Re: iptables very slow after commit 784544739a25c30637397ace5489eeb6e15d7d49

Ingo Molnar
2009-04-12T10:55:10Z

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> On Sat, Apr 11, 2009 at 09:08:54AM +0200, Ingo Molnar wrote:
> >
> > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> >
> > > I will nevertheless suggest the following egregious hack to
> > > get a consistent sample of one counter for some other CPU:
> > >
> > > a. Disable interrupts
> > > b. Atomically exchange the bottom 32 bits of the
> > > counter with the value zero.
> > > c. Atomically exchange the top 32 bits of the counter
> > > with the value zero.
> > > d. Concatenate the values obtained in (b) and (c), which
> > > is the snapshot value.
> >
> > Note, i have recently implemented full atomic64_t support on 32-bit
> > x86, for the perfcounters code, based on the CMPXCHG8B instruction.
> >
> > Which, while not the lightest of instructions, is still much better
> > than the sequence above.
> >
> > So i think a better approach would be to also add a dumb generic
> > implementation for atomic64_t (using a global lock or so), and then
> > generic code could just assume that atomic64_t always exists.
> >
> > It is far nicer - and faster as well - as the hack above, even on
> > 32-bit x86.
>
> If the generic implementation is needed only on !SMP systems, that
> could work. The architectures I would be worried about include
> powerpc and ia64, which I believe support 32-bit SMP builds.

ia64 would naturally support the CMPXCHG8B instructions.

Not sure about powerpc32. Having a lock for the library
implementation is not _that_ much of a problem. We obviously dont
want the design of Linux to be dictated by the weakest link of all
platforms, right?

Ingo