OS APIs?
Note: I am asking about VMs and on top of them, instead of what is underlying them. Do VMs not run some virtual OS on top of them? If there is no, why is there no such a need?
Thanks.
There are two kinds of virtual machines: system virtual machines and process virtual machines. System virtual machines provide a virtualization of complete instructions sets including user-mode instructions and kernel-mode instructions and therefore they can run operating systems. Process virtual machines virtualizes user-mode instructions and, usually, some system calls (such as those for managing threads, memory, and files) and therefore can only run applications or processes. That is, on top of a single process virtual machine a single app or process can run. The JVM and CLR are process virtual machines.
While in theory it is indeed possible to develop an OS to run on a process virtual machine, this is practically not useful because the performance of the programs that will run on that OS will be terrible due to the excessive layering in software.
Generally, system and process virtual machines themselves are not considered to be operating systems. However, process virtual machines do not necessarily require an OS to run on and may run on a bare-metal computer. The .NET Micro Framework is an example of such VM. Such VMs are sometimes called operating systems. Some virtual ISAs or a subset thereof have been implemented completely in hardware similar to x86 and ARM. One could develop operating systems to run on them. They are almost never used in industry because of their low performance.
An "operating system" is a a large, fuzzy ball of hairs. You got the hal, kernel, userspace... do we count some userspace libraries (typically libc) too?
With squinting you can find some comparable concepts in the JVM/JRE but generally a JVM runs on top of a bona-fide operating system and thus does not reimplement all aspects and instead simply provides platform-independent abstractions over facilities that you can find on almost all systems.
For example these days Thread usually is just java representation of native OS threads, but a JVM could choose to implement thread scheduling in userspace, and in sun's JVM did back during 1.1 times and some other JVMs still do today.
I'll answer this from the perspective of Java since I have no in-depth knowledge of .Net. I would assume, however that the CLR and JVM are similar from this point of view.
Let's start with an operating system. The purpose of this is to abstract away the hardware specific interfaces as well as providing a runtime environment for processes.
The OS uses device drivers to provide a uniform interface to similar devices (like the processor, memory, disk drives, network cards and so on). The OS uses system calls to allow user-level code to interact with these devices. If you wrote code in C you will call 'open' then 'read' and 'write' to the device before calling 'close'. The 'ioctl' (IO control) system call is also used a lot for device control. Each OS provides a standard set of these system calls (you can run Linux on an Intel or ARM processor, but you have the same set of system calls for each distro). Incidentally, this is also how Docker works by using a standard set of system calls to enable containers to be moved from one platform to another without problem.
The OS also provides the ability to run multiple processes simultaneously. With newer, multi-core machines this really can happen in parallel but the OS also uses scheduling to share a CPU between multiple processes or threads. By switching processes or threads very quickly this gives the impression that things are happening simultaneously, even on a single processor.
Now let's look at the JVM, which is a user-level process (from the OS point of view so just like any other user application). This has been designed to abstract away CPU and operating specific functionality from the Java application. The bytecodes generated by the Java compiler do not contain any system calls. If you look at the bytecode instruction set (defined in the Java Virtual Machine Specification) you will find that the instructions provide many familiar low-level features such as loading a register, bit manipulation and so on. In addition, there are many instructions that are higher level and relate more specifically to Java; things like invokestatic that invokes a static method on a class, monitorenter. monitorexit for locking, newarray and so on.
The JVM takes these bytecodes and converts them from a CPU- and OS-independent form (that of a Virtual Machine) into the instructions for the specific CPU architecture and OS that the JVM is running on. In some cases this can be a one-to-one mapping (for things like bitwise operators), but can often be much more complex and involve the use of system calls to open files, access network interfaces, etc. The JVM also uses the OS to deal with threads created by the application. In the very early days of Java operating systems like Windows 95 did not have the concept of threads within a process so the JVM had to provide it's own implementation (this was called green threads and performed pretty badly).
To summarise the JVM takes the platform neutral bytcodes of the class files it is executing and converts them to the apprporiate native CPU instructions and system calls to make the application run. The JVM does not provide any traditional OS services, it just uses them.
I know that executable files (eg: .exe for Windows) are binaries. I know about hex files and assembly and so on. I also know about OS API's. Theoretically, I could write a web browser entirely in assembly code that uses the OS API's, assemble it with NASM and get an executable. But my question is, how do operating systems control the applications? For example, I could have a executable on Windows that writes to the video memory, and fills the screen with random stuff. I've tried this (actually) and Windows halts the application. How does it control the application? Moreover, if I have a linux executable and I attempt to run it on a Windows machine with the exact same hardware, theoretically, it should work (though it won't use any Windows API's) since the processor architecture is the same, but yet It cant. How exactly does the OS 'control' the binaries?
The operating system, in particular windows and linux protect other applications and the operating system itself from applications. So there is a protection layer that an application ideally cannot punch through running as a user.
You want to put pixels on the screen, you have to ask the operating system to do it for you. If the operating system allows a way to punch through (calling mmap perhaps having to run as root) then you can trash the computer at will, yes. The plan/design is to not let you have direct access, you are given a virtual space, a sandbox, for your application to play in, you have a virtual address space with some ram you can read and write at will, but you go outside that, you need to be shut down or at least handled in some way.
As far as what language you write applications in yes, if it is a normal windows program or linux program, then anything you can do to generate machine code for that target, asm, ada, C, etc. If you are talking specific virtual machines (java, python) then your choices get more limited. If you want to do more than play in your space, like have some sort of output other than a return value, then you have to make system calls in the way that the system requires them, which is target and operating system specific. And again, you get the right instructions or registers or memory structure or whatever is required by the operating system for that call in whatever language you are using or libraries you can link with then the rest of your program can be in whatever language, yes absolutely you want to write a web browser in assembly language using nasm, completely doable. have to create a binary format the os supports, and make the system calls as needed in the way that the system requires.
I want to understand how this applies to an operating system and also to those things that are not infact operating systems. I can't understand the difference between the three and their essence. API is the functions we can call but what is Shell? If we have an API than what exactly is the Kernel of the operating system? I understand the an operating system has a Core that is not going to change and this core does the fundamental Job of a typical OS while we may have different user interfaces like GUI or command line with the same Kernel. So the problem is I am confused how these things are different. Aaaaaaarhg!
Can the functions like printf and fopen in C be called API calls?
A command-line interface (CLI) shell is a command interpreter, i.e. the program that either processes the command you enter in your command line (aka terminal) or processes shell scripts (text files containing commands) (batch mode). In early Unix times, it used to be the unique way for users to interact with their machines. Nowadays, graphical user interfaces (GUIs) are becoming the preferred type of shell for most users.
A kernel is a low level program interfacing with the hardware (CPU, RAM, disks, network, ...) on top of which applications are running. It is the lowest level program running on computers although with virtualization you can have multiple kernels running on top of virtual machines which themselves run on top of another operating system.
An API is a generic term defining the interface developers have to use when writing code using libraries and a programming language. Kernels have no APIs as they are not libraries. They do have an ABI, which, beyond other things, define how do applications interact with them through system calls. Unix application developers use the standard C library (eg: libc, glibc) to build ABI compliant binaries. printf(3) and fopen(3) are not wrappers to system calls but (g)libc standard facilities. The low level system calls they eventually use are write(2) and open(2) and possibly others like brk, mmap. The number in parentheses is a convention to tell in what manual the command is to be found.
The first volume of the Unix manual pages contains the shell commands.
The second one contains the system call wrappers like write and open. They form the interface to the kernel.
The third one contains the standard library (including the Unix standard API) functions (excluding system calls) like fopen and printf. These are not wrappers to specific system calls but just code using system calls when required.
The Shell is the way to communicate with the OS and kernel by command line. The Shell does this by also calling the API.
The kernel is indeed the core of the OS and does memory management, task scheduling, handles with filesystems, I/O handling,...
All the things that the kernel does, can in some way be invoced by the API the OS provides.
printf and fopen are wraps around the system calls (API) provided by the OS and kernel
Shell: It is like a command line interface to your operating system. Commands like ls, ps, kill and many more can be used to request to complete the specific operation to the OS. It is like "cmd" on windows.
Kernel: It is the main code of any operating system. Any request you give on shell or through GUI (like memory allocation, opening a file etc) are finally fulfilled by Kernel.
And yes, the calls you mentioned are regarded as API calls. The request to these calls are also handled by Kernel. Please go the below link to find API calls in unix.
http://www.mkssoftware.com/docs/api_index.asp
This is the overall picture in a unix os:
Applications => (shell+library routines) => system calls => kernel
look the final request handler is the Kernel.
Thx!
Consider an example, You are watching the movie is on shell and actually process done over hardware is the kernel. shell is approximately work as that of os for user and software interface and kernel work as that of os for software and hardware.
In this coming semester, I am starting some research on large-scale distributed computing with MPI. What I am looking for help with is the initial stages, specifically getting a solid development environment set up. Does anyone have any recommendations for good tools to use for this?
I am also curious as to whether there exists a kind of simulator that would allow be to write MPI and distribute it to virtual (rather than physical) nodes.
You could download a MPI library such as Open-MPI, MPICH, etc. and run it on a multi-core system (such as a recent desktop) with number of processes = number of cores. They would operate without a network interconnect (for instance, over shared memory). That should be enough to explore initially.
If you really want multiple nodes, you can experiment with multiple VMs with a VM network before actually moving on to a physical cluster. One of the VMs would have to be configured to act like a NFS server and the rest of the VMs could mount your home directories over NFS.
Depends on what is your favourite language. I dove into MPI using python and the pypar module. It lets you concentrate on MPI procedures without worrying too much about pointers and complicated c / c++ stuff. MPI on a single machine is programmed no differently from MPI on 100s. Getting cross machine setups is more about what MPI implementation and operating systems you use.
If hardware support is a must for virtualization, how can Java Virtual Machines run on machines without support for virtualization ? Or is JVM not a virtual machine ?
A JVM is not virtual in the same sense as a VirtualBox or VMWare virtual machine. It is a 'machine' that implements the Java bytecode, not a virtualized version of actual hardware.
The term-of-art 'virtual machine' was coined a very long time ago for the following scenario:
make up a computer, like Knuth's MIX.
write a computer program that implements the made-up computer.
run programs
When this virtual machine runs, it's a completely ordinary program, running completely in user mode. It needs no special help from the hardware or operating system to work reasonably well. This is especially true of the JVM, since the Java byte code does not deal with low-level hardware I/O or other things which are hard to simulate.
Later, historically, (to pick a particular instance), IBM invented VM/370. VM/370 uses the other sense of the term 'virtual machine'. In this later sense, the hardware and operating system cooperate to allow a single physical machine to host multiple virtual instances of (more or less) the same architecture, in which multiple copies of the whole operating system are written as if they are running on more or less bare hardware. Later, the X86 was designed with features to facilitate this.
So, yes, any virtual machine is making use of some physical hardware, unless you implement it with pieces of paper passed around a table (pace John Searle). But when the virtual machine bears no resemblance to the machine it is running on, then there's no need for special help from the operating system and hardware, and no need for anything as complex as VM/370, or VMware.
If hardware support is a must for virtualization, ...
Let me stop you right there :-)
There is a difference in concept between the JVM (software virtualization) and (for example) a VMWare VM (hardware-assisted virtualization).
The JVM (and other software-based VMMs such as the ones that allow to to emulate x86 on Solaris hardware - I think Bochs and possibly DosBox fall into this category) runs like any other application, using the operating system to gain access to the hardware, or emulating its own hardware purely in software.
VMWare, and the other VMMs optimised for speed, rely on hardware support. In other words, they run on the hardware as if they have full access to the hardware and, only when they try to do something they're not supposed to does the OS captures that attempt and fake it.
That's why VMWare runs so much faster than the software-only emulators. It's because, for the vast majority of the time, it's actually running on the real hardware.
The JVM is a virtual machine, but it doesn't require any additional support from the Operating System. Instead of virtualising instructions for a particular CPU it executes java bytecode.
The JVM is a virtual machine for running Java, in other words it emulates a machine which would be capable of running java. It is a confusing choice of names, but it comes from the general meaning of "machine" not from the more common Virtual Machine meaning.
The JVM, like a regular VM emulates the execution of instructions, but in the case of the JVM the instructions being emulated are Java Instructions, and in the case of a VM they are Hardware Instructions as would be executed by an OS running on the same hardware.
Yes the JVM does access hardware, however this is why you install a MAC or WINDOWS JVM since the instructions are translated by the JVM and acted upon depending on the installation of the JVM, for example, open file dialog on mac opens the mac dialog and windows JVM opens the windows dialog.
So its not being virtualized by the system, but the bytecode is being virtualized by the JVM you installed. It's basically like an application that reads something(bytecode) and does something(access hardware, or other stuff).
It should be noted that nothing stipulates that a JVM does not (have to) have HW virtualization access. There are notable exceptions, but to which the answered poster alluded, few CPs exist that run Java bytecode natively. Maybe someday a Java bytecode HAL or TIMI will be commonplace to put the JVM into the same class as the formalized HW virtualization?