[Speed] Performance difference in call_method()

[Victor Stinner]

Hi,

I noticed a temporary performance peak in the call_method:

https://speed.python.org/timeline/#/?exe=4&ben=call_method&env=1&revs=50&equid=off&quarts=on&extr=on

The difference is major: 17 ms => 29 ms, 70% slower!

I expected a temporary issue on the server used to run benchmarks,
but... I reproduced the result on the server.

Recently, the performance of call_method() changed in CPython default
from 17 ms to 28 ms (well, the exact value is variable: 25 ms, 28 ms,
29 ms, ...) and then back to 17 ms:

(1) ce85a1f129e3: 17 ms => 83877018ef97 (Oct 18): 25 ms

https://hg.python.org/cpython/rev/83877018ef97

(2) 3e073e7b4460: 28 ms => 204a43c452cc (Oct 22): 17 ms

https://hg.python.org/cpython/rev/204a43c452cc

None of these revisions modify code used in the call_method()
benchmark, so I guess that it's yet another compiler joke.


On my laptop and my desktop PC, I'm unable to reproduce the issue: the
performance is the same (I tested ce85a1f129e3, 83877018ef97,
204a43c452cc). These PC uses Fedora 24, GCC 6.2.1. CPUs:

* laptop: Intel(R) Core(TM) i7-3520M CPU @ 2.90GHz
* desktop: Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz


The speed-python runs Ubuntu 14.04, GCC 4.8.4-2ubuntu1~14.04. CPU:
"Intel(R) Xeon(R) CPU X5680  @ 3.33GHz".


call_method() benchmark is a microbenchmark which seems to depend a
lot of very low level stuff like CPU L1 cache. Maybe the impact on the
compiler is more important on speed-python which has an older CPU,
than my more recent hardware. Maybe GCC 6.2 produces more efficient
machine code than GCC 4.8.


I expect that PGO would "fix" the call_method() performance issue, but
PGO compilation fails on Ubuntu 14.04 with a compiler error :-p A
solution would be to upgrade the OS of this server.

Victor