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[Python-Dev] mmap & munmap loop (Was: Compact ordered set


It happened very accidentally.  Since venv is used,
many paths in the interpreter is changed.  So how memory
is used are changed.

Let's reproduce the accident.

$ cat m2.py
import pickle, sys

LIST = pickle.dumps([[0]*10 for _ in range(10)], pickle.HIGHEST_PROTOCOL)

N = 1000
z = [[0]*10 for _ in range(N)]

if '-c' in sys.argv:
    sys._debugmallocstats()
    sys.exit()

for _ in range(100000):
    pickle.loads(LIST)

$ /usr/bin/time python3 m2.py
0.42user 0.00system 0:00.43elapsed 99%CPU (0avgtext+0avgdata
9100maxresident)k
0inputs+0outputs (0major+1139minor)pagefaults 0swaps

There are only 1139 faults.  It is less than 100000.

$ /usr/bin/time python3 m2.py -c
...
14 unused pools * 4096 bytes       =               57,344
...

adjust N im m2.py until it shows "0 unused pools".
In my case, N=1390.

$ /usr/bin/time python3 m2.py
0.51user 0.33system 0:00.85elapsed 99%CPU (0avgtext+0avgdata
9140maxresident)k
0inputs+0outputs (0major+201149minor)pagefaults 0swaps

200000 faults!
It seems two page fault / loop.  (2 pools are used and returned).


On Wed, Feb 27, 2019 at 7:51 PM Victor Stinner <vstinner at redhat.com> wrote:

> Sorry, I didn't get a coffee yet: more *often* in a venv.
>
> Le mer. 27 f?vr. 2019 ? 11:32, Victor Stinner <vstinner at redhat.com> a
> ?crit :
> >
> > Any idea why Python calls mmap+munmap more even in a venv?
> >
> > Victor
> >
> > Le mer. 27 f?vr. 2019 ? 10:00, INADA Naoki <songofacandy at gmail.com> a
> ?crit :
> > >
> > > >
> > > > > Ah, another interesting point, this huge slowdown happens only
> when bm_pickle.py
> > > > > is executed through pyperformance.  When run it directly, slowdown
> is
> > > > > not so large.
> > > >
> > > > pyperformance runs benchmarks in a virtual environment. I don't know
> > > > if it has any impact on bm_pickle.
> > > >
> > > > Most pyperformance can be run outside a virtual env if required
> > > > modules are installed on the system. (bm_pickle only requires the
> > > > stdlib and perf.)
> > > >
> > >
> > > Bingo!
> > >
> > > Without venv:
> > >
> > > unpickle: Mean +- std dev: 26.9 us +- 0.0 us
> > > % time     seconds  usecs/call     calls    errors syscall
> > > ------ ----------- ----------- --------- --------- ----------------
> > >  28.78    0.000438           0      1440           read
> > >  27.33    0.000416           1       440        25 stat
> > >   9.72    0.000148           1       144           mmap
> > > ...
> > >   0.79    0.000012           1        11           munmap
> > >
> > > With venv:
> > >
> > > % time     seconds  usecs/call     calls    errors syscall
> > > ------ ----------- ----------- --------- --------- ----------------
> > >  57.12    0.099023           2     61471           munmap
> > >  41.87    0.072580           1     61618           mmap
> > >   0.23    0.000395           1       465        27 stat
> > >
> > > unpickle and unpickle_list creates massive same-sized objects, then
> all objects are
> > > removed.  If all pools in the arena is freed, munmap is called.
> > >
> > > I think we should save some arenas to reuse.  On recent Linux,
> > > we may be able to use MADV_FREE instead of munmap.
> > >
> >
> >
> > --
> > Night gathers, and now my watch begins. It shall not end until my death.
>
>
>
> --
> Night gathers, and now my watch begins. It shall not end until my death.
>


-- 
INADA Naoki  <songofacandy at gmail.com>
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