linux-kernel - Re: [PATCH] SLUB use cmpxchg

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Message-ID: <20070822000323.GG29691@Krystal>
Date:	Tue, 21 Aug 2007 20:03:23 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <clameter@....com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com
Subject: Re: [PATCH] SLUB use cmpxchg_local

* Christoph Lameter (clameter@....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

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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