[jira] [Created] (ARROW-17441) [Python] Memory kept after del and pool.released_unused()

[Will Jones (Jira)]

Will Jones created ARROW-17441:
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             Summary: [Python] Memory kept after del and pool.released_unused()
                 Key: ARROW-17441
                 URL: https://issues.apache.org/jira/browse/ARROW-17441
             Project: Apache Arrow
          Issue Type: Improvement
          Components: Python
    Affects Versions: 9.0.0
            Reporter: Will Jones


I was trying reproduce another issue involving memory pools not releasing 
memory, but encountered this confusing behavior: if I create a table, then call 
{{{}del table{}}}, and then {{{}pool.release_unused(){}}}, I still see 
significant memory usage. On mimalloc in particular, I see no meaningful drop 
in memory usage on either call.

Am I missing something?
{code:python}
import os
import psutil
import time
import gc
process = psutil.Process(os.getpid())
import numpy as np
from uuid import uuid4


import pyarrow as pa

def gen_batches(n_groups=200, rows_per_group=200_000):
    for _ in range(n_groups):
        id_val = uuid4().bytes
        yield pa.table({
            "x": np.random.random(rows_per_group), # This will compress poorly
            "y": np.random.random(rows_per_group),
            "a": pa.array(list(range(rows_per_group)), type=pa.int32()), # This 
compresses with delta encoding
            "id": pa.array([id_val] * rows_per_group), # This compresses with 
RLE
        })

def print_rss():
    print(f"RSS: {process.memory_info().rss:,} bytes")

print(f"memory_pool={pa.default_memory_pool().backend_name}")
print_rss()
print("reading table")
tab = pa.concat_tables(list(gen_batches()))
print_rss()
print("deleting table")
del tab
gc.collect()
print_rss()
print("releasing unused memory")
pa.default_memory_pool().release_unused()
print_rss()
print("waiting 10 seconds")
time.sleep(10)
print_rss()
{code}
{code:none}
> ARROW_DEFAULT_MEMORY_POOL=mimalloc python test_pool.py && \
    ARROW_DEFAULT_MEMORY_POOL=jemalloc python test_pool.py && \
    ARROW_DEFAULT_MEMORY_POOL=system python test_pool.py
memory_pool=mimalloc
RSS: 44,449,792 bytes
reading table
RSS: 1,819,557,888 bytes
deleting table
RSS: 1,819,590,656 bytes
releasing unused memory
RSS: 1,819,852,800 bytes
waiting 10 seconds
RSS: 1,819,852,800 bytes
memory_pool=jemalloc
RSS: 45,629,440 bytes
reading table
RSS: 1,668,677,632 bytes
deleting table
RSS: 698,400,768 bytes
releasing unused memory
RSS: 699,023,360 bytes
waiting 10 seconds
RSS: 699,023,360 bytes
memory_pool=system
RSS: 44,875,776 bytes
reading table
RSS: 1,713,569,792 bytes
deleting table
RSS: 540,311,552 bytes
releasing unused memory
RSS: 540,311,552 bytes
waiting 10 seconds
RSS: 540,311,552 bytes
{code}



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