Re: [PATCH] SLUB use cmpxchg_local

Mathieu Desnoyers
2007-08-22T00:03:34Z

* Christoph Lameter (clameter@sgi.com) wrote:
> On Tue, 21 Aug 2007, Mathieu Desnoyers wrote:
>
> > Are you running a UP or SMP kernel ? If you run a UP kernel, the
> > cmpxchg_local and cmpxchg are identical.
>
> UP.
>
> > Oh, and if you run your tests at boot time, the alternatives code may
> > have removed the lock prefix, therefore making cmpxchg and cmpxchg_local
> > exactly the same.
>
> Tests were run at boot time.
>
> That still does not explain kmalloc not showing improvements.
>

Hrm, weird.. because it should. Here are the numbers I posted
previously:

The measurements I get (in cycles):
enable interrupts (STI) disable interrupts (CLI) local CMPXCHG
IA32 (P4) 112 82 26
x86_64 AMD64 125 102 19

So both AMD64 and IA32 should be improved.

So why those improvements are not shown in your test ? A few possible
causes:

- Do you have any CONFIG_DEBUG_* options activated ? smp_processor_id()
may end up being more expensive in these cases.
- Rounding error.. you seem to round at 0.1ms, but I keep the values in
cycles. The times that you get (1.1ms) seems strangely higher than
mine, which are under 1000 cycles on a 3GHz system (less than 333ns).
I guess there is both a ms - ns error there and/or not enough
precision in your numbers.

Mathieu