Re: [rfc][patch 1/2] mnt_want_write speedup 1

Nick Piggin
2008-12-19T06:50:37Z

On Thu, Dec 18, 2008 at 10:34:52PM -0800, Dave Hansen wrote:
> On Fri, 2008-12-19 at 07:19 +0100, Nick Piggin wrote:
> > Hi. Fun, chasing down performance regressions.... I wonder what people think
> > about these patches? Is it OK to bloat struct vfsmount? Any races?
>
> Very cool stuff, Nick. I especially like how much it simplifies things
> and removes *SO* much code.

Thanks.

> Bloating the vfsmount was one of the things that really, really tried to
> avoid. When I start to think about the SGI machines, it gets me really
> worried. I went to a lot of trouble to make sure that the per-vfsmount
> memory overhead didn't scale with the number of cpus.

Well, OTOH, the SGI machines have a lot of memory ;) I *think* that
not many systems probably have thousands of mounts (given that the
mount hashtable is fixed sized single page), but I might be wrong
which is why I ask here.

Let's say a 4096 CPU machine with one mount for each CPU (4096 mounts),
I think should only use about 128MB total for the counters. OK, yes
that is a lot ;) but not exactly insane for such machine size.

Say for 32 CPU system with 10,000 mounts, it's 9MB.

> > This could
> > be made even faster if mnt_make_readonly could tolerate a really high latency
> > synchronize_rcu()... can it?)
>
> Yes, I think it can tolerate it. There's a lot of work to do, and we
> already have to go touch all the other per-cpu objects. There also
> tends to be writeout when this happens, so I don't think a few seconds,
> even, will be noticed.

That would be good. After the first patch, mnt_want_write still shows up
on profiles and almost oall the hits come right after the msync from
the smp_mb there.

It would be really nice to use RCU here. I think it might allow us to
eliminate the memory barriers.

> > This patch speeds up lmbench lat_mmap test by about 8%. lat_mmap is set up
> > basically to mmap a 64MB file on tmpfs, fault in its pages, then unmap it.
> > A microbenchmark yes, but it exercises some important paths in the mm.
>
> Do you know where the overhead actually came from? Was it the
> spinlocks? Was removing all the atomic ops what really helped?

I thnk about 95% of the unhalted cycles were hit against the two
instructions after the call to spin_lock. It wasn't actually flipping
the write counter per-cpu cache as far as I could see. I didn't save
the instruction level profiles, but I'll do another run if people
think it will be sane to use RCU here.

> I'll take a more in-depth look at your code tomorrow and see if I see
> any races.

Thanks.