1. Instead, when a thread needs system resources, it sends the request to the runtime, and the latter obtains system resources through corresponding system calls. (1) When there are multiple processors, multiple threads of a process can execute simultaneously. (4) Allow each process to customize its own scheduling algorithm, and thread management is more flexible. The overall throughput of the server is much greater. In the many-to-many model, a program can have as many threads as are appropriate without making the process too heavy or burdensome. The ability to share an LWP with several user-level threads. Threads also share most of the operating system state of a process, can open files and permit other threads to read them, and allow different processes to synchronize with each other in varying degrees. If one thread is blocked using a system call, the entire process will be suspended. (Why user-level thread blocking will affect the entire process) In addition, page failure will also cause the same problem. In this implementation, each user-level thread created by the application is known to the kernel, and all threads can access the kernel at the same time. Note that it is not necessary for calls made from only the main thread to the unsafe library to go through a lock, since calls to the library are single-threaded through the library; only the main thread ever calls the library. See "Programming Compute-Bound, Parallellized Java Applications" for more information. However, if you cannot avoid referencing errno-unsafe libraries, the following workaround may help: Enable the main thread to enter the Java application and arrange for all calls to the unsafe library to be routed through the main thread. In an MT kernel such as the one used by the Solaris operating environment, each kernel thread is a single flow of control within the kernel's address space. Do not depend on this extension, as it may be discontinued. Additionally, using the extended, I/O interruption semantics of the interrupt method makes the code non-portable across different JVMs. The following unique features of Java on Solaris threads are available by default to all Java applications on Solaris: Unbound Solaris threads: A Java thread is essentially the same as an unbound Solaris thread, with the inherent advantages of unbound threads. The thread can wait for the main thread to issue the call and return the results to it. 
The two-level model allows the kernel to accommodate the concurrency demands of all program types without blocking or otherwise restricting thread access to system services. (3) The cost of thread management such as creating and destroying threads and thread switching costs is much less than that of kernel threads. 
This feature is useful to real-time applications running on multiprocessor systems. The ability to bind a user-level thread (through native methods) to an LWP that is dedicated to a single processor. 
The Solaris MT kernel is one of the most important components of the Solaris operating environment, enabling the Solaris system to be the only standard operating environment that provides this level of concurrency, sophistication, and efficiency. It is fully preemptive, providing real-time responsiveness. Thread binding is useful to applications that need to maintain strict control over their own concurrency, such as those that require real-time response. Automatic concurrency control for unbound threads. Therefore, all Java threads are unbound by default. The two-level threads model - the Solaris system's proven MT implementation that enables processes to use an unlimited number of threads for optimal performance. Because the green threads library was user-level and the Solaris system could process only one green thread at a time, Solaris handled the Java runtime as a many-to-one threading implementation (refer to "Many-to-One Model (Green Threads)" . Many user threads are multiplexed on a pool of kernel LWPs. Synchronization primitives which have inter-process scopes. Kernel-level threads: Switching is controlled by the kernel. Threads require less overhead to create, maintain, and manage than a traditional process. Threads can be used to simplify the structure of complex applications, such as server-class and multimedia applications. As a result, there were several problems: Java applications could not inter-operate with existing MT applications in the Solaris environment. Additionally, only one thread at a time can access the kernel, so only one schedulable entity is known to the operating system.
The runtime environment, as implemented by a threads library, multiplexes and schedules runnable threads onto "execution resources," the LWPs. This improves the efficiency of hardware subsystems by providing concurrency and parallelism in computation, networking, display, and I/O. The Solaris operating environment provides the best performance, tools, support, and flexibility in developing MT applications. There is no need to switch between user mode and core mode, and the speed is fast. Thread synchronization functions can be used to provide enhanced process-to-process communication. Therefore, an attempt to use priorities to schedule execution on any multi-tasked, multiprocessor system is not likely to succeed. Binding a user-level thread to an LWP establishes an exclusive connection between the two. Better communication. The many-to-many model, also called the two-level model, minimizes programming effort while reducing the cost and weight of each thread. unbound threads. The real solution is to ensure that an MT Java application that also uses native code by way of native methods is link with MT-safe (or at least errno--safe libraries). Thread groups provide a mechanism for collecting multiple threads into a single object and manipulating those threads all at once, rather than individually. The Java Language Specification (JLS), defines it to interrupt a target thread only if and when it calls the wait() method. The implementation of threads can be divided into two categories: User-Level Thread and Kernel-Level Thread. With a native-threaded JVM such as 1.1, libc stores system call error code in a thread-specific errno. . MT programming enables you to speed up applications and to leverage the parallelism of hardware and the efficiencies of objects. (5) The table space and stack space that threads can use are more than that of kernel-level threads. There is no Java API to perform the binding. When multiple processors are available, those concurrent but independent actions can take place in parallel. Windows thread is like this. Application processes use the thread library to provide functions for creating, synchronizing, scheduling, and managing threads to control user threads. Unbound user-level threads defer control of their concurrency to the threads library, which automatically expands and shrinks the pool of LWPs to meet the demands of the application's The initial implementation of Java threads on the Solaris system was many-to-one. The Java on Solaris environment supports the many-to-many threads model. (4) In a system with only user-level threads, CPU scheduling is still based on the process as a unit, and multiple threads in a running process are controlled by the user program to control the rotation of threads; in a system with kernel support threads, the CPU The scheduling is based on threads , and the thread scheduler of the OS is responsible for thread scheduling. These functions all reside in the user space as the interface between the user thread and the kernel. The use of this method is generally discouraged; it is not currently specified as particularly useful. The Solaris operating environment utilizes the following significant MT advances: The Solaris MT kernel - the essential component in a complete implementation of an MT architecture. For example, you can start or suspend all the threads within a group with a single method call. Most multithreading models fall into one of the following categories of threading implementation: Implementations of the many-to-one model (many user threads to one kernel thread) allow the application to create any number of threads that can execute concurrently. Extremely lightweight user threads that can be created, used, and discarded in very large numbers without consuming excessive system resources or degrading system performance. The user-level thread kernel switching is controlled by the user-mode program itself, which does not require kernel intervention, and reduces the consumption of entering and leaving the kernel mode, but it cannot make good use of multi-core CPUs. With its multiple kernel threads, it enables parallelism on multiprocessor machines. For example, if a thread makes a JNI call, the JVM can marshal all JNI arguments and put them in a queue serviced by the main thread. Refer to Table 4-1. In this model, a user-level threads library provides sophisticated scheduling of user-level threads above kernel threads. Kernel thread: created and revoked by the operating system kernel. The kernel needs to manage only the threads that are currently active. The Java on Solaris threaded model delivers the best combination of speed, concurrency, functionality, and kernel resource utilization. You simply write applications using threads. Therefore, the library can't access the thread-specific errno and its errno-dependent response to a failed system call would be incorrect. The Solaris two-level model delivers unprecedented high levels of flexibility for meeting many different programming requirements. Improved server responsiveness. LWPs allow you to rapidly create very fast and cheap threads through a portable application-level interface. Under multiple processors, the threads in the same process only focus on content that can be time-shared and reused under the same processor, Reference : https://blog.csdn.net/landai2011/article/details/78072897. (2) The creation, cancellation, and scheduling of user-level threads do not require the support of the OS kernel, and are processed at the language (such as Java) level; while the kernel supports the creation, cancellation and scheduling of threads that require the support of the OS kernel, and It is basically the same as process creation, cancellation, and scheduling. The POSIX pthreads standard - an implementation of the MT interface defined by the IEEE POSIX 1003.1c specification.
Daemon threads are useful to libraries that need to create threads which are unknown to applications. (2) It can be implemented in an operating system that does not support threads. Do this only if absolutely unavoidable. (1) Thread scheduling does not require the kernel to directly participate, and the control is simple. The number of processors available to a process is dynamic and unpredictable. The following are the differences between user-level threads and kernel-level threads: (1) Kernel support threads are perceptible to the OS kernel, while user-level threads are not perceptible to the OS kernel. Previous to Java on Solaris 2.6 software, the Java runtime used a user-level threads library called "green threads", part of the Java runtime thread and system support layer. Direct native support for Java's daemon threads. Many concurrent compute operations and I/O requests within a single process. A single-threaded process follows a single sequence of control while executing. A many-to-many implementation at the user level reduces programming effort as it lifts restrictions on the number of threads that can be effectively used in an application. (5) The program entity of user-level threads is a program that runs in user mode, while the program entity of kernel-supported threads is a program that can run in any state. The Java on Solaris operating environment is the first many-to-many commercial implementation of Java on an MT operating system. At present, Linux pthreads generally do this. "Programming Compute-Bound, Parallellized Java Applications", 2010, Oracle Corporation and/or its affiliates. The many-to-many model (many user-level threads to many kernel-level threads) avoids many of the limitations of the one-to-one model, while extending multithreading capabilities even further. This concurrent activity speeds applications up - one of the main benefits of multithreading. All execution entities are built using kernel threads, which represent a fully preemptive, real-time "nucleus" within the kernel. However, on the Solaris platform, the semantics have been extended so that it also interrupts the target thread's I/O calls. The one-to-one model (one user thread to one kernel thread) is among the earliest implementations of true multithreading. The runtime system puts a thread into a thread group during thread construction. (6) There can only be one thread running at the same time in the same process. The threads library implements a user-level scheduler that is separate from the system scheduler. Simple routines can be written for each activity, making complex programs easier to design and code, and more adaptive to a wide variation in user demands. As a result, this multithreading model provides limited concurrency and does not exploit multiprocessors. This is the difference between having to write your own hypervisor and kernel threads. In a many-to-one (user-level threads) implementation, all threads activity is restricted to user space. Though accessing Solaris-specific features from Java applications is not recommended, here is a list of those features to illustrate the richness of the Solaris MT architecture: The ability to define bound or unbound threads for user or system level control of application concurrency. Daemon threads are threads that run in the background and have dedicated exit semantics enabling them to terminate independently of the processes that use them. This workaround is not trivial and may cause deadlocks if not carefully programmed. Consequently, many implementations of this model, such as Windows NT and the OS/2 threads package, limit the number of threads supported on the system. The thread priorities available to Java threads on a native threaded JVM should be treated as hints to the scheduler, especially if the threads are compute-bound. Certain programs, such as window programs, demand heavy logical parallelism. Java thread groups are implemented by the ThreadGroup [(in the API reference documentation)] class in the java.lang package. User threads run on an intermediate system. The LWP can obtain the services provided by the kernel through system calls, and the user threads in the process can be associated with the LWP through multiplexing to obtain the services of the kernel.
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