Implement zero copy writes in SelectorSocketTransport in asyncio

[kumaraditya303]

BPO	47010
Nosy	@gvanrossum, @asvetlov, @1st1, @jakirkham, @kumaraditya303
PRs	GH-91166: zero copy SelectorSocketTransport transport implementation #31871

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-03-14.09:18:41.974>
labels = ['3.11', 'expert-asyncio', 'performance']
title = 'Implement zero copy writes in SelectorSocketTransport in asyncio'
updated_at = <Date 2022-03-21.23:44:35.077>
user = 'https://github.com/kumaraditya303'

bugs.python.org fields:

activity = <Date 2022-03-21.23:44:35.077>
actor = 'jakirkham'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['asyncio']
creation = <Date 2022-03-14.09:18:41.974>
creator = 'kumaraditya'
dependencies = []
files = []
hgrepos = []
issue_num = 47010
keywords = ['patch']
message_count = 2.0
messages = ['415124', '415131']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'asvetlov', 'yselivanov', 'jakirkham', 'kumaraditya']
pr_nums = ['31871']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'resource usage'
url = 'https://bugs.python.org/issue47010'
versions = ['Python 3.11']

[kumaraditya303]

Currently, _SelectorSocketTransport transport creates a copy of the data before sending which in case of large amount of data, can create multiple giga bytes copies of data before sending.

Script demonstrating current behavior:

import asyncio
import memory_profiler

@memory_profiler.profile
async def handle_echo(reader: asyncio.StreamReader, writer: asyncio.StreamWriter):
    data = b'x' * 1024 * 1024 * 1000  # 1000 MiB payload
    writer.write(data)
    await writer.drain()
    writer.close()

async def main():
    server = await asyncio.start_server(
        handle_echo, '127.0.0.1', 8888)
    addrs = ', '.join(str(sock.getsockname()) for sock in server.sockets)
    print(f'Serving on {addrs}')

    async with server:
        asyncio.create_task(server.start_serving())
        reader, writer = await asyncio.open_connection('127.0.0.1', 8888)
        while True:
            data = await reader.read(1024 * 1024 * 100)
            if not data:
                break

asyncio.run(main())

Memory profile result:

Filename: test.py

Line #    Mem usage    Increment  Occurrences   Line Contents
=============================================================
     4     17.7 MiB     17.7 MiB           1   @memory_profiler.profile
     5                                         async def handle_echo(reader: asyncio.StreamReader, writer: asyncio.StreamWriter):
     6   1017.8 MiB   1000.1 MiB           1       data = b'x' * 1024 * 1024 * 1000 bpo-1000 MiB payload
     7   2015.3 MiB    997.5 MiB           1       writer.write(data)
     8   2015.3 MiB   -988.1 MiB           2       await writer.drain()
     9   1027.1 MiB   -988.1 MiB           1       writer.close()

------------------------------------------------------------------------

To make it zero copy, python's buffer protocol can be used and use memory views of data to save RAM. The writelines method currently joins all the data before sending whereas it can use socket.sendmsg to make it more memory efficient.

Links:

• writelines -

  cpython/Lib/asyncio/transports.py

  Line 116 in 2153daf

  def writelines(self, list_of_data):

• socket.sendmsg - https://docs.python.org/3/library/socket.html#socket.socket.sendmsg

• memory_profiler -
  https://pypi.org/project/memory-profiler/

[asvetlov]

Known problem, PR is welcome!
I expect the fix is not trivial.

[kumaraditya303]

New benchmark comparing various chunk and packet sizes:

import asyncio
from pyperf import Runner


async def handle_echo(reader: asyncio.StreamReader, writer: asyncio.StreamWriter, chunks: int, packet_size: int):
    data = b'x' * packet_size
    for _ in range(chunks):
        writer.write(data)
        await writer.drain()
    writer.close()
    await writer.wait_closed()


async def main(chunks: int, packet_size: int):
    server = await asyncio.start_server(
        lambda reader, writer: handle_echo(reader, writer, chunks, packet_size), '127.0.0.1', 8882)

    async with server:
        asyncio.create_task(server.start_serving())
        reader, writer = await asyncio.open_connection('127.0.0.1', 8882)
        while True:
            data = await reader.read(packet_size)
            if not data:
                break
        writer.close()
        await writer.wait_closed()

if __name__ == '__main__':
    runner = Runner()
    for chunks in [10, 100]:
        for packet_size in [1024, 1024 ** 2, 1024 ** 2 * 10]:
            runner.bench_async_func(
                f'echo {chunks} -- {packet_size}', main, chunks, packet_size)

Fallback implementation with send:

Benchmark	base	send
echo 10 -- 1024	1.11 ms	951 us: 1.16x faster
echo 10 -- 1048576	12.3 ms	9.89 ms: 1.24x faster
echo 10 -- 10485760	165 ms	80.9 ms: 2.04x faster
echo 100 -- 1024	1.51 ms	1.56 ms: 1.03x slower
echo 100 -- 1048576	98.6 ms	81.0 ms: 1.22x faster
echo 100 -- 10485760	1.31 sec	764 ms: 1.72x faster
Geometric mean	(ref)	1.35x faster

Faster Implementation with sendmsg:

Benchmark	base	sendmsg
echo 10 -- 1024	1.11 ms	924 us: 1.20x faster
echo 10 -- 1048576	12.3 ms	9.34 ms: 1.32x faster
echo 10 -- 10485760	165 ms	77.7 ms: 2.12x faster
echo 100 -- 1024	1.51 ms	1.58 ms: 1.05x slower
echo 100 -- 1048576	98.6 ms	74.5 ms: 1.32x faster
echo 100 -- 10485760	1.31 sec	717 ms: 1.83x faster
Geometric mean	(ref)	1.41x faster

Memory usage:

Benchmark	base-memory	sendmsg-memory
echo 10 -- 1048576	14.9 MB	13.8 MB: 1.08x faster
echo 10 -- 10485760	47.7 MB	22.8 MB: 2.09x faster
echo 100 -- 1048576	15.2 MB	13.7 MB: 1.10x faster
echo 100 -- 10485760	51.6 MB	22.9 MB: 2.25x faster
Geometric mean	(ref)	1.33x faster

Benchmark hidden because not significant (2): echo 10 -- 1024, echo 100 -- 1024

The new implementation is faster on both send and sendmsg across the board with #31871 and avoids memory copying so "zero copy writes".