Re: [RFC] mmaped copy too slow?

[KOSAKI Motohiro]

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


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