Re: [RFC] mmaped copy too slow?

KOSAKI Motohiro
2008-01-17T03:24:21Z

Hi

> > One thing you could also try is to pass MAP_POPULATE to mmap so that the
> > page tables are filled in at the time of the mmap, avoiding a lot of
> > page faults later.
> >
>
> OK, I will test your idea and report about tomorrow.
> but I don't think page fault is major performance impact.

I got more interesting result :)
MAP_POPULATE is harmful result at large copy.

1G copy
elapse(sec)
--------------------------------------------
mmap 71.54
mmap + madvice 69.63
mmap + populate 100.87
mmap + populate + madvice 101.16

more detail:
time command output of mmap copy
0.50user 3.59system 1:11.54elapsed 5%CPU (0avgtext+0avgdata 0maxresident)k
2101192inputs+2097160outputs (32776major+491573minor)pagefaults 0swaps

time command output of mmap+populate copy
0.53user 5.13system 1:40.87elapsed 5%CPU (0avgtext+0avgdata 0maxresident)k
4200808inputs+2097160outputs (49164major+737340minor)pagefaults 0swaps

input blocks increase about x2.
in fact, mmap(MAP_POPULATE) read disk to memory and drop it just after,
thus read again.

of cource, when copy file size is enough small, MAP_POPULATE is effective.

100M copy
elapse(sec)
--------------------------------------------
mmap 7.38
mmap + madvice 7.29
mmap + populate 7.13
mmap + populate + madvice 6.65

- kosaki