[Python-Dev] Store startup modules as C structures for 20%+ startup speed improvement?

Larry Hastings larry at hastings.org
Fri Sep 14 17:27:37 EDT 2018

What follows is the text of issue 34690:


The PR is here:


I don't know if we should be discussing this here on python-dev, or on 
bpo, or on Zulip, or on the soon-to-be-created Discourse. But maybe we 
can talk about it somewhere!



This patch was sent to me privately by Jeethu Rao at Facebook. It's a 
change they're working with internally to improve startup time.  What 
I've been told by Carl Shapiro at Facebook is that we have their 
blessing to post it publicly / merge it / build upon it for CPython.  
Their patch was written for 3.6, I have massaged it to the point where 
it minimally works with 3.8.

What the patch does: it takes all the Python modules that are loaded as 
part of interpreter startup and deserializes the marshalled .pyc file 
into precreated objects stored as static C data.  You add this .C file 
to the Python build.  Then there's a patch to Python itself (about 250 
lines iirc) that teaches it to load modules from these data structures.

I wrote a quick dumb test harness to compare this patch vs 3.8 stock.  
It runs a command line 500 times and uses time.perf_counter to time the 
process.  On a fast quiescent laptop I observe a 21-22% improvement:

cmdline: ['./python', '-c', 'pass']
500 runs:

   average time 0.006302303705982922
           best 0.006055746000129147
          worst 0.00816565500008437

   average time 0.007969956444008858
           best 0.007829047999621253
          worst 0.008812210000542109

improvement 0.20924239043734505 %

cmdline: ['./python', '-c', 'import io']
500 runs:

   average time 0.006297688038004708
           best 0.005980765999993309
          worst 0.0072462130010535475

   average time 0.007996319670004595
           best 0.0078091849991324125
          worst 0.009175700999549008

improvement 0.21242667903482038 %

The downside of the patch: for these modules it ignores the Python files 
on disk--it doesn't even stat them.  If you add stat calls you lose half 
of the speed improvement.  I believe they added a work-around, where you 
can set a flag (command-line? environment variable? I don't know, I 
didn't go looking for it) that tells Python "don't use the frozen 
modules" and it loads all those files from disk.

I don't propose to merge the patch in its current state.  I think it 
would need a lot of work both in terms of "doing things the way Python 
does it" as well as just code smell (the serializer is implemented in 
both C and Python and jumps back and forth, also the build process for 
the serialized modules is pretty tiresome).

Is it worth working on?

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