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Implement spend_cpu_cycles in assembly

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The compiler was unrolling it previously, so we're doing assembly now for consistency.
This commit is contained in:
Andrew Noyes
2025-09-05 15:16:49 -04:00
parent ffe7ab0a3e
commit 0357a41dd8
10 changed files with 84 additions and 28 deletions
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2
benchmarks/bench_cpu_work.cpp
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@@ -5,7 +5,7 @@
#include "../src/cpu_work.hpp"
int main(int argc, char *argv[]) {
int iterations = DEFAULT_HEALTH_CHECK_ITERATIONS; // Default: 7000
int iterations = DEFAULT_HEALTH_CHECK_ITERATIONS;
if (argc > 1) {
try {

2
config.md
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@@ -51,7 +51,7 @@ Controls benchmarking and health check behavior.
| Parameter | Type | Default | Description |
|-----------|------|---------|-------------|
| `ok_resolve_iterations` | integer | `7000` | CPU-intensive loop iterations for `/ok` requests in resolve stage. 0 = health check only, 7000 = default benchmark load (~650ns, 1M req/s) |
| `ok_resolve_iterations` | integer | `4000` | CPU-intensive loop iterations for `/ok` requests in resolve stage. 0 = health check only, 4000 = default benchmark load (~650ns, 1M req/s) |
## Example Configuration

2
config.toml
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@@ -29,6 +29,6 @@ keepalive_interval_seconds = 30
[benchmark]
# CPU-intensive loop iterations for /ok requests in resolve stage
# 0 = health check only (no CPU work)
# 7000 = default benchmark load (650ns CPU work, 1M req/s)
# 4000 = default benchmark load (740ns CPU work, 1M req/s)
# Higher values = more CPU stress testing
ok_resolve_iterations = 0

4
src/config.hpp
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@@ -79,8 +79,8 @@ struct SubscriptionConfig {
*/
struct BenchmarkConfig {
/// CPU-intensive loop iterations for /ok requests in resolve stage
/// 0 = health check only, 7000 = default benchmark load (650ns, 1M req/s)
int ok_resolve_iterations = 7000;
/// 0 = health check only, 4000 = default benchmark load (740ns, 1M req/s)
int ok_resolve_iterations = 0;
};
/**

67
src/cpu_work.cpp
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@@ -1,10 +1,61 @@
#include "cpu_work.hpp"
void spend_cpu_cycles(int iterations) {
// Perform CPU-intensive work that cannot be optimized away
// Use inline assembly to prevent compiler optimization
for (int i = 0; i < iterations; ++i) {
// Simple loop with inline assembly barrier to prevent optimization
asm volatile("");
}
}
#if defined(__x86_64__) || defined(__amd64__)
// x86-64 file-scoped assembly implementation
#ifdef __APPLE__
asm(".text\n"
".globl _spend_cpu_cycles\n"
"_spend_cpu_cycles:\n"
" test %edi, %edi\n" // Test if iterations <= 0
" jle .L_end\n" // Jump to end if <= 0
".L_loop:\n" // Loop start
" dec %edi\n" // Decrement iterations
" jnz .L_loop\n" // Jump back if not zero
".L_end:\n" // End
" ret\n" // Return
);
#else
asm(".text\n"
".globl spend_cpu_cycles\n"
".type spend_cpu_cycles, @function\n"
"spend_cpu_cycles:\n"
" test %edi, %edi\n" // Test if iterations <= 0
" jle .L_end\n" // Jump to end if <= 0
".L_loop:\n" // Loop start
" dec %edi\n" // Decrement iterations
" jnz .L_loop\n" // Jump back if not zero
".L_end:\n" // End
" ret\n" // Return
".size spend_cpu_cycles, .-spend_cpu_cycles\n");
#endif
#elif defined(__aarch64__)
// ARM64 file-scoped assembly implementation
#ifdef __APPLE__
asm(".text\n"
".globl _spend_cpu_cycles\n"
"_spend_cpu_cycles:\n"
" cmp w0, wzr\n" // Compare iterations with zero
" b.le .L_end\n" // Branch to end if <= 0
".L_loop:\n" // Loop start
" subs w0, w0, #1\n" // Decrement iterations and set flags
" b.ne .L_loop\n" // Branch back if not zero
".L_end:\n" // End
" ret\n" // Return
);
#else
asm(".text\n"
".globl spend_cpu_cycles\n"
".type spend_cpu_cycles, %function\n"
"spend_cpu_cycles:\n"
" cmp w0, wzr\n" // Compare iterations with zero
" b.le .L_end\n" // Branch to end if <= 0
".L_loop:\n" // Loop start
" subs w0, w0, #1\n" // Decrement iterations and set flags
" b.ne .L_loop\n" // Branch back if not zero
".L_end:\n" // End
" ret\n" // Return
".size spend_cpu_cycles, spend_cpu_cycles\n");
#endif
#endif

5
src/cpu_work.hpp
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@@ -1,5 +1,7 @@
#pragma once
extern "C" {
/**
* @brief Perform CPU-intensive work for benchmarking and health check purposes.
*
@@ -10,6 +12,7 @@
* @param iterations Number of loop iterations to perform
*/
void spend_cpu_cycles(int iterations);
}
/**
* @brief Default CPU work iterations for health check benchmarking.
@@ -18,4 +21,4 @@ void spend_cpu_cycles(int iterations);
* /ok health check resolve stage. This value provides 650ns of CPU work
* and achieves 1M requests/second throughput through the 4-stage pipeline.
*/
constexpr int DEFAULT_HEALTH_CHECK_ITERATIONS = 7000;
constexpr int DEFAULT_HEALTH_CHECK_ITERATIONS = 4000;

5
src/http_handler.cpp
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@@ -79,6 +79,11 @@ void HttpHandler::on_connection_closed(Connection &conn) {
void HttpHandler::on_write_buffer_drained(
std::unique_ptr<Connection> &conn_ptr) {
// Reset arena after all messages have been written for the next request
auto *state = static_cast<HttpConnectionState *>(conn_ptr->user_data);
if (state) {
TRACE_EVENT("http", "reply",
perfetto::Flow::Global(state->http_request_id));
}
on_connection_closed(*conn_ptr);
conn_ptr->reset();
on_connection_established(*conn_ptr);

2
src/http_handler.hpp
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@@ -88,7 +88,7 @@ struct HttpHandler : ConnectionHandler {
resolveThread = std::thread{[this]() {
pthread_setname_np(pthread_self(), "txn-resolve");
for (;;) {
auto guard = commitPipeline.acquire<1, 0>();
auto guard = commitPipeline.acquire<1, 0>(/*maxBatch*/ 1);
if (process_resolve_batch(guard.batch)) {
return; // Shutdown signal received
}

2
test_benchmark_config.toml
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@@ -30,4 +30,4 @@ keepalive_interval_seconds = 30
[benchmark]
# Use original benchmark load for testing
ok_resolve_iterations = 7000
ok_resolve_iterations = 4000

21
threading_performance_report.md
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@@ -2,7 +2,7 @@
## Summary
WeaselDB's /ok health check endpoint achieves 1M requests/second with 650ns of configurable CPU work per request through the 4-stage commit pipeline, while maintaining 0% CPU usage when idle. The configurable CPU work serves both as a health check (validating the full pipeline) and as a benchmarking tool for measuring per-request processing capacity.
WeaselDB's /ok health check endpoint achieves 1M requests/second with 740ns of configurable CPU work per request through the 4-stage commit pipeline, while maintaining 0% CPU usage when idle. The configurable CPU work serves both as a health check (validating the full pipeline) and as a benchmarking tool for measuring per-request processing capacity.
## Performance Metrics
@@ -22,9 +22,9 @@ WeaselDB's /ok health check endpoint achieves 1M requests/second with 650ns of c
**Health Check Pipeline (/ok endpoint)**:
- **Throughput**: 1.0M requests/second
- **Configurable CPU work**: 650ns (7000 iterations, validated with nanobench)
- **Configurable CPU work**: 740ns (4000 iterations, validated with nanobench)
- **Theoretical maximum CPU time**: 1000ns (1,000,000,000ns ÷ 1,000,000 req/s)
- **CPU work efficiency**: 65% (650ns ÷ 1000ns)
- **CPU work efficiency**: 74% (740ns ÷ 1000ns)
- **Pipeline stages**: Sequence (noop) → Resolve (CPU work) → Persist (response) → Release (cleanup)
- **CPU usage when idle**: 0%
@@ -41,12 +41,9 @@ WeaselDB's /ok health check endpoint achieves 1M requests/second with 650ns of c
- **Maintained**: 100,000 spin iterations necessary to prevent thread descheduling
- **Result**: Same throughput with more efficient spinning
**Stage-0 Batch Size Optimization**:
- **Changed**: Stage-0 max batch size from unlimited to 1
**Resolve Batch Size Optimization**:
- **Changed**: Resolve max batch size from unlimited to 1
- **Mechanism**: Single-item processing checks for work more frequently, keeping the thread in fast coordination paths instead of expensive spin/wait cycles
- **Profile evidence**: Coordination overhead reduced from ~11% to ~5.6% CPU time
- **Result**: Additional 12.7% increase in serial CPU budget (488ns → 550ns)
- **Overall improvement**: 38.9% increase from baseline (396ns → 550ns)
### Request Flow
@@ -56,8 +53,8 @@ I/O Threads (8) → HttpHandler::on_batch_complete() → Commit Pipeline
↑ ↓
| Stage 0: Sequence (noop)
| ↓
| Stage 1: Resolve (650ns CPU work)
| (spend_cpu_cycles(7000))
| Stage 1: Resolve (740ns CPU work)
| (spend_cpu_cycles(4000))
| ↓
| Stage 2: Persist (generate response)
| (send "OK" response)
@@ -71,8 +68,8 @@ I/O Threads (8) → HttpHandler::on_batch_complete() → Commit Pipeline
## Test Configuration
- Server: test_benchmark_config.toml with 8 io_threads, 8 epoll_instances
- Configuration: `ok_resolve_iterations = 7000` (650ns CPU work)
- Configuration: `ok_resolve_iterations = 4000` (740ns CPU work)
- Load tester: targeting /ok endpoint
- Benchmark validation: ./bench_cpu_work 7000
- Benchmark validation: ./bench_cpu_work 4000
- Build: ninja
- Command: ./weaseldb --config test_benchmark_config.toml