Why python doesn't have Garbage Collector thread?

ghz 12hours ago ⋅ 1 views

Java has daemon thread to monitor memory usage and do the gc task. From jstack I see

"main" #1 prio=5 os_prio=0 tid=0x00007f34b000e000 nid=0x808 waiting on condition [0x00007f34b6f02000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
    at java.lang.Thread.sleep(Native Method)
....
"GC task thread#0 (ParallelGC)" os_prio=0 tid=0x00007f34b0023000 nid=0x809 runnable 
"GC task thread#1 (ParallelGC)" os_prio=0 tid=0x00007f34b0024800 nid=0x80a runnable 
"GC task thread#2 (ParallelGC)" os_prio=0 tid=0x00007f34b0026800 nid=0x80b runnable

But speaking of python, I wrote a

#!/usr/bin/env python
import gc
import time 
gc.enable()
while True:
    print "This prints once a minute."
    time.sleep(60)

I saw the the python process has only one thread,

$ cat /proc/1627/status
Name:   python
...
Threads:    1

The question is, why python doesn't have gc thread like Java? Then which thread does the gc task?

Answer

Great question! The reason Python doesn't have a dedicated garbage collection (GC) thread like Java is due to the differences in how the two languages handle memory management and garbage collection. Let's break this down:

1. Python's Garbage Collection Mechanism

In Python, garbage collection is primarily handled by the reference counting mechanism and cyclic garbage collection (GC) for objects involved in reference cycles.

  • Reference Counting: Every Python object has an associated reference count. When an object's reference count drops to zero (i.e., no more references to it exist), Python immediately deallocates that object. This is very efficient and happens in the thread that is executing the code.

  • Cyclic Garbage Collection: Python uses an additional garbage collector for reference cycles (i.e., when objects reference each other in a cycle, such as a -> b -> c -> a). These cycles can't be cleaned up by reference counting alone, so the cyclic GC runs periodically to detect and break cycles. This is handled by the main thread of the Python program, not by a separate GC thread.

2. Why Python Doesn't Need a Daemon GC Thread

  • Single-threaded nature: Python typically operates in a single-threaded environment (especially with the Global Interpreter Lock, or GIL in CPython). The GC works in the main thread as part of the normal execution flow. The cyclic garbage collector is invoked either explicitly (via gc.collect()) or implicitly when the interpreter detects the need (e.g., when the program allocates more memory or when a threshold is crossed).

  • Performance Considerations: Unlike Java, where GC operations can take a significant amount of time and can be parallelized across multiple threads to minimize pauses, Python's memory management is simpler, and the GC operations are less complex. The cyclic GC runs in the main thread during idle times, usually triggered when the interpreter is not executing user code or when a manual gc.collect() is invoked. This is generally not a performance bottleneck.

3. The Role of the GC in Python

  • In your Python example, when you call gc.enable() and run your script, Python will automatically perform garbage collection to clean up objects and handle cycles during the execution of the program. However, there is no separate thread dedicated to running GC tasks; instead, GC runs in the main thread when certain thresholds are reached (e.g., after a certain number of allocations or after an object has been deallocated).

  • In CPython, the cyclic GC runs periodically. The main thread checks whether garbage collection is needed, and if it is, the collection happens in the same thread. The Python interpreter will pause execution of your program only for a brief time while it runs the GC, but it won't create a separate daemon thread for GC like Java does.

4. Can You See Python's GC in Action?

If you're running a long-lived process and want to see the GC activity, you can manually trigger garbage collection in Python and observe it. For example:

import gc
import time

# Enable the garbage collector
gc.enable()

# Trigger GC explicitly
gc.collect()

while True:
    print("Running main loop...")
    time.sleep(60)  # This will keep the process alive
    # You can force a GC run here if needed:
    gc.collect()

You can also set the GC debug flags to get detailed logs on when GC is occurring:

import gc
gc.set_debug(gc.DEBUG_LEAK)

5. How Python's GC Affects Threads

  • Since Python uses reference counting and cyclic GC (as opposed to Java's more complex GC algorithms), there's no need for background GC threads. Python's GC behavior, including collecting objects and handling cycles, happens in the same thread that's executing your Python code. This keeps the implementation simpler, but it also means that garbage collection might cause occasional pauses during the program's execution.

  • If you're running Python with multiple threads (with threading module, for example), the GC will still occur in the main thread unless you explicitly manage it with multiple threads, but the GC will interact with objects that are shared across threads in the normal way.

6. How Does Java's GC Work Differently?

  • Java (specifically in the JVM) uses a multi-threaded garbage collection system, which can run in parallel or concurrently with the application's main threads. This is a more sophisticated setup designed to optimize the pauses that occur during garbage collection by distributing the work across multiple threads, often with dedicated GC threads (e.g., GC task thread as seen in your jstack output).

  • Java also uses generational garbage collection (young, old, and permanent generations) and has various types of collectors (like ParallelGC, G1GC, ZGC, etc.) that determine how GC threads behave. These GC threads are more complex and often require a separate daemon thread to handle the GC tasks in the background without blocking application threads.

Conclusion:

In summary, Python doesn't need a separate GC thread because:

  • The reference counting and cyclic GC mechanisms are simpler than Java's GC model.
  • Python runs its garbage collection in the main thread, typically during idle times or when memory pressure reaches certain thresholds.
  • The GC mechanism is tied to Python's simpler, single-threaded execution model (especially in CPython).

If you need to monitor or control garbage collection in Python, you can interact with the gc module, but there is no need for a dedicated background thread like in Java.

#py