[issue36012] Investigate slow writes to class variables

[Raymond Hettinger]

Change by Raymond Hettinger :


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

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[issue36012] Investigate slow writes to class variables

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset d8b9e1fc2e45d2bc3f4a9737c375f2adb8a8c7de by Raymond Hettinger 
(Stefan Behnel) in branch 'master':
bpo-36012: Avoid linear slot search for non-dunder methods (GH-11907)
https://github.com/python/cpython/commit/d8b9e1fc2e45d2bc3f4a9737c375f2adb8a8c7de


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[issue36012] Investigate slow writes to class variables

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Thanks Neil.  The tooling does indeed look nice.

I added your PyUnicode_InternInPlace() suggestion to the PR.

At this point, the PR looks ready-to-go unless any of you think we've missed 
some low-hanging fruit.

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[issue36012] Investigate slow writes to class variables

[Neil Schemenauer]

Neil Schemenauer  added the comment:

BTW, 'perf report [...]' has a really neat annotated assembly view.  Scroll to 
the function you are interested in and press 'a'.  Press 't' to toggle the time 
units (left side numbers).  I'm attaching a screenshot of the disassembly of 
the lookdict function.  The time units are sample accounts.  Each count is a 
point where the profiler woke up on that specific instruction.

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Added file: https://bugs.python.org/file48157/perf-screenshot.png

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[issue36012] Investigate slow writes to class variables

[Neil Schemenauer]

Neil Schemenauer  added the comment:

Some profiling using 'perf'.  This is for cpython 
63fa1cfece4912110ce3a0ff11fb3ade3ff5e756.

  children  self
  [...]
+   97.27% 0.00%  run_mod (inlined)
+   88.53% 6.33%  PyObject_SetAttr
+   79.34% 6.80%  type_setattro
+   43.92%43.92%  update_slot
+   26.87% 5.84%  _PyObject_GenericSetAttrWithDict
+   14.62% 6.52%  insertdict
+8.80% 8.80%  lookdict_unicode_nodummy
+6.57% 0.00%  _Py_DECREF (inlined)
+5.19% 5.19%  PyUnicode_InternInPlace
+3.57% 3.57%  _PyObjectDict_SetItem
+3.38% 3.38%  _PyType_Lookup
+1.71% 0.00%  _Py_INCREF (inlined)
+1.42% 0.00%  _Py_XDECREF (inlined)
  [...]

After applying PR 11907:

  children  self
  [...]
+   94.76% 0.00%  run_mod (inlined)
+   75.22% 6.77%  PyObject_SetAttr
+   64.65%13.08%  type_setattro
+   47.51%11.96%  _PyObject_GenericSetAttrWithDict
+   22.99%13.95%  insertdict
+   10.10%10.10%  lookdict_unicode_nodummy
+9.47% 9.47%  PyUnicode_InternInPlace
+7.10% 7.10%  _PyObjectDict_SetItem
+7.02% 7.02%  _PyType_Lookup
+6.52% 0.00%  _Py_DECREF (inlined)
+3.17% 0.00%  _Py_INCREF (inlined)
+3.16% 0.00%  _Py_XDECREF (inlined)
+2.59% 0.00%  PyDict_SetItem (inlined)
+1.50% 0.00%  is_dunder_name (inlined)
  [...]

The PyUnicode_InternInPlace() can mostly be eliminated by testing 
PyUnicode_CHECK_INTERNED() first (we already have called PyUnicode_Check() on 
it).  That only gives a 7% speedup on my machine though.  The is_dunder_name() 
is a much bigger optimization.

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[issue36012] Investigate slow writes to class variables

[STINNER Victor]

Change by STINNER Victor :


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nosy:  -vstinner

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[issue36012] Investigate slow writes to class variables

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Wow, I didn't expect to get an immediate win this of this magnitude :-)

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[issue36012] Investigate slow writes to class variables

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

This are the timings that I am measuring with PR 11907:

Variable and attribute read access:
   5.7 ns   read_local
   5.9 ns   read_nonlocal
  16.2 ns   read_global
  24.5 ns   read_builtin
  20.9 ns   read_classvar_from_class
  20.0 ns   read_classvar_from_instance
  29.7 ns   read_instancevar
  24.7 ns   read_instancevar_slots
  22.9 ns   read_namedtuple
  36.8 ns   read_boundmethod

Variable and attribute write access:
   6.9 ns   write_local
   6.9 ns   write_nonlocal
  26.7 ns   write_global
  65.4 ns   write_classvar <- Down from 120.6 ns
  49.5 ns   write_instancevar
  34.5 ns   write_instancevar_slots

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[issue36012] Investigate slow writes to class variables

[Stefan Behnel]

Stefan Behnel  added the comment:

It turns out that "update_slot()" is always called, even when we are not 
updating a slot name (which is always a special dunder-name). The linear search 
for names in "update_slots()" is a huge waste of time here, and 
short-circuiting out of it when the name does not start with "_" cuts the 
overall update time by 50%. I pushed a PR.

Another improvement would be a sub-linear algorithm for searching the slot 
name, but that's a bigger change.

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nosy: +scoder

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[issue36012] Investigate slow writes to class variables

[Stefan Behnel]

Change by Stefan Behnel :


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keywords: +patch
pull_requests: +11932
stage:  -> patch review

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[issue36012] Investigate slow writes to class variables

[Raymond Hettinger]

Raymond Hettinger  added the comment:

> can you include your python 2.7 runs? for me it looks similar

It will give similar results unless you switch to old-style classes (edit out 
the inheritance from object).

class A:
pass

A.x = 1

---
$ python2.7 var_access_benchmark.py
Variable and attribute read access:
   6.7 ns   read_local
  10.9 ns   read_global
  18.9 ns   read_builtin
  24.4 ns   read_classvar_from_class
  30.2 ns   read_classvar_from_instance
  22.7 ns   read_instancevar
  25.5 ns   read_instancevar_slots
  99.3 ns   read_namedtuple
  40.9 ns   read_boundmethod

Variable and attribute write access:
   8.2 ns   write_local
  18.7 ns   write_global
  32.9 ns   write_classvar <== Not formerly an outlier
  32.5 ns   write_instancevar
  31.2 ns   write_instancevar_slots

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[issue36012] Investigate slow writes to class variables

[Yasser Alshalaan]

Yasser Alshalaan  added the comment:

can you include your python 2.7 runs? for me it looks similar

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nosy: +Yasser Alshalaan

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[issue36012] Investigate slow writes to class variables

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

It seems 50% of the overhead (50ns) is due to two reasons:

- 30-40% is due to the call to update_slot(type, name) after the item is set in 
the class dictionary.

- 70-60% is due to all the extra work from _PyObject_GenericSetAttrWithDict 
until it ends calling  _PyObjectDict_SetItem with the slot. If in the code of 
typeobject.c:type_setattro you change:

res = _PyObject_GenericSetAttrWithDict((PyObject *)type, name, value, NULL);

by 

PyObject* dictptr = _PyObject_GetDictPtr(type);
res = _PyObjectDict_SetItem(type, dictptr, name, value);

and delete the update_slot(type, name) call afterwards, the times are reduced 
to 50ns.

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[issue36012] Investigate slow writes to class variables

[Raymond Hettinger]

New submission from Raymond Hettinger :

Benchmark show what writes to class variables are anomalously slow.

class A(object):
pass

A.x = 1   # This write is 3 to 5 times slower than other writes.

FWIW, the same operation for old-style classes in Python 2.7 was several times 
faster.

We should investigate to understand why the writes are so slow.  There might be 
a good reason or there might be an opportunity for optimization.

-
$ python3.8 Tools/scripts/var_access_benchmark.py
Variable and attribute read access:
   4.3 ns   read_local
   4.6 ns   read_nonlocal
  14.5 ns   read_global
  19.0 ns   read_builtin
  18.4 ns   read_classvar_from_class
  16.2 ns   read_classvar_from_instance
  24.7 ns   read_instancevar
  19.7 ns   read_instancevar_slots
  19.5 ns   read_namedtuple
  26.4 ns   read_boundmethod

Variable and attribute write access:
   4.4 ns   write_local
   5.1 ns   write_nonlocal
  18.2 ns   write_global
 103.9 ns   write_classvar <== Outlier
  35.4 ns   write_instancevar
  25.6 ns   write_instancevar_slots

--
components: Interpreter Core
messages: 335714
nosy: nascheme, pablogsal, rhettinger, vstinner
priority: low
severity: normal
status: open
title: Investigate slow writes to class variables
type: performance
versions: Python 3.8

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