On my system, {}  has become significantly slower in 3.3:

$ python3.2 -m timeit -n 1000000 '{}'
1000000 loops, best of 3: 0.0314 usec per loop
$ python3.3 -m timeit -n 1000000 '{}'
1000000 loops, best of 3: 0.0892 usec per loop
$ hg id -i
ee7b713fec71+
$ ./python -m timeit -n 1000000 '{}'
1000000 loops, best of 3: 0.0976 usec per loop

Is this because of the dict randomization?

I confirm that.

$ ./python -m timeit -n 1000000 '{};{};{};{};{};{};{};{};{};{}'

2.6: 0.62 usec
2.7: 0.57 usec
3.1: 0.55 usec
3.2: 0.48 usec
3.3: 1.5 usec

Randomization is not affecting it.

Ah, this is an effect of PEP 412.  The difference exists only for creating dicts up to 5 items (inclusive).

May be using the free list for keys will restore performance.

Does this regression impact any real-world program?

> Does this regression impact any real-world program?

That is a blow-off response.  A huge swath of the language is affected by dictionary performance (keyword args, module lookups, attribute lookup, etc).  Most programs will be affected to some degree -- computationally bound programs will notice more and i/o bound programs won't notice at all.

> > Does this regression impact any real-world program?
> 
> That is a blow-off response.  A huge swath of the language is affected
> by dictionary performance (keyword args, module lookups, attribute
> lookup, etc).

I was merely suggesting to report actual (non-micro) benchmark numbers.
This issue is about dict creation, not dict lookups.

As I understand, the new dict created on every call of function with keyword arguments.  This slow down every such call about 0.1 µsec.  This is about 10% of int('42', base=16).  In the sum, some programs can slow down to a few percents.

Direct comparison of 3.2 and 3.3 is meaningless because of the influence of other factors (first of all PEP 393).

> As I understand, the new dict created on every call of function with
> keyword arguments.  This slow down every such call about 0.1 µsec.
> This is about 10% of int('42', base=16).

Ok, but `int('42', base=16)` is about the fastest function call with
keyword arguments one can think about :-) In other words, the overhead
will probably not be noticeable for most function calls.

> Ok, but `int('42', base=16)` is about the fastest function call with
> keyword arguments one can think about :-)

Not as fast as a call without keywords, `int('42', 16)`. :-(  But this is a different issue.

Here is a really simple patch.  It speed up to 1.9x an empty dict creation (still 1.6x slower than 3.2).  Make your measurements of real-world programs.

Ok, here are some benchmark results with your patch:

### call_method ###
Min: 0.313816 -> 0.305150: 1.03x faster
Avg: 0.316292 -> 0.305678: 1.03x faster
Significant (t=21.07)
Stddev: 0.00188 -> 0.00050: 3.7398x smaller
Timeline: http://tinyurl.com/couzwbe

### call_method_unknown ###
Min: 0.323125 -> 0.312034: 1.04x faster
Avg: 0.325697 -> 0.313115: 1.04x faster
Significant (t=17.62)
Stddev: 0.00258 -> 0.00099: 2.6059x smaller
Timeline: http://tinyurl.com/clwacj3

### call_simple ###
Min: 0.215008 -> 0.220773: 1.03x slower
Avg: 0.215666 -> 0.221657: 1.03x slower
Significant (t=-23.28)
Stddev: 0.00062 -> 0.00078: 1.2721x larger
Timeline: http://tinyurl.com/cv6gxft

### fastpickle ###
Min: 0.554353 -> 0.569768: 1.03x slower
Avg: 0.558192 -> 0.574705: 1.03x slower
Significant (t=-6.90)
Stddev: 0.00363 -> 0.00393: 1.0829x larger
Timeline: http://tinyurl.com/c8s2dqk

### float ###
Min: 0.292259 -> 0.279563: 1.05x faster
Avg: 0.296011 -> 0.286154: 1.03x faster
Significant (t=2.99)
Stddev: 0.00417 -> 0.00609: 1.4585x larger
Timeline: http://tinyurl.com/c34ofe5

### iterative_count ###
Min: 0.115091 -> 0.118527: 1.03x slower
Avg: 0.116174 -> 0.119266: 1.03x slower
Significant (t=-6.82)
Stddev: 0.00068 -> 0.00075: 1.1148x larger
Timeline: http://tinyurl.com/bsmuuso

### json_dump_v2 ###
Min: 2.591838 -> 2.652666: 1.02x slower
Avg: 2.599335 -> 2.687225: 1.03x slower
Significant (t=-8.09)
Stddev: 0.00715 -> 0.02322: 3.2450x larger
Timeline: http://tinyurl.com/clxutkt

### json_load ###
Min: 0.378329 -> 0.370132: 1.02x faster
Avg: 0.379840 -> 0.371222: 1.02x faster
Significant (t=13.68)
Stddev: 0.00112 -> 0.00085: 1.3237x smaller
Timeline: http://tinyurl.com/c7ycte2

### nbody ###
Min: 0.259945 -> 0.251294: 1.03x faster
Avg: 0.261186 -> 0.251843: 1.04x faster
Significant (t=14.95)
Stddev: 0.00125 -> 0.00062: 2.0274x smaller
Timeline: http://tinyurl.com/cg8z6yg

### nqueens ###
Min: 0.295304 -> 0.275767: 1.07x faster
Avg: 0.298382 -> 0.278839: 1.07x faster
Significant (t=10.19)
Stddev: 0.00244 -> 0.00353: 1.4429x larger
Timeline: http://tinyurl.com/bo2dn4x

### pathlib ###
Min: 0.100494 -> 0.102371: 1.02x slower
Avg: 0.101787 -> 0.104876: 1.03x slower
Significant (t=-8.15)
Stddev: 0.00103 -> 0.00159: 1.5432x larger
Timeline: http://tinyurl.com/cb9w79t

### richards ###
Min: 0.166939 -> 0.172828: 1.04x slower
Avg: 0.167945 -> 0.174199: 1.04x slower
Significant (t=-9.65)
Stddev: 0.00092 -> 0.00112: 1.2241x larger
Timeline: http://tinyurl.com/cdros5x

### silent_logging ###
Min: 0.070485 -> 0.066857: 1.05x faster
Avg: 0.070538 -> 0.067041: 1.05x faster
Significant (t=44.64)
Stddev: 0.00003 -> 0.00017: 5.4036x larger
Timeline: http://tinyurl.com/buz5h9j

### 2to3 ###
0.490925 -> 0.478927: 1.03x faster

### chameleon ###
Min: 0.050694 -> 0.049364: 1.03x faster
Avg: 0.051030 -> 0.049759: 1.03x faster
Significant (t=8.45)
Stddev: 0.00041 -> 0.00041: 1.0036x larger
Timeline: b'http://tinyurl.com/d5czdjt'

### mako ###
Min: 0.183295 -> 0.177191: 1.03x faster
Avg: 0.184944 -> 0.179300: 1.03x faster
Significant (t=15.54)
Stddev: 0.00119 -> 0.00137: 1.1555x larger
Timeline: b'http://tinyurl.com/dx6xrvx'


In the end, it's mostly a wash.

Antoine, I would consider this a performance regression to solve for 3.3.1. Small dictionary creation is everywhere in CPython.

> Antoine, I would consider this a performance regression to solve for
> 3.3.1. Small dictionary creation is everywhere in CPython.

Again, feel free to provide real-world benchmark numbers proving the
regression. I've already posted some figures.

> In the end, it's mostly a wash.

Yes, it's predictable. The gain is too small and undistinguished on a background of random noise. May be more long-running benchmark will show more reliable results, but in any case, the gain is small.

My long-running hard-optimized simulation is about 5% faster on 3.2 or patched 3.4 comparing to 3.3. But this is not a typical program.

The patch gives a measurable speedup (./python is a patched 3.3.0+). IMO we should apply it. It's small and I can see no harm, too.

$ for PY in python2.7 python3.2 python3.3 ./python; do cmd="$PY -R -m timeit -n 10000000 '{};{};{};{};{};{};{};{};{};{}'"; echo $cmd; eval $cmd; done
python2.7 -R -m timeit -n 10000000 '{};{};{};{};{};{};{};{};{};{}'
10000000 loops, best of 3: 0.162 usec per loop
python3.2 -R -m timeit -n 10000000 '{};{};{};{};{};{};{};{};{};{}'
10000000 loops, best of 3: 0.142 usec per loop
python3.3 -R -m timeit -n 10000000 '{};{};{};{};{};{};{};{};{};{}'
10000000 loops, best of 3: 0.669 usec per loop
./python -R -m timeit -n 10000000 '{};{};{};{};{};{};{};{};{};{}'
10000000 loops, best of 3: 0.381 usec per loop

$ for PY in python2.7 python3.2 python3.3 ./python; do cmd="$PY -R -m timeit -n 10000000 'int(\"1\", base=16)'"; echo $cmd; eval $cmd; done
python2.7 -R -m timeit -n 10000000 'int("1", base=16)'
10000000 loops, best of 3: 0.268 usec per loop
python3.2 -R -m timeit -n 10000000 'int("1", base=16)'
10000000 loops, best of 3: 0.302 usec per loop
python3.3 -R -m timeit -n 10000000 'int("1", base=16)'
10000000 loops, best of 3: 0.477 usec per loop
./python -R -m timeit -n 10000000 'int("1", base=16)'
10000000 loops, best of 3: 0.356 usec per loop

So what we should do with this?

You said it yourself: """The gain is too small and undistinguished on a background of random noise""". Closing would be reasonable, unless someone exhibits a reasonable benchmark where there is a significant difference.

In any case, it's too late for perf improvements on 3.4.

This patch looks correct and like the right thing to do.  I recommend applying it to 3.5.

Antoine, are you still oppose to this patch?

The patch adds complication to the already complicated memory management of dicts. It increases maintenance burden in critical code.

Have we found any case where it makes a tangible difference?
(I'm not talking about timeit micro-benchmarks)

Closed in the favor of issue23601.

Serhiy Storchaka:
> Closed in the favor of issue23601.

Cool!