Multithreading models and its types, comparison between process and thread.

Multithreading Models

Depending on the support for user and kernel threads, three different multithreading models are :-

1.      Many-to-one model

2.      One-to-one model

3.      Many-to-many model

1.      Many-to-one model  

·        In many-to-one model, many user level threads are mapped to one kernel level thread. (see fig).

·        Thread management is done in user space.

·        If any of the threads makes a blocking system call, the entire process will be blocked.

·        Multiple threads are unable to run in parallel on multiprocessor as only on thread can access the kernel at a time.

·        For example Green thread-a thread library of Solaris 2 uses this model.

2.      One-to-one model

·        In this model, each user thread is mapped to one kernel thread hence the name one-to-one model (see fig.).

·        In this model, when one threads makes a blocking system call, the other threads continue to run. Therefore it provides more concurrency than many to-one model.

·        It also allows the multiple threads to run on multiprocessors.

·        The drawback of this model is that each user thread requires its corresponding kernel thread. Thus the overhead of creating kernel thread burdens the performance of an application.

·        The operating systems that implement this model are Windows NT, Window 2000 and OS/2.

3.      Many-to-many model

·        Many-to-many model multiplexes many user level treads to a smaller or equal number of kernel threads (see fig.)

·        The number of kernel threads created depends on a particular application or particular machine.

·        Although, this model allows the creation of multiple kernel threads, true concurrency cannot be achieved by this model as the kernel can schedule only one thread at a time.

·        In this model, developer can create as many user threads as necessary.

·        Also, when a thread performs blocking system call the kernel can schedule another thread for execution.

·        This model is supported by Soalris 2 operating system. .

Comparisons between process and threads:-

Sno.	Process	Thread
1.	Each process has its own distinguish address space in memory.	All threads is a process share the same address space
2.	Process require more system resources than threads	Threads require less system resourse than process
3.	Process are independent of one another.	Unlike process, threads are not independent of one another.
4.	New process takes more time for creation, it also takes its more time to terminate a process	New threads take less time for creation than new process. Moreover threads also take less time to terminate.
5.	It takes more time to switch between two process.	It takes less time to switch between two threads of a same process.
6.	Process are more robust in comparison to threads	Threads are less robust.