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When f_trace is NULL, only PyTrace_CALL tracing events trigger the invocation of
the trace function (see trace_trampoline).
maybe_call_line_trace() does quite some work though _PyCode_CheckLineNumber to
find out if the instruction should be traced, and all this work is useless when f_trace
is NULL. The patch checks for f_trace on all tracing events that are not
PyTrace_CALL.
The performance gain with the following test is about 30%. The first test is
with python on the default branch, the second one with the patch applied.
Note: a breakpoint is set at line 1 to ensure that tracing is enabled, without
any breakpoint, pdb sets the trace function to None after a continue.
$ ./python -m pdb /tmp/foo.py
> /tmp/foo.py(1)<module>()
-> import timeit
(Pdb) break 1
Breakpoint 1 at /tmp/foo.py:1
(Pdb) continue
9.226385930000106
The program finished and will be restarted
> /tmp/foo.py(1)<module>()
-> import timeit
(Pdb) q
[62276 refs]
$ ./python -m pdb /tmp/foo.py 
> /tmp/foo.py(1)<module>()
-> import timeit
(Pdb) break 1
Breakpoint 1 at /tmp/foo.py:1
(Pdb) continue
7.199809867001022
The program finished and will be restarted
> /tmp/foo.py(1)<module>()
-> import timeit
(Pdb)

> When f_trace is NULL, only PyTrace_CALL tracing events trigger the invocation
> of the trace function (see trace_trampoline).
This may be confusing. I meant that when f_trace is NULL, PyTrace_LINE,
PyTrace_RETURN and PyTrace_EXCEPTION are not traced.

New changeset 1fb26db7d719 by Benjamin Peterson in branch 'default':
improve tracing performance when f_trace is NULL (closes #16672)
http://hg.python.org/cpython/rev/1fb26db7d719 

Attached is a patch for the current head of 2.7.
It would nice to have this patch on 2.7 too.
With this patch, an implementation of pdb running on 2.7 with an
extension module, runs at 1.2 times the speed of the interpreter when
the trace function is active (see
http://code.google.com/p/pdb-clone/wiki/Performances). The performance
gain is 30%.

Benjamin, ans the previous commiter, could you possibly check the 2.7 proposed patch?

This patch causes test_hotshot to fail.

I don't think performance patches should be committed to bugfix branches (especially 2.7 which is in slow maintenance mode). Recommend closing.

That, too.

One may argue that this is not only a performances patch and that it
fixes the wasting of cpu resources when tracing is on. Wasting cpu
resources is a bug. Anyway, this is fine with me to close this minor
issue on 2.7.
The test_hotshot test is ok on my linux box and with the patch applied
on 2.7 head. Just curious to know what the problem is.
And thanks for applying the patch to 3.4.

The patch applied to the default branch should be reverted.
The 2.7 _hotshot extension module follows the specifications of PyEval_SetTrace:
 """Set the tracing function to func. This is similar to PyEval_SetProfile(),
 except the tracing function does receive line-number events."""
 ^^^^^^^^^^^^
The 2.7 patch breaks test_hotshot because PyTrace_LINE events are not sent
anymore when f_trace is NULL.
The first patch (already applied to the default branch), breaks also existing
applications that expect to receive line events when they don't care to
implement the following semantics defined by sys.settrace:
 """The trace function is invoked (with event set to 'call') whenever a new
 local scope is entered; it should return a reference to a local trace
 function to be used that scope, or None if the scope shouldn’t be
 traced."""
Those applications want to receive unconditionally line debug events.
Attached is a patch that reverts the patch on the default branch.

So, Xavier, are you saying that you are reverting the patch?.
Could be possible to provide a "good" patch, with a correct test of the situation you describe?
Or are you suggesting just revert this and close this bugentry for good?.

New changeset cd87afe18ff8 by Benjamin Peterson in branch 'default':
revert #16672 for incorrect semantics
http://hg.python.org/cpython/rev/cd87afe18ff8 

It is not possible to improve the performances of the trace function set with
sys.settrace without breaking backward compatibility for PyEval_SetTrace or
without introducing a new PyEval_xxx of some sort.
Yes, I suggest to revert this patch.

status should be close, I guess.

A patch proposed in issue 20041 provides a backward compatible solution to this performance enhancement.