macmon causes MLX Metal GPU crash during inference — recommend mactop instead

[PixelMyoos]

Bug Report: macmon GPU monitoring triggers SIGABRT in libmlx.dylib during active inference

Environment

• Hardware: Mac17,6 (MacBook Pro M5, 128GB)
• OS: macOS 26.4.1 (25E253)
• Python: 3.12.13 (Homebrew)
• Stack: Exo (ai.hermes.gateway-ss coalition), oMLX paged inference, libmlx.dylib
• Triggered by: Running macmon (the system monitor recommended in Exo's README) while MLX inference is active

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What Happened

Launching macmon while Exo/oMLX is serving inference causes a hard crash in Python (PID 39036) with SIGABRT / Abort trap: 6.

Crash thread (Thread 52 — com.Metal.CompletionQueueDispatch):

mlx::core::gpu::check_error(MTLCommandBuffer)
  → cxa_throw  (C++ exception thrown inside Metal async dispatch boundary)
  → std::terminate()  (exception cannot propagate across dispatch queue)
  → abort()
  → SIGABRT

Full stack:

0  libsystem_kernel.dylib   pthread_kill
1  libsystem_pthread.dylib  pthread_kill
2  libsystem_c.dylib        abort
3  libc++abi.dylib          abort_message
4  libc++abi.dylib          demangling_terminate_handler
5  libobjc.A.dylib          objc_terminate
6  libc++abi.dylib          std::terminate()
7  libc++abi.dylib          __cxa_throw
8  libmlx.dylib             mlx::core::gpu::check_error(MTLCommandBuffer*)
9  libmlx.dylib             (Metal CompletionHandler block)
10 Metal                    -[MTLCommandBuffer didCompleteWith...]
11 IOGPU                    IOGPUNotificationQueueDispatchAvailableCompletionNotifications

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Root Cause

macmon reads Apple Silicon performance counters and GPU utilization metrics via IOKit/IOGPUFamily — the same interfaces Metal uses internally. When macmon samples the GPU concurrently with an in-flight Metal command buffer (during MLX inference), the GPU returns an error state that mlx::core::gpu::check_error detects and attempts to throw as a C++ exception. Because this happens inside a GCD async dispatch block (com.Metal.CompletionQueueDispatch), the exception cannot propagate, triggering std::terminate() → abort().

This is not a user error — Exo's own documentation and README point users toward macmon as the recommended Apple Silicon monitoring tool. Users following those instructions will hit this crash reliably on M3/M4/M5 systems running active inference.

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Reproduction Steps

1. Start Exo with MLX inference engine on Apple Silicon (M3/M4/M5)
2. Load a model and begin inference (active Metal command buffers in flight)
3. Launch macmon in a separate terminal
4. Observe Python crash with SIGABRT — Abort trap: 6

---

Recommendation: Replace macmon with mactop

mactop is a drop-in alternative that:

• Uses the same sysinfo / powermetrics data sources as Activity Monitor
• Does not directly query IOGPUFamily in a way that interferes with active Metal sessions
• Has been used extensively on the same M5 hardware alongside active MLX inference with zero crashes

Suggested README change:

macmon → mactop for real-time Apple Silicon GPU/ANE/CPU monitoring during Exo inference

Install:

brew install mactop
sudo mactop   # requires sudo for power metrics

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Impact

• Any user following Exo's documented toolchain on Apple Silicon who uses macmon for monitoring while running inference is at risk of hard Python crashes mid-session
• Crash is non-recoverable (process terminates, active inference context lost)
• No warning or graceful error — just Abort trap: 6

---

Crash Report Excerpt (Thread 52 — crashing thread)

Exception Type:  EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Termination Reason: Namespace SIGNAL, Code 6, Abort trap: 6
Triggered by Thread: 52, Dispatch Queue: com.Metal.CompletionQueueDispatch

Thread 52 Crashed:
0   libsystem_kernel.dylib   pthread_kill + 8
1   libsystem_pthread.dylib  pthread_kill + 296
2   libsystem_c.dylib        abort + 148
3   libc++abi.dylib          abort_message + 132
4   libc++abi.dylib          demangling_terminate_handler + 272
5   libobjc.A.dylib          objc_terminate + 172
6   libc++abi.dylib          std::terminate() + 16
7   libc++abi.dylib          __cxa_throw + 92
8   libmlx.dylib             mlx::core::gpu::check_error(MTLCommandBuffer*) + 244
9   libmlx.dylib             (Metal completion handler block)

Coalition: ai.hermes.gateway-ss | Hardware: Mac17,6 | macOS 26.4.1