10x Slowdown in series with sqrt

[oscarbenjamin]

There is a significant slowdown in

sympy/core/tests/test_power.py::test_issue_6068

I've bisected that to commit 8a06c07 in this #19369.

The content of the test is

from sympy import *

x = Symbol('x')
assert sqrt(sin(x)).series(x, 0, 7) == \
    sqrt(x) - x**Rational(5, 2)/12 + x**Rational(9, 2)/1440 - \
    x**Rational(13, 2)/24192 + O(x**7)
assert sqrt(sin(x)).series(x, 0, 9) == \
    sqrt(x) - x**Rational(5, 2)/12 + x**Rational(9, 2)/1440 - \
    x**Rational(13, 2)/24192 - 67*x**Rational(17, 2)/29030400 + O(x**9)
assert sqrt(sin(x**3)).series(x, 0, 19) == \
    x**Rational(3, 2) - x**Rational(15, 2)/12 + x**Rational(27, 2)/1440 + O(x**19)
assert sqrt(sin(x**3)).series(x, 0, 20) == \
    x**Rational(3, 2) - x**Rational(15, 2)/12 + x**Rational(27, 2)/1440 - \
    x**Rational(39, 2)/24192 + O(x**20)

In sympy 1.6 that takes 5 seconds whereas on master it takes 45 seconds.

Originally posted by @oscarbenjamin in #19369 (comment)

[oscarbenjamin]

CC @sachin-4099

[oscarbenjamin]

I've been wondering whether the test script should fail if a test becomes unexpectedly slow. I was thinking that we could require all not slow tests to run in under 10 seconds and fail the tests if any exceeds 20 seconds which would have picked this up.

[sachin-4099]

This is not happening on my master:

In [8]:  %time assert sqrt(sin(x)).series(x, 0, 7) == sqrt(x) - x**Rational(5, 2)/12 + x**Rational(9, 2)/1440 - x**Rational(13, 2)/24192 + O(x**7)
Wall time: 234 ms

In [9]: %time assert sqrt(sin(x)).series(x, 0, 9) == sqrt(x) - x**Rational(5, 2)/12 + x**Rational(9, 2)/1440 - x**Rational(13, 2)/24192 - 67*x**Rational(17, 2)/29030400 + O(x**9)
Wall time: 203 ms

In [10]: %time assert sqrt(sin(x**3)).series(x, 0, 19) == x**Rational(3, 2) - x**Rational(15, 2)/12 + x**Rational(27, 2)/1440 + O(x**19)
Wall time: 223 ms

In [11]: %time assert sqrt(sin(x**3)).series(x, 0, 20) == x**Rational(3, 2) - x**Rational(15, 2)/12 + x**Rational(27, 2)/1440 - x**Rational(39, 2)/24192 + O(x**20)
Wall time: 218 ms

Is this related to a specific version of python?

[jksuom]

This is what I get:

In [11]: from sympy.core.tests.test_power import *

In [12]: %time test_issue_6068()
CPU times: user 1.19 s, sys: 0 ns, total: 1.19 s
Wall time: 1.19 s

[oscarbenjamin]

I just updated my master and it finishes in 1.5 seconds now. I'm not sure how to reproduce this any more...