From Fedora Project Wiki

Python built with gcc -O3


Instead of Fedora's default -O2 compiler flag, we will use -O3 to build CPython. This only impacts the interpreter and Python standard library, not any 3rd party extension modules built as RPM or on developer machines. This aligns with the way Python is built upstream. According to our performance measurements, it makes Python significantly faster (pyperformance geometric mean: 1.04x faster).


Current status

Detailed Description

We will replace the -O2 compiler flag with -O3 when building the python3.13 package. This change may be backported to older Pythons if desired. Python 3.13 should be the main Python version in Fedora 41+.

The Fedora packaging guidelines about compiler flags explicitly say:

> Overriding these flags for performance optimizations (for instance, -O3 instead of -O2) is generally discouraged. If you can present benchmarks that show a significant speedup for this particular code, this could be revisited on a case-by-case basis.

This change proposal presents such benchmarks and a case for Python to use -O3.

This change is limited to CPython interpreter and extension modules from the Python standard library only thanks to Changes/Python_Extension_Flags_Reduction (since Fedora 39). Other Python extension modules will remain bulidng as before, e.g. in RPM packages, they will still be built with -O2, unless Fedora changes that globally. The extension modules built with -O2 still work with Python built with -O3.


Questions asked and answered:

How much larger is Python at -O3 compared to -O2?

-O2 RPM size:

  • python3-3.12.2-3.fc41.x86_64 32638
  • python3-libs-3.12.2-3.fc41.x86_64 42888846

-O3 RPM size:

  • python3-3.12.2-3.O3.fc41.x86_64 32638
  • python3-libs-3.12.2-3.O3.fc41.x86_64 43389702

Difference of python3-libs:

500856 == 489 kB = 1.1678 % increase in size of pytohn3-libs itself or 1.1669 % of python3-libs+python3 combined size.

Benefit to Fedora

Upstream already builds Python with -O3 by default. Fedora's Python built with -O3 is faster (1.04x):

Benchmark with python3.12-3.12.2-3.fc41
Benchmark -O2 -O3 Change Significance
2to3 465 ms 446 ms 1.04x faster Significant (t=21.72)
async_generators 853 ms 784 ms 1.09x faster Significant (t=36.61)
async_tree_cpu_io_mixed 1.19 sec 1.11 sec 1.08x faster Significant (t=13.38)
async_tree_cpu_io_mixed_tg 1.17 sec 1.09 sec 1.08x faster Significant (t=18.69)
async_tree_eager 202 ms 189 ms 1.07x faster Significant (t=7.99)
async_tree_eager_cpu_io_mixed 727 ms 664 ms 1.09x faster Significant (t=18.56)
async_tree_eager_cpu_io_mixed_tg 633 ms 558 ms 1.13x faster Significant (t=24.53)
async_tree_eager_io 1.72 sec 1.68 sec 1.03x faster Significant (t=6.13)
async_tree_eager_io_tg 1.65 sec 1.62 sec 1.02x faster Significant (t=4.65)
async_tree_eager_memoization 437 ms 422 ms 1.04x faster Significant (t=5.09)
async_tree_eager_memoization_tg 330 ms 322 ms 1.03x faster Significant (t=2.60)
async_tree_eager_tg 137 ms 125 ms 1.09x faster Significant (t=16.94)
async_tree_io 1.64 sec 1.60 sec 1.02x faster Significant (t=9.49)
async_tree_io_tg 1.65 sec 1.61 sec 1.02x faster Not significant
async_tree_memoization 895 ms 871 ms 1.03x faster Significant (t=3.73)
async_tree_memoization_tg 848 ms 836 ms 1.01x faster Not significant
async_tree_none 718 ms 700 ms 1.03x faster Significant (t=6.90)
async_tree_none_tg 686 ms 659 ms 1.04x faster Significant (t=13.11)
asyncio_tcp 757 ms 748 ms 1.01x faster Not significant
asyncio_tcp_ssl 2.58 sec 2.56 sec 1.01x faster Not significant
asyncio_websockets 419 ms 418 ms 1.00x faster Not significant
bench_mp_pool 10.7 ms 10.7 ms 1.00x faster Not significant
bench_thread_pool 1.62 ms 1.61 ms 1.01x faster Not significant
chameleon 12.2 ms 12.0 ms 1.02x faster Not significant
chaos 113 ms 105 ms 1.07x faster Significant (t=46.23)
comprehensions 37.4 us 35.1 us 1.07x faster Significant (t=49.72)
coroutines 42.4 ms 41.4 ms 1.02x faster Significant (t=18.68)
coverage 109 ms 104 ms 1.05x faster Significant (t=33.91)
create_gc_cycles 1.84 ms 1.79 ms 1.02x faster Significant (t=5.50)
crypto_pyaes 141 ms 127 ms 1.11x faster Significant (t=86.61)
dask 766 ms 769 ms 1.00x slower Not significant
deepcopy 619 us 614 us 1.01x faster Not significant
deepcopy_memo 71.3 us 68.3 us 1.04x faster Significant (t=26.58)
deepcopy_reduce 5.62 us 5.56 us 1.01x faster Not significant
deltablue 5.76 ms 5.49 ms 1.05x faster Significant (t=7.97)
django_template 62.8 ms 59.7 ms 1.05x faster Significant (t=27.05)
docutils 4.38 sec 4.29 sec 1.02x faster Significant (t=11.25)
fannkuch 706 ms 667 ms 1.06x faster Significant (t=75.80)
float 144 ms 137 ms 1.05x faster Significant (t=24.66)
gc_traversal 5.73 ms 5.81 ms 1.01x slower Not significant
generators 56.0 ms 58.2 ms 1.04x slower Significant (t=-16.25)
genshi_text 40.8 ms 39.5 ms 1.03x faster Significant (t=17.64)
genshi_xml 88.2 ms 86.3 ms 1.02x faster Significant (t=6.96)
go 223 ms 217 ms 1.03x faster Significant (t=19.92)
hexiom 10.3 ms 9.76 ms 1.05x faster Significant (t=42.15)
html5lib 109 ms 108 ms 1.01x faster Not significant
json_dumps 17.4 ms 16.3 ms 1.06x faster Significant (t=45.38)
json_loads 44.2 us 42.3 us 1.04x faster Significant (t=27.71)
logging_format 12.9 us 12.4 us 1.04x faster Significant (t=9.81)
logging_silent 176 ns 174 ns 1.01x faster Not significant
logging_simple 11.4 us 11.0 us 1.03x faster Significant (t=9.94)
mako 19.2 ms 18.1 ms 1.06x faster Significant (t=54.89)
mdp 4.46 sec 4.33 sec 1.03x faster Significant (t=30.14)
meteor_contest 189 ms 167 ms 1.13x faster Significant (t=60.31)
nbody 157 ms 153 ms 1.03x faster Significant (t=4.34)
nqueens 153 ms 140 ms 1.09x faster Significant (t=63.60)
pathlib 32.9 ms 32.6 ms 1.01x faster Not significant
pickle 18.6 us 16.0 us 1.16x faster Significant (t=23.88)
pickle_dict 45.8 us 44.6 us 1.03x faster Significant (t=16.51)
pickle_list 6.86 us 6.59 us 1.04x faster Significant (t=19.65)
pickle_pure_python 515 us 505 us 1.02x faster Not significant
pidigits 285 ms 284 ms 1.00x faster Not significant
pprint_pformat 2.72 sec 2.54 sec 1.07x faster Significant (t=40.28)
pprint_safe_repr 1.34 sec 1.25 sec 1.08x faster Significant (t=58.43)
pyflate 738 ms 724 ms 1.02x faster Not significant
python_startup 15.5 ms 15.3 ms 1.01x faster Not significant
python_startup_no_site 11.2 ms 11.0 ms 1.01x faster Not significant
raytrace 549 ms 514 ms 1.07x faster Significant (t=45.37)
regex_compile 245 ms 233 ms 1.05x faster Significant (t=13.30)
regex_dna 269 ms 268 ms 1.00x faster Not significant
regex_effbot 4.83 ms 4.95 ms 1.03x slower Significant (t=-12.52)
regex_v8 33.7 ms 33.1 ms 1.02x faster Not significant
richards 75.7 ms 71.9 ms 1.05x faster Significant (t=18.30)
richards_super 85.2 ms 81.4 ms 1.05x faster Significant (t=31.25)
scimark_fft 662 ms 587 ms 1.13x faster Significant (t=71.10)
scimark_lu 199 ms 190 ms 1.04x faster Significant (t=26.77)
scimark_monte_carlo 123 ms 117 ms 1.05x faster Significant (t=37.45)
scimark_sor 217 ms 210 ms 1.04x faster Significant (t=10.68)
scimark_sparse_mat_mult 8.51 ms 7.42 ms 1.15x faster Significant (t=62.99)
spectral_norm 196 ms 183 ms 1.07x faster Significant (t=95.78)
sqlalchemy_declarative 239 ms 234 ms 1.02x faster Significant (t=4.81)
sqlalchemy_imperative 33.1 ms 33.4 ms 1.01x slower Not significant
sqlglot_normalize 197 ms 187 ms 1.05x faster Significant (t=39.81)
sqlglot_optimize 97.1 ms 91.3 ms 1.06x faster Significant (t=47.14)
sqlglot_parse 2.29 ms 2.18 ms 1.05x faster Significant (t=14.70)
sqlglot_transpile 2.79 ms 2.67 ms 1.04x faster Significant (t=11.76)
sqlite_synth 3.97 us 3.90 us 1.02x faster Not significant
sympy_expand 833 ms 802 ms 1.04x faster Significant (t=19.41)
sympy_integrate 34.7 ms 33.8 ms 1.03x faster Significant (t=9.99)
sympy_str 511 ms 489 ms 1.04x faster Significant (t=18.17)
sympy_sum 286 ms 278 ms 1.03x faster Significant (t=14.46)
telco 12.6 ms 11.7 ms 1.08x faster Significant (t=9.31)
tomli_loads 3.91 sec 3.56 sec 1.10x faster Significant (t=46.29)
tornado_http 213 ms 212 ms 1.01x faster Not significant
typing_runtime_protocols 214 us 196 us 1.09x faster Significant (t=24.74)
unpack_sequence 70.5 ns 66.0 ns 1.07x faster Significant (t=8.58)
unpickle 24.3 us 22.0 us 1.10x faster Significant (t=10.67)
unpickle_list 7.44 us 8.61 us 1.16x slower Significant (t=-45.10)
unpickle_pure_python 390 us 360 us 1.08x faster Significant (t=37.48)
xml_etree_generate 160 ms 145 ms 1.10x faster Significant (t=44.33)
xml_etree_iterparse 189 ms 180 ms 1.05x faster Significant (t=20.16)
xml_etree_parse 275 ms 257 ms 1.07x faster Significant (t=20.58)
xml_etree_process 106 ms 98.6 ms 1.08x faster Significant (t=46.73)
Geometric mean 1.04x faster

Generated by pyperformance run -o Ox.json and pyperformance compare -O table O2.json O3.json on Fedora 40 x86_64 with rawhide-built Python, python3.12-3.12.2-3.fc41, on Lenovo X1 Carbon 3rd gen.

The benchmark was performed on Python 3.12 because it uses 3rd party Python packages lacking support for Python 3.13. Once it is possible to run such a benchmark for Python 3.13, we will do so.

The benchmark was performed on x86_64. Until somebody presents a contradicting benchmark (or gives explicit reason for us to measure it), we believe the change makes sense on all architectures.

Benchmark attempt in Koji on all architectures shows good results on all architectures, for details, see


  • Other developers: no action expected, report bugs when found
  • Release engineering: no action expected
  • Policies and guidelines: this change is following the spirit of the guidelines
  • Trademark approval: not needed for this Change
  • Alignment with Community Initiatives: faster Python, happier users, more contributors?

Upgrade/compatibility impact

None expected.

How To Test

To verify this change has landed, inspect the build.log of python3.13. It should be built with gcc ... -O3.

To test this change, test Fedora as you would normally do and assert there are no regressions.

Run benchmarks, and report slowdowns if found.

User Experience

Faster Python, faster Fedora.


Contingency Plan

  • Contingency mechanism: revert the change, rebuild Python
  • Contingency deadline: Final Freeze
  • Blocks release? No


N/A (not a System Wide Change)

Release Notes