Re: Subject: [RFC MM] mmap_sem scaling: Use mutex and percpu counter instead

[KOSAKI Motohiro]

> On Fri, 6 Nov 2009, Andi Kleen wrote:
> 
> > On Fri, Nov 06, 2009 at 12:08:54PM -0500, Christoph Lameter wrote:
> > > On Fri, 6 Nov 2009, Andi Kleen wrote:
> > >
> > > > Yes but all the major calls still take mmap_sem, which is not ranged.
> > >
> > > But exactly that issue is addressed by this patch!
> >
> > Major calls = mmap, brk, etc.
> 
> Those are rare. More frequently are for faults, get_user_pages and
> the like operations that are frequent.
> 
> brk depends on process wide settings and has to be
> serialized using a processor wide locks.
> 
> mmap and other address space local modification may be able to avoid
> taking mmap write lock by taking the read lock and then locking the
> ptls in the page struct relevant to the address space being modified.
> 
> This is also enabled by this patchset.

Andi, Why do you ignore fork? fork() hold mmap_sem write-side lock and
it is one of critical path.
Ah yes, I know HPC workload doesn't call fork() so frequently, I mean
typical desktop and small server case.

I agree with cristoph halfly. if the issue is only in mmap, it isn't
so important.

Probably, I haven't catch your mention.


Plus, most critical mmap_sem issue is not locking cost itself. In stree workload,
the procss grabbing mmap_sem frequently sleep. and fair rw-semaphoe logic
frequently prevent reader side locking.
At least, this improvement doesn't help google like workload.

Thanks.


> > Only for page faults, not for anything that takes it for write.
> >
> > Anyways the better reader lock is a step in the right direction, but
> > I have my doubts it's a good idea to make write really slow here.
> 
> The bigger the system the larger the problems with mmap. This is one key
> scaling issue important for the VM. We can work on that. I have a patch
> here that restricts the per cpu checks to only those cpus on which the
> process has at some times run before.



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