Memory-Safe Python Script Patterns

Battle-tested patterns for keeping Python scripts alive under systemd MemoryMax constraints. Extracted from repair_direct_parquet.py (24-worker parallel repair) and exness_tick_cache_seeder.py (10-symbol daily seeder) after 5 OOM optimization cycles on a 62 GB GPU workstation.

Core insight: Python's garbage collector frees objects, but the C allocator (glibc ptmalloc2) does NOT return freed pages to the OS. Without explicit malloc_trim(0), RSS only grows — even after del and gc.collect(). mimalloc with MIMALLOC_PURGE_DELAY helps but explicit purge is faster.

Self-Evolving Skill: This skill improves through use. If instructions are wrong, parameters drifted, or a workaround was needed — fix this file immediately, don't defer. Only update for real, reproducible issues.

The 7 Patterns

1. Cached Allocator Purge

The most important pattern. Cache the ctypes library handle on first call so subsequent purges are a single FFI invocation with zero allocation overhead.

import ctypes
import gc
import sys

_purge_lib = None
_purge_method = None  # "mimalloc" | "glibc" | "none"

def _force_allocator_purge():
    """Force mimalloc/glibc to return freed pages to the OS."""
    global _purge_lib, _purge_method

    if sys.platform != "linux":
        return

    if _purge_method is None:
        try:
            _purge_lib = ctypes.CDLL("libmimalloc.so.2")
            _purge_method = "mimalloc"
        except OSError:
            try:
                _purge_lib = ctypes.CDLL("libc.so.6")
                _purge_method = "glibc"
            except OSError:
                _purge_method = "none"

    if _purge_method == "mimalloc":
        _purge_lib.mi_collect(ctypes.c_bool(True))
    elif _purge_method == "glibc":
        _purge_lib.malloc_trim(0)

def _force_gc():
    """Python GC + allocator purge. Call every 50 iterations + between work units."""
    gc.collect()
    _force_allocator_purge()

Why cached handle matters: ctypes.CDLL("libc.so.6") calls dlopen() which itself allocates memory. Calling it 1400 times in a loop is counterproductive. Cache it once.

Why prefer mimalloc: When LD_PRELOAD=libmimalloc.so.2 is active, glibc's malloc_trim is a no-op because mimalloc intercepted all allocations. mi_collect(True) is the correct purge for mimalloc.

2. HTTP Response Lifecycle

Close responses immediately after extracting the content you need. The requests library holds the response body, connection pool references, and urllib3 internal state.

# CORRECT: extract content, close, delete
resp = requests.get(url, timeout=60)
if resp.status_code != 200:
    resp.close()
    return None

content = resp.content  # Extract what you need
resp.close()            # Release connection pool reference
del resp                # Drop the Python object

# Process content...
del content             # Release after processing

# WRONG: response lives until end of function scope
resp = requests.get(url, timeout=60)
data = parse(resp.content)  # resp still alive, holding ~18 MB
return data                 # resp GC'd eventually... maybe

Why this matters: Each requests.Response holds content (the full body), a reference to the urllib3.HTTPResponse, and the connection pool's PoolManager. With 4 concurrent workers processing 1400 URLs, unclosed responses accumulate hundreds of MB.

3. Explicit Object Deletion

Don't rely on Python's GC for large objects. Use del immediately after the object is no longer needed.

# After writing a DataFrame to Parquet
_atomic_write_parquet(df, path)
del df  # Only reference gone → immediate refcount GC

# After extracting data from a ZIP
with zipfile.ZipFile(io.BytesIO(zip_bytes)) as zf:
    df = pl.read_csv(zf.open(zf.namelist()[0]), ...)
del zip_bytes  # Release raw ZIP content after parsing

# After processing a list of work items
results = process_all(missing_days)
del missing_days  # Release the 1400-element date list

When to del: any object larger than ~1 MB that you're done with. DataFrames, byte strings from HTTP responses, ZIP contents, large lists.

4. Periodic GC Cadence

Call _force_gc() at two levels:

# Level 1: Every 50 iterations within a work unit
for i, item in enumerate(items):
    process(item)
    if (i + 1) % 50 == 0:
        _force_gc()

# Level 2: Between major work units
for symbol in symbols:
    seed_symbol(symbol)
    _force_gc()  # Release all per-symbol state before next symbol

Why 50: Empirically validated on a 32-core workstation. At 100, RSS drifts too high before purge. At 25, the purge overhead is measurable (~2% throughput loss). 50 is the sweet spot from repair_direct_parquet.py.

5. ThreadPoolExecutor Cleanup

After the executor exits, explicitly clean up residual state.

with concurrent.futures.ThreadPoolExecutor(max_workers=4) as ex:
    pending = {}
    # ... bounded future submission pattern ...

# After pool exits:
del pending     # Future objects hold references to results
del missing     # Work item list
_force_gc()     # Release worker thread memory + allocator pages

For advanced cases (DB connections in workers), close thread-local resources explicitly:

pool.shutdown(wait=False, cancel_futures=True)
for t in threading.enumerate():
    if t.name.startswith("ThreadPoolExecutor"):
        _close_worker_cache()  # Close DB connections
gc.collect()
_force_allocator_purge()

6. Thread-Local Connection Reuse

Never create database connections or HTTP sessions inside a loop. Use threading.local() to get one connection per worker thread.

import threading

_thread_local = threading.local()

def _get_worker_cache():
    """One DB connection per worker thread, reused across all iterations."""
    cache = getattr(_thread_local, "cache", None)
    if cache is None:
        cache = DatabaseClient()
        _thread_local.cache = cache
    return cache

def _close_worker_cache():
    """Explicit cleanup at shutdown."""
    cache = getattr(_thread_local, "cache", None)
    if cache is not None:
        cache.close()
        _thread_local.cache = None

Why this prevents fd exhaustion: Each urllib3.PoolManager(maxsize=20) holds up to 20 file descriptors. Creating a new one per iteration in a 24-worker pool exhausts ulimit -n 1024 within minutes. Thread-local reuse keeps fd count at ~4N+50 for N workers.

7. systemd Service Configuration

[Service]
# Memory limits — hard kill prevents runaway RSS
MemoryHigh=2G        # Soft limit: triggers reclaim pressure
MemoryMax=4G         # Hard limit: SIGKILL on breach
MemorySwapMax=0      # No swap escape — fail fast, don't thrash

# mimalloc: replaces glibc ptmalloc2, returns freed pages faster
Environment=LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libmimalloc.so.2
Environment=MIMALLOC_PURGE_DELAY=1000

# OOM priority (lower = more likely to survive)
OOMScoreAdjust=-200
ManagedOOMMemoryPressure=kill

MemoryHigh vs MemoryMax: MemoryHigh triggers kernel memory reclaim (cgroup pressure) — the process slows but survives. MemoryMax is a hard SIGKILL. Set MemoryHigh at 50-66% of MemoryMax so the kernel gets a chance to reclaim before killing.

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Anti-Patterns

Anti-Pattern	Why It Fails	Fix
ctypes.CDLL("libc.so.6") inside a loop	dlopen() allocates memory; 1000 calls wastes ~50 MB	Cache the handle in a module global
requests.get() without resp.close()	Response body + connection pool held until GC	resp.close() + del resp immediately after extracting content
No gc.collect() between work units	Cyclic references accumulate across symbols/batches	_force_gc() between every major work unit
New DB connection per loop iteration	Each connection = 20 fds via urllib3 PoolManager	threading.local() for one-per-thread reuse
Raising MemoryMax to fix OOM	Masks the leak; RSS will grow to fill any limit	Fix the leak first. The fix is always one of patterns 1-6
del df without gc.collect()	Refcount frees the object, but glibc holds the pages	del + gc.collect() + _force_allocator_purge()
MemorySwapMax not set	Process swaps to disk instead of dying; thrashes for hours	Set MemorySwapMax=0 — fail fast, don't thrash

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Diagnostic Checklist

When a script gets SIGKILL (status=9) under systemd:

1. Confirm it's OOM: journalctl --user -u service.service | grep -E "killed|signal|KILL"
2. Check peak RSS: systemctl --user status service.service | grep Memory (shows peak)
3. Profile steady-state RSS: Run the script manually, check /proc/PID/status for VmRSS at 3 time points 30s apart
4. Check fd count: ls /proc/PID/fd | wc -l — if >500, suspect connection churn (Pattern 6)
5. Check allocator: Is LD_PRELOAD=libmimalloc.so.2 in the service file? If glibc, check if malloc_trim is called
6. Add periodic logging: logger.info("RSS=%d MB", psutil.Process().memory_info().rss // 1048576) every 50 iterations

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Reference Implementations

Script	Patterns Used	RSS Profile
scripts/repair_direct_parquet.py	All 7 patterns	Starts 3 GB, plateaus ~13 GB with 24 workers
scripts/exness_tick_cache_seeder.py	Patterns 1-5, 7	Flat 163 MB across 10 symbols x 1400 days
scripts/tick_cache_seeder.py	Patterns 3, 7	Peak 2.5 GB with mimalloc (was 4.47 GB with glibc)

Post-Execution Reflection

After this skill completes, check before closing:

1. Did the command succeed? — If not, fix the instruction or error table that caused the failure.
2. Did parameters or output change? — If the underlying tool's interface drifted, update Usage examples and Parameters table to match.
3. Was a workaround needed? — If you had to improvise (different flags, extra steps), update this SKILL.md so the next invocation doesn't need the same workaround.

Only update if the issue is real and reproducible — not speculative.