Have shutil.copytree(), copy() and copystat() use cached scandir() stat()s

[giampaolo]

BPO	33695
Nosy	@brettcannon, @ncoghlan, @vstinner, @giampaolo, @benjaminp, @tarekziade, @benhoyt, @serhiy-storchaka, @1st1
PRs	bpo-33695 shutil.copytree() + os.scandir() cache #7874 bpo-35652 Add use_srcentry parameter to shutil.copytree() II #11425 bpo-35652: shutil.copytree(copy_function=...): pass DirEntry instead of path str  #11997 bpo-38688: Consume iterator and create list of entries to prevent infinite loop #17098
Files	bench.py bpo-33695.patch

^{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 = 'https://github.com/giampaolo'
closed_at = <Date 2018-11-12.14:19:18.809>
created_at = <Date 2018-05-30.12:22:35.667>
labels = ['3.8', 'library', 'performance']
title = 'Have shutil.copytree(), copy() and copystat() use cached scandir() stat()s'
updated_at = <Date 2019-11-09.13:25:26.220>
user = 'https://github.com/giampaolo'

bugs.python.org fields:

activity = <Date 2019-11-09.13:25:26.220>
actor = 'kinow'
assignee = 'giampaolo.rodola'
closed = True
closed_date = <Date 2018-11-12.14:19:18.809>
closer = 'giampaolo.rodola'
components = ['Library (Lib)']
creation = <Date 2018-05-30.12:22:35.667>
creator = 'giampaolo.rodola'
dependencies = []
files = ['47624', '47625']
hgrepos = []
issue_num = 33695
keywords = ['patch']
message_count = 13.0
messages = ['318175', '320303', '320304', '321852', '322872', '322873', '322901', '322912', '322933', '322975', '322984', '328267', '329732']
nosy_count = 10.0
nosy_names = ['brett.cannon', 'ncoghlan', 'vstinner', 'giampaolo.rodola', 'benjamin.peterson', 'tarek', 'stutzbach', 'benhoyt', 'serhiy.storchaka', 'yselivanov']
pr_nums = ['7874', '11425', '11997', '17098']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue33695'
versions = ['Python 3.8']

[giampaolo]

Patch in attachment makes shutil.copytree() use os.scandir() and (differently from bpo-33414) DirEntry instances are passed around so that cached stat()s are used also from within copy2() and copystat() functions. The number of times the filesystem gets accessed via os.stat() is therefore reduced quite consistently. A similar improvement can be done for rmtree() (but that's for another ticket). Patch and benchmark script are in attachment.

Linux (+13.5% speedup)
======================

--- without patch:

./python  bench.py 
Priming the system's cache...
7956 files and dirs, repeat 1/3... min = 0.551s
7956 files and dirs, repeat 2/3... min = 0.548s
7956 files and dirs, repeat 3/3... min = 0.548s
best result = 0.548s

--- with patch:

    $ ./python  bench.py 
    Priming the system's cache...
    7956 files and dirs, repeat 1/3... min = 0.481s
    7956 files and dirs, repeat 2/3... min = 0.479s
    7956 files and dirs, repeat 3/3... min = 0.474s
    best result = 0.474s

Windows (+17% speedup)
======================

--- without patch:

./python  bench.py 
Priming the system's cache...
7956 files and dirs, repeat 1/3... min = 9.015s
7956 files and dirs, repeat 2/3... min = 8.747s
7956 files and dirs, repeat 3/3... min = 8.614s
best result = 8.614s

--- with patch:

    $ ./python  bench.py 
    Priming the system's cache...
    7956 files and dirs, repeat 1/3... min = 7.827s
    7956 files and dirs, repeat 2/3... min = 7.369s
    7956 files and dirs, repeat 3/3... min = 7.153s
    best result = 7.153s

Windows SMB share (+30%)
========================

--- without patch:

C:\Users\user\Desktop\cpython>PCbuild\win32\python.exe bench.py
Priming the system's cache...
7956 files and dirs, repeat 1/3... min = 46.853s
7956 files and dirs, repeat 2/3... min = 46.330s
7956 files and dirs, repeat 3/3... min = 44.720s
best result = 44.720s

--- with patch:

C:\Users\user\Desktop\cpython>PCbuild\win32\python.exe bench.py
Priming the system's cache...
7956 files and dirs, repeat 1/3... min = 31.729s
7956 files and dirs, repeat 2/3... min = 30.936s
7956 files and dirs, repeat 3/3... min = 30.936s
best result = 30.936s

Number of stat() syscalls (-38%)
================================

--- without patch:

    $ strace ./python bench.py  2>&1 | grep "stat(" | wc -l
    324808
    
--- with patch:

    $ strace ./python bench.py  2>&1 | grep "stat(" | wc -l
    198768

[giampaolo]

PR at: #7874.
I re-ran benchmarks since shutil code changed after bpo-33695. Linux went from +13.5% to 8.8% and Windows went from +17% to 20.7%.
In the PR I explicitly avoided using a context manager around os.scandir() for now so that patch it's easier to review (will add it before pushing).

Linux (+8.8%)
=============

without patch:
$ ./python bench-copytree.py
Priming the system's cache...
7956 files and dirs, repeat 1/3... min = 0.604s
7956 files and dirs, repeat 2/3... min = 0.603s
7956 files and dirs, repeat 3/3... min = 0.601s

with patch:
    $ ./python  bench-copytree.py 
    Priming the system's cache...
    7956 files and dirs, repeat 1/3... min = 0.557s
    7956 files and dirs, repeat 2/3... min = 0.548s
    7956 files and dirs, repeat 3/3... min = 0.548s
    best result = 0.548s

Windows (+20.7%)
================

without patch:
C:\Users\user\Desktop>cpython\PCbuild\win32\python.exe cpython\bench-copytree.py
Priming the system's cache...
7956 files and dirs, repeat 1/3... min = 8.275s
7956 files and dirs, repeat 2/3... min = 8.018s
7956 files and dirs, repeat 3/3... min = 7.978s
best result = 7.978s

With patch:
C:\Users\user\Desktop>cpython\PCbuild\win32\python.exe cpython\bench-copytree.py
Priming the system's cache...
7956 files and dirs, repeat 1/3... min = 6.609s
7956 files and dirs, repeat 2/3... min = 6.609s
7956 files and dirs, repeat 3/3... min = 6.609s
best result = 6.609s

[giampaolo]

I re-ran benchmarks since shutil code changed after bpo-33695.

Sorry, I meant bpo-33671.

[giampaolo]

Unless somebody has complaints I think I'm gonna merge this soon.

[serhiy-storchaka]

I'm not convinced that this change should be merged. The benefit is small, and 1) it is only for an artificial set of tiny files, 2) the benchmarking ignores the real IO, it measures the work with a cache. When copy real files (/usr/include or Lib/) with dropped caches the difference is insignificant. On other hand, this optimization makes the code more complex. It can make the case with specifying the ignore argument slower.

[serhiy-storchaka]

For dropping disc caches on Linux run

    with open('/proc/sys/vm/drop_caches', 'ab') as f: f.write(b'3\n')

before every test.

[giampaolo]

I agree the provided benchmark on Linux should be more refined. And I'm not sure if "echo 3 | sudo tee /proc/sys/vm/drop_caches" before running it is enough honestly.

The main point here is the reduction of stat() syscalls (-38%) and that can make a considerable difference, especially with network filesystems. That's basically the reason why scandir() was introduced in the first place and used in os.walk() glob.glob() and shutil.rmtree(), so I'm not sure why we should use a different rationale for shutil.copytree().

[serhiy-storchaka]

os.walk() and glob.glob() used *only* stat(), opendir() and readdir() syscalls (and stat() syscalls dominated). The effect of reducing the number of the stat() syscalls is significant. shutil.rmtree() uses also the unlink() syscall. Since it is usually cheap (but see bpo-32453), the benefit still is good, but not such large. Actually I had concerns about using scandir() in shutil.rmtree().

shutil.copytree() needs to open, read, and write files. This is not so cheap, and the benefit of reducing the number of the stat() syscalls is hardly noticed in real cases. shutil.copytree() was not converted to using scandir() intentionally.

[vstinner]

When I worked on the os.scandir() implementation, I recall that an interesting test was NFS. Depending on the configuration, stat() in a network filesystem can be between very slow and slow.

[giampaolo]

Yes, file copy (open() + read() + write()) is of course more expensive than just "reading" a tree (os.walk(), glob()) or deleting it (rmtree()) and the "pure file copy" time adds up to the benchmark. And indeed it's not an coincidence that bpo-33671 (which replaced read() + write() with sendfile()) shaved off a 5% gain from the benchmark I posted initially for Linux.

Still, in a 8k small-files-tree scenario we're seeing ~9% gain on Linux, 20% on Windows and 30% on a SMB share on localhost vs. VirtualBox. I do not consider this a "hardly noticeable gain" as you imply: it is noticeable, exponential and measurable, even with cache being involved (as it is).

Note that the number of stat() syscalls per file is being reduced from 6 to 1 (or more if follow_symlinks=False), and that is the real gist here. That *does* make a difference on a regular Windows fs and makes a huge difference with network filesystems in general, as a simple stat() call implies access to the network, not the disk.

[1st1]

Depending on the configuration, stat() in a network filesystem can be between very slow and slow.

+1. I also quickly glanced over the patch and I think it looks like a clear win.

[giampaolo]

@serhiy: I would like to proceed with this. Do you have further comments? Do you prefer to bring this up on python-dev for further discussion?