On Mon, Jul 01, 2013 at 09:16:46AM -0700, Dave Hansen wrote:
> 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.
Thanks for your explanation.
FWIW, it would be great if we can let MAP_POPULATE flag support shared
mappings because in our product system there has a lot of applications
that uses mmap(2) and then pre-faults this mapping. Currently these
applications need to pre-fault the mapping manually.
Regards,
- Zheng
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