Re: page fault fastpath: Increasing SMP scalability by introducing pte locks?

[Andi Kleen]

On Sun, Aug 15, 2004 at 04:55:57PM -0700, Christoph Lameter wrote:
> On Mon, 16 Aug 2004, Andi Kleen wrote:
> 
> > Christoph Lameter <clameter@xxxxxxx> writes:
> >
> > > On Sun, 15 Aug 2004, David S. Miller wrote:
> > >
> > >>
> > >> Is the read lock in the VMA semaphore enough to let you do
> > >> the pgd/pmd walking without the page_table_lock?
> > >> I think it is, but just checking.
> > >
> > > That would be great.... May I change the page_table lock to
> > > be a read write spinlock instead?
> >
> > That's probably not a good idea. r/w locks are extremly slow on
> > some architectures. Including ia64.
> 
> I was thinking about a read write spinlock not an readwrite
> semaphore. Look at include/asm-ia64/spinlock.h.

I was also talking about rw spinlocks.

> The implementations are almost the same. Are you sure
> about this?

Yes. Try the cat /proc/net/tcp test. It will take >100k read locks
for the TCP listen hash table, and on bigger ppc64 and ia64 machines this
can take nearly a second of system time.

-Andi
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