Re: [RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting

[Dave Hansen <dave@sr71.net>]

On 06/28/2013 07:20 PM, Zheng Liu wrote:
>> > IOW, a process needing to do a bunch of MAP_POPULATEs isn't
>> > parallelizable, but one using this mechanism would be.
> I look at the code, and it seems that we will handle MAP_POPULATE flag
> after we release mmap_sem locking in vm_mmap_pgoff():
> 
>                 down_write(&mm->mmap_sem);
>                 ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
>                                     &populate);
>                 up_write(&mm->mmap_sem);
>                 if (populate)
>                         mm_populate(ret, populate);
> 
> Am I missing something?

I went and did my same test using mmap(MAP_POPULATE)/munmap() pair
versus using MADV_POPULATE in 160 threads in parallel.

MADV_POPULATE was about 10x faster in the threaded configuration.

With MADV_POPULATE, the biggest cost is shipping the mmap_sem cacheline
around so that we can write the reader count update in to it.  With
mmap(), there is a lot of _contention_ on that lock which is much, much
more expensive than simply bouncing a cacheline around.