Quartz 4

thread debugging

2 min read

thread debuging

Using top to Monitor Threads

When debugging multi-threaded programs, top can help monitor individual threads within a process. You can launch your program in the background and immediately attach top to it:

./program & top -H -p $!

Explanation:

  • ./program & → Runs the program in the background.
  • $! → Captures the PID of the last background process.
  • top -H -p $! → Shows the threads (-H flag) of the process with the given PID.

Inspecting Memory Mappings with /proc/$PID/maps

To analyze memory allocations of a running process, use /proc/$PID/maps:

./program & export PID=$! && cat /proc/$PID/maps

Explanation:

  • ./program & → Runs the program in the background.
  • export PID=$! → Stores the PID for later use.
  • cat /proc/$PID/maps → Displays the memory layout of the process.

Understanding /proc/$PID/maps

This file contains memory regions mapped into the process’s address space. Example output:

00400000-00452000 r-xp 00000000 fd:01 123456 /home/user/program
00651000-00652000 r--p 00051000 fd:01 123456 /home/user/program
00652000-00653000 rw-p 00052000 fd:01 123456 /home/user/program
7f8c3b000000-7f8c3b021000 rw-p 00000000 00:00 0 
7f8c3b021000-7f8c3b800000 ---p 00000000 00:00 0

Key Fields:

  • Address range (e.g., 00400000-00452000) → Start and end addresses.
  • Permissions:
    • r → Read
    • w → Write
    • x → Execute
    • p → Private mapping
  • Offset (e.g., 00000000) → File offset for mapped files.
  • Device & inode (e.g., fd:01 123456) → Mapped file details.
  • File mapping (e.g., /home/user/program) → The associated file if applicable.

Practical Use Cases

  • Tracking high CPU usage in threads → top -H -p $PID
  • Inspecting memory layout for debugging segmentation faults → /proc/$PID/maps
  • Checking shared libraries and stack mappings → Look for mapped .so files and [stack]

This is useful when debugging memory-related issues and analyzing thread behavior in multi-threaded applications.

ps to Inspect Threads and CPU Usage

./program & ps -T -p $!
  • -T → Show threads of the given PID.
  • -p → Attach to a specific process.

perf for CPU Profiling

./program & export PID=$! && perf record -g -p $PID perf report
  • -g → Capture call graphs.
  • -p → Attach to a process.

strace to Trace System Calls of Threads

./program & strace -f -p $!
  • -f → Follow threads.
  • -p → Attach to a process.

valgrind helgrind ofc 🙃

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