feat: rewrite about page for 2026. (#21)

Signed-off-by: jackfiled <xcrenchangjun@outlook.com>
Reviewed-on: #21

source/posts/hpc-2025-program-smp-platform.md

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+---
+title: High Performance Computing 25 SP Programming SMP Platform
+date: 2025-05-10T00:17:26.5784020+08:00
+tags:
+- 高性能计算
+- 学习资料
+---
+
+
+
+Sharing address space brings simplification.
+
+<!--more-->
+
+### Shared Address Space Programming Paradigm
+
+Vary on mechanism for data sharing, concurrency models and support for synchronization.
+
+- Process based model
+- Lightweight processes and threads.
+
+### Thread
+
+A thread is a single stream of control in the flow of a program.
+
+**Logical memory model of a thread**:
+
+All memory is globally accessible to every thread and threads are invoked as function calls.
+
+![image-20250327200344104](./hpc-2025-program-smp-platform/image-20250327200344104.webp)
+
+Benefits of threads:
+
+- Take less time to create a new thread than a new process.
+- Less time to terminate a thread than a process.
+
+The taxonomy of thread:
+
+- User-Level Thread(ULT): The kernel is not aware of the existence of threads. All thread management is done by the application using a thread library. Thread switching does not require kernel mode privileges and scheduling is application specific.
+- Kernel-Level Thread(KLT): All thread management is done by kernel. No thread library but an API to the kernel thread facility. Switching between threads requires the kernel and schedule on a thread basis.
+- Combined ULT and KLT approaches. Combines the best of both approaches.
+
+![image-20250403183104279](./hpc-2025-program-smp-platform/image-20250403183104279.webp)
+
+## PThread Programming
+
+Potential problems with threads:
+
+- Conflicting access to shared memory.
+- Race condition occur.
+- Starvation
+- Priority inversion
+- Deadlock
+
+### Mutual Exclusion
+
+Mutex locks: implementing critical sections and atomic operations.
+
+Two states: locked and unlocked. At any point of time, only one thread can lock a mutex lock.
+
+### Producer-Consumer Work Queues
+
+The producer creates tasks and inserts them into a work-queue.
+
+The consumer threads pick up tasks from the task queue and execute them.
+
+Locks represent serialization points since critical sections must be executed by threads one after the other.
+
+**Important**: Minimize the size of critical sections.
+
+### Condition Variables for Synchronization
+
+The `pthread_mutex_trylock` alleviates the idling time but introduce the overhead of polling for availability of locks.
+
+An interrupt driven mechanism as opposed to a polled mechanism as the availability is signaled.
+
+A **condition variable**: a data object used for synchronizing threads. Block itself until specified data reaches a predefined state. 
+
+When a thread performs a condition wait, it's not runnable as not use any CPU cycles but a mutex lock consumes CPU cycles as it polls for the locks.
+
+**Common Errors**: One cannot assume any order of execution, must be explicitly established by mutex, condition variables and joins.
+
+## MPI Programming
+
+Low cost message passing architecture.
+
+![image-20250403191254323](./hpc-2025-program-smp-platform/image-20250403191254323.webp)
+
+Mapping of MPI Processes:
+
+MPI views the processes as a one-dimensional topology. But in parallel programs, processes are arranged in higher-dimensional topologies. So it is required to map each MPI process to a process in the higher dimensional topology.
+
+Non-blocking Send and Receive:
+
+`MPI_ISend` and `MPI_Irecv` functions allocate a request object and return a pointer to it.
+
+## OpenMP
+
+A standard for directive based parallel programming.
+
+Thread based parallelism and explicit parallelism.
+
+Use fork-join model:
+
+![image-20250403195750934](./hpc-2025-program-smp-platform/image-20250403195750934.webp)
+