Re: [PATCH] SLUB use cmpxchg_local

Mathieu Desnoyers
2007-08-21T23:39:49Z

* Christoph Lameter (clameter@sgi.com) wrote:
> On Tue, 21 Aug 2007, Mathieu Desnoyers wrote:
>
> > SLUB Use cmpxchg() everywhere.
> >
> > It applies to "SLUB: Single atomic instruction alloc/free using
> > cmpxchg".
>
> > +++ slab/mm/slub.c 2007-08-20 18:42:28.000000000 -0400
> > @@ -1682,7 +1682,7 @@ redo:
> >
> > object[c->offset] = freelist;
> >
> > - if (unlikely(cmpxchg_local(&c->freelist, freelist, object) != freelist))
> > + if (unlikely(cmpxchg(&c->freelist, freelist, object) != freelist))
> > goto redo;
> > return;
> > slow:
>
> Ok so regular cmpxchg, no cmpxchg_local. cmpxchg_local does not bring
> anything more? My measurements did not show any difference. I measured on
> Athlon64. What processor is being used?
>

This patch only cleans up the tree before proposing my cmpxchg_local
changes. There was an inconsistent use of cmpxchg/cmpxchg_local there.

Using cmpxchg_local vs cmpxchg has a clear impact on the fast paths, as
shown below: it saves about 60 to 70 cycles for kmalloc and 200 cycles
for the kmalloc/kfree pair (test 2).

Pros :
- we can use barrier() instead of rmb()
- cmpxchg_local is faster

Con :
- we must disable preemption

I use a 3GHz Pentium 4 for my tests.

Results (compared to cmpxchg_local numbers) :

SLUB Performance testing
========================
1. Kmalloc: Repeatedly allocate then free test
(kfree here is slow path)

* cmpxchg
kmalloc(8) = 271 cycles kfree = 645 cycles
kmalloc(16) = 158 cycles kfree = 428 cycles
kmalloc(32) = 153 cycles kfree = 446 cycles
kmalloc(64) = 178 cycles kfree = 459 cycles
kmalloc(128) = 247 cycles kfree = 481 cycles
kmalloc(256) = 363 cycles kfree = 605 cycles
kmalloc(512) = 449 cycles kfree = 677 cycles
kmalloc(1024) = 626 cycles kfree = 810 cycles
kmalloc(2048) = 681 cycles kfree = 869 cycles
kmalloc(4096) = 471 cycles kfree = 575 cycles
kmalloc(8192) = 666 cycles kfree = 747 cycles
kmalloc(16384) = 736 cycles kfree = 853 cycles

* cmpxchg_local
kmalloc(8) = 83 cycles kfree = 363 cycles
kmalloc(16) = 85 cycles kfree = 372 cycles
kmalloc(32) = 92 cycles kfree = 377 cycles
kmalloc(64) = 115 cycles kfree = 397 cycles
kmalloc(128) = 179 cycles kfree = 438 cycles
kmalloc(256) = 314 cycles kfree = 564 cycles
kmalloc(512) = 398 cycles kfree = 615 cycles
kmalloc(1024) = 573 cycles kfree = 745 cycles
kmalloc(2048) = 629 cycles kfree = 816 cycles
kmalloc(4096) = 473 cycles kfree = 548 cycles
kmalloc(8192) = 659 cycles kfree = 745 cycles
kmalloc(16384) = 724 cycles kfree = 843 cycles

2. Kmalloc: alloc/free test

*cmpxchg
kmalloc(8)/kfree = 321 cycles
kmalloc(16)/kfree = 308 cycles
kmalloc(32)/kfree = 311 cycles
kmalloc(64)/kfree = 310 cycles
kmalloc(128)/kfree = 306 cycles
kmalloc(256)/kfree = 325 cycles
kmalloc(512)/kfree = 324 cycles
kmalloc(1024)/kfree = 322 cycles
kmalloc(2048)/kfree = 309 cycles
kmalloc(4096)/kfree = 678 cycles
kmalloc(8192)/kfree = 1027 cycles
kmalloc(16384)/kfree = 1204 cycles

* cmpxchg_local
kmalloc(8)/kfree = 112 cycles
kmalloc(16)/kfree = 103 cycles
kmalloc(32)/kfree = 103 cycles
kmalloc(64)/kfree = 103 cycles
kmalloc(128)/kfree = 112 cycles
kmalloc(256)/kfree = 111 cycles
kmalloc(512)/kfree = 111 cycles
kmalloc(1024)/kfree = 111 cycles
kmalloc(2048)/kfree = 121 cycles
kmalloc(4096)/kfree = 650 cycles
kmalloc(8192)/kfree = 1042 cycles
kmalloc(16384)/kfree = 1149 cycles