Async Runtime Performance Optimization: Fix CPU Thrashing Issues#626
Async Runtime Performance Optimization: Fix CPU Thrashing Issues#626
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Actionable comments posted: 4
🧹 Nitpick comments (5)
rust-executor/src/holochain_service/mod.rs (1)
764-821: Test doesn’t exercise termination paths; consider an additional coverageThe perf test is fine, but add a test that all inputs are closed and the loop terminates (i.e.,
else => breakis hit). This would guard against regressions like the unconditional sleep arm above.rust-executor/src/js_core/mod.rs (1)
296-301: Minor: centralize backoff durationsConsider constants for 1ms and 10ms backoffs to avoid magic numbers and ease tuning across loops.
Also applies to: 374-377, 421-428
rust-executor/src/ai_service/mod.rs (3)
41-43: Global rate limiter looks good; consider bounded cleanup for abandoned modelsIf a download never reaches 100%, entries linger until process exit. Optional: periodically evict entries older than N minutes.
155-207: Rate limiting logic is sound; minor polish on status flagsCurrently every allowed progress event publishes
downloaded=false, loaded=false, even at 100% where status is “Loaded”. Consider aligning flags with status (e.g.,downloaded=trueat 100%, optionallyloaded=trueonly after model is ready to serve).Apply this diff at publish time for 100%:
- publish_model_status(model_id.clone(), progress, &status, false, false).await; + let (downloaded, loaded) = if progress >= 100.0 { (true, false) } else { (false, false) }; + publish_model_status(model_id.clone(), progress, &status, downloaded, loaded).await;
1395-1449: Test verifies the map timing, not the end-to-end progress pathThis is fine as a unit of the limiter, but it won’t catch regressions in
handle_progressbehavior (e.g., cleanup on 100%). Consider an integration-y async test that invokeshandle_progresstwice under 100 ms (expect one publish), then once at >=100% (expect publish and cleanup).I can draft a minimal test that injects a mock
publish_model_statusvia a trait to assert call counts without touching the DB/pubsub. Want me to sketch it?
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📒 Files selected for processing (3)
rust-executor/src/ai_service/mod.rs(4 hunks)rust-executor/src/holochain_service/mod.rs(2 hunks)rust-executor/src/js_core/mod.rs(3 hunks)
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rust-executor/src/holochain_service/mod.rs (1)
rust-executor/src/js_core/mod.rs (1)
start(296-471)
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| // Add a gentle backoff when no signals are available to prevent busy-waiting | ||
| _ = tokio::time::sleep(tokio::time::Duration::from_millis(1)) => { | ||
| // This provides a small backoff to prevent excessive CPU usage | ||
| // when no signals are being processed | ||
| } | ||
| else => break, |
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Unreachable else branch due to always-enabled sleep arm in select!
Because the sleep arm is always enabled, else => break can never fire, so this loop won’t terminate even if all streams are closed, leaving the task alive forever.
Apply this diff to gate the sleep arm and keep the else reachable:
- _ = tokio::time::sleep(tokio::time::Duration::from_millis(1)) => {
+ // Back off only while at least one input is still open
+ _ = tokio::time::sleep(tokio::time::Duration::from_millis(1)),
+ if !new_app_ids_receiver.is_closed() || !streams.is_empty()
+ => {
// This provides a small backoff to prevent excessive CPU usage
// when no signals are being processed
}📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| // Add a gentle backoff when no signals are available to prevent busy-waiting | |
| _ = tokio::time::sleep(tokio::time::Duration::from_millis(1)) => { | |
| // This provides a small backoff to prevent excessive CPU usage | |
| // when no signals are being processed | |
| } | |
| else => break, | |
| // Add a gentle backoff when no signals are available to prevent busy-waiting | |
| // Back off only while at least one input is still open | |
| _ = tokio::time::sleep(tokio::time::Duration::from_millis(1)), | |
| if !new_app_ids_receiver.is_closed() || !streams.is_empty() | |
| => { | |
| // This provides a small backoff to prevent excessive CPU usage | |
| // when no signals are being processed | |
| } | |
| else => break, |
🤖 Prompt for AI Agents
In rust-executor/src/holochain_service/mod.rs around lines 128-133, the
tokio::select! sleep arm is always enabled so the `else => break` branch is
unreachable; gate the sleep arm with a condition (e.g., `if sleep_enabled` or
`if !all_streams_closed`) so it only participates while there are active
streams, update that boolean when streams are closed (or set an Option to None),
and leave the `else => break` intact so the loop can terminate when all streams
are closed.
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but we could get streams in later through apps that get installed. it would break if we would have left the loop before, no?
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@lucksus You're absolutely right! I apologize for the incorrect suggestion.
Looking at the architecture more carefully, this signal processing loop is designed to run for the entire lifetime of the Holochain service, and new apps can be installed at any time. If the loop were to exit when all current streams are closed, there would be no mechanism to handle streams from apps that get installed later.
The current implementation with the unconditional sleep arm is actually correct because:
- The loop needs to stay alive indefinitely to handle future app installations
- The
new_app_ids_receiverchannel remains open for the service lifetime - New apps can be installed at any point, requiring the loop to add their streams
- The 1ms sleep prevents busy-waiting while keeping the task responsive
The "unreachable else branch" I flagged isn't actually a problem here - it's intentional that the loop never exits through that path. The loop should only terminate when the entire service shuts down.
Thank you for the correction! The current code is working as intended.
🐰 curious head tilt
/| /|
( ._.)
o_(")(")
✏️ Learnings added
Learnt from: lucksus
PR: coasys/ad4m#626
File: rust-executor/src/holochain_service/mod.rs:128-133
Timestamp: 2025-09-15T17:50:36.703Z
Learning: In the Holochain service signal processing loop in rust-executor/src/holochain_service/mod.rs, the tokio::select! loop with an unconditional sleep arm is intentionally designed to never reach the `else => break` branch. This keeps the loop alive indefinitely to handle streams from apps that may be installed later during the service lifetime, via the new_app_ids_receiver channel.
Note: Learnings are effective only in the context of similar code segments. To apply general coding guidelines, please configure review instructions. You can manage existing learnings in the app.
| } else { | ||
| // No more requests available, add a small delay to prevent busy-waiting | ||
| tokio::time::sleep(tokio::time::Duration::from_millis(1)).await; | ||
| } |
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Do not spin when the request channel is closed; break instead of sleeping
recv().await returns None only when the channel is closed. Sleeping here leaks the loop forever and needlessly consumes wakeups.
Apply this diff:
- } else {
- // No more requests available, add a small delay to prevent busy-waiting
- tokio::time::sleep(tokio::time::Duration::from_millis(1)).await;
+ } else {
+ // Channel closed — exit the loop
+ break;
}📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| } else { | |
| // No more requests available, add a small delay to prevent busy-waiting | |
| tokio::time::sleep(tokio::time::Duration::from_millis(1)).await; | |
| } | |
| } else { | |
| // Channel closed — exit the loop | |
| break; | |
| } |
🤖 Prompt for AI Agents
In rust-executor/src/js_core/mod.rs around lines 288 to 291, the code currently
sleeps when recv().await yields None which causes the loop to spin indefinitely
after the request channel is closed; change the logic so that when recv().await
returns None you break out of the loop instead of calling tokio::time::sleep,
and only perform a short sleep when recv() returns Some but there are no work
items to process (i.e., keep the sleep in the branch handling an empty queue,
not in the channel-closed branch), ensuring the loop cleanly exits when the
channel is closed.
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2 participants
Overview
This PR addresses critical performance issues in AD4M that were causing 100% CPU usage and system slowdowns. The fixes target two primary sources of async runtime thrashing:
yield_now()in async loopsProblem Analysis
Issue 1: Kalosm Progress Thrashing 🔥 Primary Culprit
During AI model downloads, kalosm would call progress callbacks hundreds or thousands of times per second. Each callback spawned a new
tokio::spawn(handle_progress(...))task, leading to:Issue 2: Holochain Signal Busy-Waiting
Multiple async loops used
tokio::task::yield_now()inappropriately, causing busy-waiting:yield_now()in signal processingyield_now()when no requests availableyield_now()when no module requests existyield_now()when no signals availableWhy
yield_now()is problematic: It immediately yields control but schedules the task to run again ASAP, creating busy-waiting that consumes CPU cycles even when idle.Solutions Implemented
1. Kalosm Progress Rate Limiting
Core Implementation: Added a global rate limiter that prevents progress updates more frequent than every 100ms.
Key Features:
2. Holochain Signal Processing Optimization
Core Fix: Replaced
yield_now()calls with appropriate conditionaltokio::time::sleep()delays.Holochain Service Signal Loop
JS Core Execution Patterns
Delay Strategy:
Technical Implementation Details
Files Modified
rust-executor/src/ai_service/mod.rs:PROGRESS_RATE_LIMITERglobal statehandle_progressfunctionrust-executor/src/holochain_service/mod.rs:yield_nowimportrust-executor/src/js_core/mod.rs:Testing
Rate Limiting Test
Signal Processing Test
Expected Performance Impact
Before Fixes
After Fixes
Runtime Architecture Impact
Thread and Runtime Analysis
The fixes address systemic issues in AD4M's complex runtime architecture:
#[tokio::main(flavor = "multi_thread")]new_current_thread)Estimated Thread Reduction: 50-100+ threads → More efficient utilization
Runtime Efficiency: Better task scheduling across all runtimes
Monitoring and Verification
Debug Logging
Enable monitoring with:
Rate Limiting Logs:
Publishing model status for {model}: {progress}% - {status}Rate limited progress update for {model}: {progress}%Cleaned up rate limiter entry for completed model: {model}Performance Metrics
Backwards Compatibility
✅ Zero Breaking Changes
Future Enhancements
Potential Further Optimizations
tokio::spawnwith bounded channelsMonitoring Improvements
Verification Checklist
Summary
This PR resolves the primary causes of CPU thrashing in AD4M:
Expected Results:
The fixes are production-ready and should immediately resolve the 100% CPU usage issues while maintaining all existing functionality.
Summary by CodeRabbit
Bug Fixes
Refactor
Tests