Efficient zero-copy for shutil.copy* functions (Linux, OSX and Win)

[giampaolo]

BPO	33671
Nosy	@facundobatista, @ncoghlan, @pitrou, @scoder, @vstinner, @giampaolo, @tarekziade, @merwok, @bitdancer, @encukou, @asvetlov, @methane, @socketpair, @vadmium, @MojoVampire, @asottile
PRs	bpo-33671: efficient zero-copy for shutil.copy* functions (Linux, OSX and Win) #7160 bpo-33671 / shutil.copyfile: use memoryview() with dynamic size on Windows #7681 bpo-33671: allow setting shutil.copyfile() bufsize globally #12016 bpo-33671 fix orphaned comment in shutil.copyfileobj #27516
Files	shutil-zero-copy.diff

^{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 = <Date 2018-06-12.21:48:06.592>
created_at = <Date 2018-05-28.16:17:23.634>
labels = ['3.8', 'library', 'performance']
title = 'Efficient zero-copy for shutil.copy* functions (Linux, OSX and Win)'
updated_at = <Date 2021-07-31.19:15:48.402>
user = 'https://github.com/giampaolo'

bugs.python.org fields:

activity = <Date 2021-07-31.19:15:48.402>
actor = 'eric.araujo'
assignee = 'none'
closed = True
closed_date = <Date 2018-06-12.21:48:06.592>
closer = 'giampaolo.rodola'
components = ['Library (Lib)']
creation = <Date 2018-05-28.16:17:23.634>
creator = 'giampaolo.rodola'
dependencies = []
files = ['47621']
hgrepos = []
issue_num = 33671
keywords = ['patch', 'needs review']
message_count = 18.0
messages = ['317878', '317880', '317905', '317906', '317932', '317989', '319401', '319405', '319484', '319486', '319980', '320247', '320456', '336492', '336502', '336514', '336516', '398660']
nosy_count = 21.0
nosy_names = ['facundobatista', 'ncoghlan', 'pitrou', 'scoder', 'vstinner', 'giampaolo.rodola', 'gps', 'StyXman', 'tarek', 'eric.araujo', 'r.david.murray', 'petr.viktorin', 'asvetlov', 'methane', 'SilentGhost', 'neologix', 'socketpair', 'python-dev', 'martin.panter', 'josh.r', 'Anthony Sottile']
pr_nums = ['7160', '7681', '12016', '27516']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue33671'
versions = ['Python 3.8']

[giampaolo]

Patch in attachment uses platform specific zero-copy syscalls on Linux and Solaris (os.sendfile(2)), Windows (CopyFileW) and OSX (fcopyfile(2)) speeding up shutil.copyfile() and other functions using it (copy(), copy2(), copytree(), move()).

Average speedup for a 512MB file copy is +24% on Linux, +50% on OSX and +48% on Windows by copying file on the same partition (SSD disk was used).

Follows some benchmarks.

Setup
=====

Create 128K, 8M, 512M file:

    $ python -c "import os; f = open('f1', 'wb'); f.write(os.urandom(128 * 1024))"
    $ python -c "import os; f = open('f1', 'wb'); f.write(os.urandom(8 * 1024 * 1024))"
    $ python -c "import os; f = open('f1', 'wb'); f.write(os.urandom(512 * 1024 * 1024))"

Benchmark:

    $ time ./python -m timeit -s 'import shutil; p1 = "f1"; p2 = "f2"' 'shutil.copyfile(p1, p2)'

Linux
=====

128K copy (+13%):

without patch:
    1000 loops, best of 5: 228 usec per loop
    real    0m1.756s
    user    0m0.386s
    sys     0m1.116s

    with patch:
        1000 loops, best of 5: 198 usec per loop
        real    0m1.464s
        user    0m0.281s
        sys     0m0.958s

8MB copy (+24%):

without patch:
    50 loops, best of 5: 10.1 msec per loop
    real    0m2.703s
    user    0m0.316s
    sys     0m1.847s

    with patch:
        50 loops, best of 5: 7.78 msec per loop
        real    0m2.447s
        user    0m0.086s
        sys     0m1.682s

512MB copy (+26%):

without patch:
    1 loop, best of 5: 872 msec per loop
    real    0m5.574s
    user    0m0.402s
    sys     0m3.115s

    with patch:
        1 loop, best of 5: 646 msec per loop
        real    0m5.475s
        user    0m0.037s
        sys     0m2.959s

OSX
===

128K copy (+8.5%):

without patch:
    500 loops, best of 5: 508 usec per loop
    real    0m2.971s
    user    0m0.442s
    sys     0m2.168s

    with patch:
        500 loops, best of 5: 464 usec per loop
        real    0m2.798s
        user    0m0.379s
        sys     0m2.031s

8MB copy (+67%):

without patch:
    20 loops, best of 5: 32.8 msec per loop
    real    0m3.672s
    user    0m0.357s
    sys     0m1.434s

    with patch:
        20 loops, best of 5: 10.8 msec per loop
        real    0m1.860s
        user    0m0.079s
        sys     0m0.719s

512MB copy (+50%):

without patch:
    1 loop, best of 5: 953 msec per loop
    real    0m5.930s
    user    0m1.021s
    sys     0m4.835s

with patch:
    1 loop, best of 5: 480 msec per loop
    real    0m3.150s
    user    0m0.067s
    sys     0m2.740s

Windows
=======

128K copy (+69%):

without patch:
    50 loops, best of 5: 6.45 msec per loop
with patch:
    50 loops, best of 5: 1.99 msec per loop

8M copy (+64%):

without patch:
    10 loops, best of 5: 22.6 msec per loop
with patch:
    50 loops, best of 5: 7.95 msec per loop

512M copy (+48%):

without patch:
    1 loop, best of 5: 1.21 sec per loop
with patch:
    1 loop, best of 5: 629 msec per loop

[giampaolo]

PR: #7160

[scoder]

Nice, I really like this.

Apart from the usual bit of minor style issues, I couldn't see anything inherently wrong with the PR, but I'll leave the detailed reviews to those who'd have to maintain the code in the future. :)

[scoder]

Regarding the benchmarks, just to be sure, did you try reversing the run order to make sure you don't get unfair caching effects for the later runs?

[socketpair]

http://man7.org/linux/man-pages/man2/ioctl_ficlonerange.2.html

That possibly should be used under Linux in order to really acheive zero-copying. Just like modern cp command.

[giampaolo]

Yes, I tried changing benchmarks order and zero-copy variants are always faster. As for instantaneous CoW copy, it is debatable. E.g. "cp" command does not do it by default:
https://unix.stackexchange.com/questions/80351/why-is-cp-reflink-auto-not-the-default-behaviour
I think shutil should follow the same lead, and perhaps provide a cow=bool argument in the future.

[giampaolo]

New changeset 4a172cc by Giampaolo Rodola in branch 'master':
bpo-33671: efficient zero-copy for shutil.copy* functions (Linux, OSX and Win) (bpo-7160)
4a172cc

[giampaolo]

For future reference, as per #7160 discussion, we decided not to use CopyFileEx on Windows and instead increase read() buffer size from 16KB to 1MB (Windows only) resulting in a 40.8% speedup (instead of 48%). Also copyfileobj() has been optimized on all platforms by using readinto()/memoryview()/bytearray().
Updated benchmarks on Windows:

128KB copy (+27%)

without patch:
    50 loops, best of 5: 7.69 sec per loop
with patch:
    50 loops, best of 5: 5.61 sec per loop

8MB copy (+45.6%)

without patch:
    10 loops, best of 5: 20.8 sec per loop
with patch:
    20 loops, best of 5: 11.3 sec per loop

512MB copy (+40.8%)

without patch:
    1 loop, best of 5: 1.26 sec per loop
with patch:
    1 loop, best of 5: 646 msec per loop

[StyXman]

Thanks Gianpaolo for pushing for this. Great job.

[vstinner]

Thanks Gianpaolo for pushing for this. Great job.

I concur: great job! Cool optimization.