Subject: Re: Mainline kernel OLTP performance update

From: "Zhang, Yanmin" <[email-masked]>
Date: Thu, 12 Feb 2009 13:22:33 +0800

On Sat, 2009-01-24 at 09:36 +0200, Pekka Enberg wrote:
> On Fri, 2009-01-23 at 10:22 -0500, Christoph Lameter wrote:
> >> No there is another way. Increase the allocator order to 3 for the
> >> kmalloc-8192 slab then multiple 8k blocks can be allocated from one of the
> >> larger chunks of data gotten from the page allocator. That will allow slub
> >> to do fast allocs.
> 
> On Sat, Jan 24, 2009 at 4:55 AM, Zhang, Yanmin
> <[email-masked]> wrote:
> > After I change kmalloc-8192/order to 3, the result(pinned netperf UDP-U-4k)
> > difference between SLUB and SLQB becomes 1% which can be considered as fluctuation.
> 
> Great. We should fix calculate_order() to be order 3 for kmalloc-8192.
> Are you interested in doing that?
Pekka,

Sorry for the late update.
The default order of kmalloc-8192 on 2*4 stoakley is really an issue of calculate_order.

slab_size	order		name
-------------------------------------------------
4096            3               sgpool-128
8192            2               kmalloc-8192
16384           3               kmalloc-16384

kmalloc-8192's default order is smaller than sgpool-128's.

On 4*4 tigerton machine, a similiar issue appears on another kmem_cache.

Function calculate_order uses 'min_objects /= 2;' to shrink. Plus size calculation/checking
in slab_order, sometimes above issue appear.

Below patch against 2.6.29-rc2 fixes it.

I checked the default orders of all kmem_cache and they don't become smaller than before. So
the patch wouldn't hurt performance.

Signed-off-by Zhang Yanmin <[email-masked]>

---

diff -Nraup linux-2.6.29-rc2/mm/slub.c linux-2.6.29-rc2_slubcalc_order/mm/slub.c
--- linux-2.6.29-rc2/mm/slub.c	2009-02-11 00:49:48.000000000 -0500
+++ linux-2.6.29-rc2_slubcalc_order/mm/slub.c	2009-02-12 00:08:24.000000000 -0500
@@ -1856,6 +1856,7 @@ static inline int calculate_order(int si
 	min_objects = slub_min_objects;
 	if (!min_objects)
 		min_objects = 4 * (fls(nr_cpu_ids) + 1);
+	min_objects = min(min_objects, (PAGE_SIZE << slub_max_order)/size);
 	while (min_objects > 1) {
 		fraction = 16;
 		while (fraction >= 4) {
@@ -1865,7 +1866,7 @@ static inline int calculate_order(int si
 				return order;
 			fraction /= 2;
 		}
-		min_objects /= 2;
+		min_objects --;
 	}
 
 	/*