[issue8425] a -= b should be fast if a is a small set and b is a large set

[Raymond Hettinger]

Change by Raymond Hettinger :


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stage: patch review -> resolved
status: open -> closed

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset 88ea166dadb8aeb34541a0a464662dea222629e5 by Raymond Hettinger in 
branch 'master':
bpo-8425: Fast path for set inplace difference when the second set is large 
(GH-15590)
https://github.com/python/cpython/commit/88ea166dadb8aeb34541a0a464662dea222629e5


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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Raymond Hettinger]

Change by Raymond Hettinger :


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pull_request: https://github.com/python/cpython/pull/15590

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Cheryl Sabella]

Cheryl Sabella  added the comment:

@maker, would you be interested in converting your patch to a Github pull 
request?  Thanks!

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Michele Orrù]

Michele Orrù added the comment:

ping.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Raymond Hettinger]

Raymond Hettinger added the comment:

There's no rush on this.  I have other work I want to do on set objects before 
applying further optimizations, so I want to hold off on it for a bit.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Terry J. Reedy]

Changes by Terry J. Reedy tjre...@udel.edu:


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stage: needs patch - patch review

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

 I find my first patch more readable and efficient, but if these comments
 are conformant to pep7.. Attaching updated patch (issue8425.3.patch)

It was only a suggestion. Both forms are good enougth.

 Yes, but bench.py is available, fell free to run it with whatever
 configuration; my example was done with a --with-pydebug and clang
 compiler.

Yes, I ran it in release mode (gcc, 32-bit Linux) and confirm your results.

In general except minor whitespace defects last patch LGTM.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Michele Orrù]

Changes by Michele Orrù maker...@gmail.com:


Added file: http://bugs.python.org/file27293/bench.py

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Michele Orrù]

Michele Orrù added the comment:

Updated.

tumbolandia:cpython maker$ hg import --no-commit -f issue8425.2.patch  make 
-j3 /dev/null 2/dev/null
sto applicando issue8425.2.patch
tumbolandia:cpython maker$ ./python.exe bench.py starting...
setting up tests...
testing...
big_timer_no_intersection 0.000546158852762
big_timer_subtraction 382.0618862710003
small_timer 0.0034195990001535392
big_timer 603.510513244
std_timer_subtraction 0.000686513778934
big_timer_reversed 292.4404268074
std_timer 8.092500047496287e-05
[44705 refs]
tumbolandia:cpython maker$ hg co -C  make -j3 /dev/null 2/dev/null1 files 
updated, 0 files merged, 0 files removed, 0 files unresolved
tumbolandia:cpython maker$ ./python.exe bench.py 
starting...
setting up tests...
testing...
big_timer_subtraction 611.292889542
big_timer 465.6846392586
small_timer 0.00261835360109
big_timer_reversed 256.5112134430001
std_timer 0.00011092699969594833
big_timer_no_intersection 0.0005648139995173551
std_timer_subtraction 2.861284273945e-05
[44705 refs]

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I added a few comments in Rietveld.

Did you run benchmarks in debug mode?

In order for results to be convenient to compare please sort it by name.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

s/it/them/

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Michele Orrù]

Michele Orrù added the comment:

 I'm not sure of the usefulness of this comment.
removing, then.

 else redundant here.

 Instead of using a local variable intersected, you can simply add else
 Py_INCREF(other) here and then decref other unconditionally. It will be
 shorter and perhaps a little clearer, but a bit slower.

I find my first patch more readable and efficient, but if these comments are 
conformant to pep7.. 
Attaching updated patch (issue8425.3.patch)


 Did you run benchmarks in debug mode?
Yes, but bench.py is available, fell free to run it with whatever 
configuration; my example was done with a --with-pydebug and clang compiler. 

 In order for results to be convenient to compare please sort it by name.
Done.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Michele Orrù]

Michele Orrù added the comment:

 I'm not sure of the usefulness of this comment.
removing, then.

 else redundant here.

 Instead of using a local variable intersected, you can simply add else
 Py_INCREF(other) here and then decref other unconditionally. It will be
 shorter and perhaps a little clearer, but a bit slower.

I find my first patch more readable and efficient, but if these comments are 
conformant to pep7.. 
Attaching updated patch (issue8425.3.patch)


 Did you run benchmarks in debug mode?
Yes, but bench.py is available, fell free to run it with whatever 
configuration; my example was done with a --with-pydebug and clang compiler. 

 In order for results to be convenient to compare please sort it by name.
Done.

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Added file: http://bugs.python.org/file27305/issue8425.3.patch

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Michele Orrù]

Changes by Michele Orrù maker...@gmail.com:


Removed file: http://bugs.python.org/file27293/bench.py

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Michele Orrù]

Changes by Michele Orrù maker...@gmail.com:


Added file: http://bugs.python.org/file27306/bench.py

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Raymond Hettinger]

Raymond Hettinger added the comment:

Please don't apply this until I've signed-off on it.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Michele Orrù]

Michele Orrù added the comment:

Reviewed with Ezio.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

 $ ./python.exe -m timeit -s s= set(range(2000)); l = set(range(2000)); 
 s=s-l

You are measure empty loop.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Michele Orrù]

Michele Orrù added the comment:

woops, sry. Re-posting the benchmark, with three tests: the first one proposed 
(@abacabadabacaba) with very large sets; another one with smaller sets.

$ ./python.exe -m timeit  -n 100 -s s= set(range(2000)); l = 
set(range(2000))   s-=l
100 loops, best of 3: 9.71 usec per loop
[48787 refs]

$ ./python.exe -m timeit  -n 100 -s s= set(range(1)); l = set(range(20))   
s-=l
100 loops, best of 3: 0.366 usec per loop


$ hg co -C
$ make -j3

[!PATCHED]--
$ ./python.exe -m timeit  -n 100 -s s= set(range(2000)); l = 
set(range(2000))   s-=l
100 loops, best of 3: 665 msec per loop
[48787 refs]

$ ./python.exe -m timeit  -n 100 -s s= set(range(1)); l = set(range(20))   
s-=l
100 loops, best of 3: 0.849 usec per loop
[48787 refs]

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

 $ ./python.exe -m timeit  -n 100 -s s= set(range(2000)); l = 
 set(range(2000))   s-=l

s is empty set after first loop. Try this:

$ ./python.exe -m timeit  -n 100 -s s= set(range(2000)); l = 
set(range(2000,2000+2000))   s-=l

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Michele, in any case you patch is not preserve set identity.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Serhiy Storchaka]

Changes by Serhiy Storchaka storch...@gmail.com:


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keywords:  -easy

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Michele Orrù]

Michele Orrù added the comment:

What do you mean by does not preserve set identity? Because I see: 

$ ./python.exe
Python 3.3.0rc2+ (default:178f9042af81+, Sep 24 2012, 18:54:31) 
[GCC 4.2.1 Compatible Apple Clang 2.1 (tags/Apple/clang-163.7.1)] on darwin
Type help, copyright, credits or license for more information.
 a = set(range(1))
[65967 refs]
 b = set(range(20))
[65989 refs]
 id(a)
4540421032
[65992 refs]
 a -= b
[65991 refs]
 id(a)
4540421032

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Sorry, I was wrong.

I think that the proposed changes should be applied in 
set_difference_update_internal() directly.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Michele Orrù]

Michele Orrù added the comment:

Something like this would also need unit tests?

$ ./python.exe -m timeit -s s= set(range(2000)); l = set(range(2000)); 
s=s-l
1000 loops, best of 3: 0.0622 usec per loop
[48787 refs]
$ ./python.exe -m timeit -s s= set(range(2000)); l = set(range(2000)); 
s-=l
1000 loops, best of 3: 0.0591 usec per loop
[48787 refs]

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Added file: http://bugs.python.org/file27265/issue8425.patch

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Michele Orrù]

Changes by Michele Orrù maker...@gmail.com:


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[issue8425] a -= b should be fast if a is a small set and b is a large set

[STINNER Victor]

Changes by STINNER Victor victor.stin...@gmail.com:


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versions: +Python 3.4 -Python 3.3

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

Deferring to 3.3.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

Any new logic should make maximum use of existing tools:

def __isub__(self, other)
if len(other)  len(self)*8:
other = self  other
. . .
# rest of isub unchanged

--
stage: patch review - needs patch

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Daniel Stutzbach]

Changes by Daniel Stutzbach dan...@stutzbachenterprises.com:


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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Mark Dickinson]

Mark Dickinson dicki...@gmail.com added the comment:

See also issue 8685.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Alexander Belopolsky]

Alexander Belopolsky belopol...@users.sourceforge.net added the comment:

The problem is apparently due to the fact that small_set -= large_set
iterates over the large set removing entries from small_set while more
efficient small_set - large_set builds a new set iterating over a
small_set and adding items that are not in the large set.  Since
lookups are O(1), it is more efficient to iterate over a small set
while looking up a large set than the other way around.  I am
attaching a minimalist patch which gives up in-place update for s1 -=
s2 if len(s1)  len(s2) and effectively replaces it with s1 = s1 - s2.
 The code can be further optimized by replicating set_difference logic
in set_isub instead of relying on fall back behavior, but the big
question here  with whether it is acceptable to give up preserving set
identity in in-place subtract to gain performance.

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Added file: http://bugs.python.org/file17282/issue8425.diff

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___Index: Objects/setobject.c
===
--- Objects/setobject.c (revision 81048)
+++ Objects/setobject.c (working copy)
@@ -1612,7 +1612,10 @@
 static PyObject *
 set_isub(PySetObject *so, PyObject *other)
 {
-if (!PyAnySet_Check(other)) {
+if (!PyAnySet_Check(other) ||
+   /* Fall back to s = s - o if len(s)  len(o) to
+  avoid interation over a large set. */
+   PySet_GET_SIZE(so)  PySet_GET_SIZE(other)) {
 Py_INCREF(Py_NotImplemented);
 return Py_NotImplemented;
 }
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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Alexander Belopolsky]

Changes by Alexander Belopolsky belopol...@users.sourceforge.net:


--
stage: needs patch - patch review

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[R. David Murray]

R. David Murray rdmur...@bitdance.com added the comment:

The answer is almost certainly no.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Alexander Belopolsky]

Alexander Belopolsky belopol...@users.sourceforge.net added the comment:

On the second thought, it is possible to preserve set identity and avoid 
iteration over a large set.   See issue8425a.diff attached.

It looks like the unit tests don't check identity preservation.  I will submit 
additional tests shortly.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Alexander Belopolsky]

Changes by Alexander Belopolsky belopol...@users.sourceforge.net:


Added file: http://bugs.python.org/file17284/issue8425-tests.diff

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Alexander Belopolsky]

Alexander Belopolsky belopol...@users.sourceforge.net added the comment:

Note that issue8425a.diff leaves small_set.difference_update(large_dict) 
unoptimized:

$ ./python.exe -m timeit -s s = {1}; l = dict.fromkeys(range(1000)); 
s.difference_update(l)
1000 loops, best of 3: 842 usec per loop
$ ./python.exe -m timeit -s s = {1}; l = dict.fromkeys(range(1000)); 
s.difference(l)
10 loops, best of 3: 13.2 usec per loop

It would be easy to add an extra type check, but I noticed that 
small_set.difference_update(large_dict.viewkeys()) is not optimized either:

$ ./python.exe -m timeit -s s = {1}; l = dict.fromkeys(range(1000)); 
s.difference(l.viewkeys())
1000 loops, best of 3: 842 usec per loop

It would be nice if there was a uniform C-API that would allow to uniformly 
check that an object supports O(1) lookup and invoke such lookup efficiently.  
I don't think such C-API exists at the moment.

I would like to hear what others have to say before adding optimizations  to 
the patch that go beyond the scope of this issue.

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Alexander Belopolsky]

Changes by Alexander Belopolsky belopol...@users.sourceforge.net:


--
nosy: +belopolsky

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Ezio Melotti]

Ezio Melotti ezio.melo...@gmail.com added the comment:

Raymond, have you forgotten to attach the patch or have you just set the stage 
to 'patch review' by mistake?

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nosy: +ezio.melotti
priority:  - normal
stage: patch review - unit test needed

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Raymond Hettinger]

Changes by Raymond Hettinger rhettin...@users.sourceforge.net:


--
stage: unit test needed - needs patch

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Raymond Hettinger]

Changes by Raymond Hettinger rhettin...@users.sourceforge.net:


--
keywords: +easy
stage:  - patch review
versions: +Python 2.7, Python 3.2 -Python 3.3

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Eugene Kapun]

New submission from Eugene Kapun abacabadabac...@gmail.com:

 small_set = set(range(2000))
 large_set = set(range(2000))
 large_set -= small_set # Fast
 small_set -= large_set # Slow, should be fast
 small_set = small_set - large_set # Fast

--
components: Interpreter Core
messages: 103343
nosy: abacabadabacaba
severity: normal
status: open
title: a -= b should be fast if a is a small set and b is a large set
type: resource usage
versions: Python 3.1

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Raymond Hettinger]

Changes by Raymond Hettinger rhettin...@users.sourceforge.net:


--
assignee:  - rhettinger
nosy: +rhettinger
versions: +Python 3.3 -Python 3.1

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[issue8425] a -= b should be fast if a is a small set and b is a large set

[Eric Smith]

Changes by Eric Smith e...@trueblade.com:


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nosy: +eric.smith

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