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78 lines
2.5 KiB
Markdown
78 lines
2.5 KiB
Markdown
# Parallel Overhead
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## Overview
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This example measures the overhead of parallel synchronization by comparing two approaches to shared state access: mutex-based locking and lock-free atomic operations. Multiple threads compute Fibonacci numbers with randomized inputs and accumulate results through either a mutex-protected shared variable or an atomic counter. By comparing the two builds, you can quantify the profiling overhead introduced by different synchronization primitives. Thread synchronization uses a `pthread_barrier_t` to ensure all threads start simultaneously.
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## Source Files
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- `parallel-overhead.cpp` - Defines `fib()` (recursive Fibonacci, `noinline`), `run()` (per-thread worker with random input variance), and barrier-synchronized thread management. The `USE_LOCKS` compile-time macro switches between mutex and atomic modes.
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## Prerequisites
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- CMake 3.25+
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- C++ compiler with pthreads support
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## Building
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**Standalone build:**
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```bash
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cmake -B <build_dir> -S <project_root>/examples/parallel-overhead
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cmake --build <build_dir>
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```
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**As part of the examples suite:**
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```bash
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cmake -B <build_dir> -S <project_root>/examples/
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cmake --build <build_dir> --target parallel-overhead parallel-overhead-locks
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```
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| Target | Description |
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| -------- | ------------- |
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| `parallel-overhead` | Lock-free atomic accumulation |
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| `parallel-overhead-locks` | Mutex-based accumulation (`USE_LOCKS=1`) |
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## Running
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```bash
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# Default: fib(10), min(16, ncpus) threads, 50000 iterations
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./parallel-overhead
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# Custom: fib(12), 8 threads, 20000 iterations
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./parallel-overhead 12 8 20000
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# Mutex variant
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./parallel-overhead-locks 12 8 20000
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```
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**Arguments:**
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| Position | Description | Default |
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| ---------- | ------------- | --------- |
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| 1 | Fibonacci number to compute | 10 |
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| 2 | Number of threads | min(16, hardware concurrency) |
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| 3 | Number of iterations per thread | 50000 |
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## Profiling with rocprofiler-systems
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Compare overhead between variants:
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```bash
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# Atomic version
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rocprof-sys-run -- ./parallel-overhead 10 8 10000
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# Mutex version
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rocprof-sys-run -- ./parallel-overhead-locks 10 8 10000
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```
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### Recommended Configuration
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| Variable | Value | Purpose |
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| ---------- | ------- | --------- |
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| `ROCPROFSYS_TRACE` | `true` | Generate Perfetto trace for timeline analysis |
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| `ROCPROFSYS_PROFILE` | `true` | Generate call-stack profile |
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| `ROCPROFSYS_USE_SAMPLING` | `ON` | Enable statistical sampling |
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| `ROCPROFSYS_SAMPLING_FREQ` | `50` | Sampling frequency (interrupts/sec) |
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