Difference Between threads and processes

Following are some of the major differences between the thread and the processes :

• Processes do not share their address space while threads executing under same process share the address space.
• From the above point its clear that processes execute independent of each other and the synchronization between processes is taken care by kernel only while on the other hand the thread synchronization has to be taken care by the process under which the threads are executing
• Context switching between threads is fast as compared to context switching between processes
• The interaction between two processes is achieved only through the standard inter process communication while threads executing under the same process can communicate easily as they share most of the resources like memory, text segment etc

User threads Vs Kernel Threads

Threads can exist in user space as well as in kernel space.

A user space threads are created, controlled and destroyed using user space thread libraries. These threads are not known to kernel and hence kernel is nowhere involved in their processing. These threads follow co-operative multitasking where-in a thread releases CPU on its own wish ie the scheduler cannot preempt the thread. Th advantages of user space threads is that the switching between two threads does not involve much overhead and is generally very fast while on the negative side since these threads follow co-operative multitasking so if one thread gets block the whole process gets blocked.

A kernel space thread is created, controlled and destroyed by the kernel. For every thread that exists in user space there is a corresponding kernel thread. Since these threads are managed by kernel so they follow preemptive multitasking where-in the scheduler can preempt a thread in execution with a higher priority thread which is ready for execution. The major advantage of kernel threads is that even if one of the thread gets blocked the whole process is not blocked as kernel threads follow preemptive scheduling while on the negative side the context switch is not very fast as compared to user space threads.

If we talk of Linux then kernel threads are optimized to such an extent that they are considered better than user space threads and mostly used in all scenarios except where prime requirement is that of cooperative multitasking.