Threads
A thread is the smallest unit of execution
in a process. In other words, a thread is a sequence of instructions within a
program that can be executed independently of other code. Here are some more
details:
Threads of Benefits
Using threads in programming can offer several benefits:
- Improved
Responsiveness:
In interactive applications, multithreading can allow a program to
continue running even if a part of it is blocked or is performing a
lengthy operation, thereby increasing responsiveness to users.
- Resource
Sharing:
Threads share the memory and the resources of the process to which they
belong by default. The benefit of this is that a program doesn't need to
be divided into separate processes to share data easily between different
tasks.
- Efficiency: Threads are more economical
than processes. They are faster to create and destroy, and they use less
system resources. Thread switching does not require a context switch (a
process that saves and restores the state), hence it's more efficient.
- Utilization
of Multiprocessor Architectures:
The real advantage of multithreading becomes apparent on multiprocessor or
multi-core systems. Here, multiple threads of a single process can run in
parallel on different cores, which can lead to a significant speedup.
- Simplicity
of Program Design:
A single-threaded process must be organized so that it can handle many
different tasks at once, such as user input, computations, and I/O. With
threads, you can design your program so that each thread handles a
specific task, which can simplify program design and maintenance.
It's worth noting that while threads can offer these benefits,
they also come with their own challenges such as synchronization issues,
deadlocks, and race conditions. Effective use of threads requires careful
program design and debugging.
Users and Kernel Threads
Threads can operate at two levels: user level and kernel level.
Here's the distinction between the two:
- User-Level
Threads: These
threads are managed by a thread library at the user level. The kernel is
not aware of these threads and hence can't directly manage them or
schedule them. All the thread management tasks like creation, scheduling,
and synchronization are done in user space by the application. The benefit
is that operations on user threads are faster because they don't require
interaction with the kernel. However, because the kernel isn't aware of
user threads, if one user thread gets blocked (such as for an I/O
operation), the entire process gets blocked.
- Kernel-Level
Threads: These
threads are managed directly by the operating system. The kernel is aware
of and schedules all kernel-level threads, so if one thread blocks, others
can continue executing. Kernel threads can also take advantage of
multiprocessor systems because the kernel has the ability to schedule
threads on different processors. However, operations on kernel threads can
be slower than user threads, because they require system calls to the
kernel, which take more time.
A hybrid approach called the Many-to-Many Model also
exists where multiple user-level threads are multiplexed to an equal or lesser
number of kernel threads. The number of kernel threads may be specific to
either a particular application or a particular machine (an
operating-system-wide setting). This model combines the benefits of both
user-level and kernel-level threads and helps to avoid the drawbacks of each.
Multithreading Models - Many to One, One to One, Many to Many
Multithreading models define the way that user threads (created
and managed by a threading library in user space) relate to kernel threads
(those that the operating system kernel has knowledge of and can schedule).
- Many-to-One
Model: In this
model, multiple user-level threads map to a single kernel thread. As such,
only one thread can access the kernel at a time, so if one user thread
performs blocking operations, the entire process is blocked. On the plus
side, thread management (creation, synchronization, destruction) is fast
and efficient since it is done entirely in user space.
- One-to-One
Model: In this
model, each user-level thread maps to a kernel thread. This model allows
multiple threads to run in parallel on multiprocessors and doesn't block
the entire process if one thread performs a blocking operation. However,
creating a user thread involves creating the corresponding kernel thread,
and because creating kernel threads can be more resource-intensive, some
systems limit the number of threads that can be created.
- Many-to-Many
Model: This
model multiplexes multiple user-level threads to a smaller or equal number
of kernel threads. This provides a good balance between the two previous
models. The kernel can schedule multiple threads in parallel on multiple
processors. Also, when a user thread performs a blocking operation, the
kernel can schedule another thread for execution.
A variant of the Many-to-Many Model is the Two-Level Model,
where the system allows a user-level thread to be bound to one kernel thread,
providing a mixture of both the Many-to-Many and One-to-One Models.
The choice of which model to use depends on factors such as the
operating system, the nature of the tasks the program needs to perform, and the
hardware.