Re: [PATCH 2/2] Futex non-page-pinning fix

[Arjan van de Ven <arjanv@redhat.com>]

On Tue, Aug 26, 2003 at 11:08:00AM +0300, Muli Ben-Yehuda wrote:
> On Tue, Aug 26, 2003 at 09:56:51AM +0200, Arjan van de Ven wrote:
> > On Tue, 2003-08-26 at 09:02, Andrew Morton wrote:
> > 
> > > umm, how about hashing only on offset into page?  That reduces the number of
> > > threads which need to be visited in futex_wake() by a factor of up to 1024.
> > 
> > How likely do you consider it that we then get a major collision?
> > I wouldn't be surprised if, say, glibc lays some hot futexes out in a
> > way that's the same for all processes in the system, like start of the
> > page.... Might as well not hash :)
> 
> How about combining something that's shared to all of the threads that
> share a futex but not system wide (the mm?) with something simple that
> won't change, like the page offset? Adding the mm into the mix wil
> make collisions harder, and limiting the buckets to the number of
> different futex offsets will make it simple and differentiate between
> different futexes in the same mm. 

The problem is that you can (and
want to) share futexes between different processes via shm....
glibc actualy uses this to implement cross-process posix mutexes (mutici?)