Re: mmap() scalability in the presence of the MAP_POPULATE flag

[Roman Dubtsov <dubtsov@gmail.com>]

On Wed, 2013-01-02 at 16:09 -0800, Michel Lespinasse wrote:
> > Is there an interest in fixing this or concurrent mmaps() from the same
> > process are too much of a corner case to worry about it?
> 
> Funny this comes up again. I actually have a patch series that is
> supposed to do that:
> [PATCH 0/9] Avoid populating unbounded num of ptes with mmap_sem held
> 
> However, the patches are still pending, didn't get much review
> (probably not enough for Andrew to take them at this point), and I
> think everyone forgot about them during the winter break.
> 
> Care to have a look at that thread and see if it works for you ?
> 
> (caveat: you will possibly also need "[PATCH 10/9] mm: make
> do_mmap_pgoff return populate as a size in bytes, not as a bool" to
> make the series actually work for you)

I applied the patches on top of 3.7.1. Here're the results for 4 threads
concurrently mmap()-ing 10 64MB buffers in a loop without munmap()-s.
The data is from a Nehalem i7-920 single-socket 4-core CPU. I've also
added the older data I have for the 3.6.11 (patched and not) for
reference.

3.6.11 vanilla, do not populate: 0.001 seconds
3.6.11 vanilla, populate via a loop: 0.216 seconds
3.6.11 vanilla, populate via MAP_POPULATE: 0.358 seconds 

3.6.11 + crude patch, do not populate: 0.002 seconds
3.6.11 + crude patch, populate via loop: 0.215 seconds
3.6.11 + crude patch, populate via MAP_POPULATE: 0.217 seconds

3.7.1 vanilla, do not populate: 0.001 seconds
3.7.1 vanilla, populate via a loop: 0.216 seconds
3.7.1 vanilla, populate via MAP_POPULATE: 0.411 seconds

3.7.1 + patch series, do not populate: 0.001 seconds
3.7.1 + patch series, populate via loop: 0.216 seconds
3.7.1 + patch series, populate via MAP_POPULATE: 0.273 seconds

So, the patch series mentioned above do improve performance but as far
as I can read the benchmarking data there's still some performance left
on the table.

Regards,
Roma