Re: [PATCH] SLAB : use a multiply instead of a divide in obj_to_index()

[Eric Dumazet <dada1@cosmosbay.com>]

On Monday 04 December 2006 17:55, Christoph Lameter wrote:
> Could you generalize the reciprocal thingy so that the division
> can be used from other parts of the kernel as well? It would be useful to
> separately get some cycle counts on a regular division compared with your
> division. If that shows benefit then we may think about using it in the
> kernel. I am a bit surprised that this is still an issue on modern cpus.

OK I added a new include file, I am not sure it is the best way...

Well, AFAIK this particular divide is the only one that hurts performance on 
my machines.

Do you have in mind another spot in kernel where we could use this reciprocal 
divide as well ?

Yes divide complexity is still an issue with modern CPUS : 
elapsed time for 10^9 loops on Pentium M 1.6 Ghz
24 s for the version using divides
3.8 s for the version using multiplies

[PATCH] SLAB : use a multiply instead of a divide in obj_to_index()

When some objects are allocated by one CPU but freed by another CPU we can 
consume lot of cycles doing divides in obj_to_index().

(Typical load on a dual processor machine where network interrupts are handled 
by one particular CPU (allocating skbufs), and the other CPU is running the 
application (consuming and freeing skbufs))

Here on one production server (dual-core AMD Opteron 285), I noticed this 
divide took 1.20 % of CPU_CLK_UNHALTED events in kernel. But Opteron are 
quite modern cpus and the divide is much more expensive on oldest 
architectures :

On a 200 MHz sparcv9 machine, the division takes 64 cycles instead of 1 cycle 
for a multiply.

Doing some math, we can use a reciprocal multiplication instead of a divide.

If we want to compute V = (A / B)  (A and B being u32 quantities)
we can instead use :

V = ((u64)A * RECIPROCAL(B)) >> 32 ;

where RECIPROCAL(B) is precalculated to ((1LL << 32) + (B - 1)) / B

Note :

I wrote pure C code for clarity. gcc output for i386 is not optimal but 
acceptable :

mull   0x14(%ebx)
mov    %edx,%eax // part of the >> 32
xor     %edx,%edx // useless 
mov    %eax,(%esp) // could be avoided
mov    %edx,0x4(%esp) // useless
mov    (%esp),%ebx

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>

--- linux-2.6.19/include/linux/reciprocal_div.h	1970-01-01 01:00:00.000000000 +0100
+++ linux-2.6.19-ed/include/linux/reciprocal_div.h	2006-12-04 19:01:44.000000000 +0100
@@ -0,0 +1,30 @@
+#ifndef _LINUX_RECIPROCAL_DIV_H
+#define _LINUX_RECIPROCAL_DIV_H
+
+/*
+ * Define the reciprocal value of B so that
+ * ((u32)A / (u32)B) can be replaced by :
+ * (((u64)A * RECIPROCAL_VALUE(B)) >> 32)
+ * If RECIPROCAL_VALUE(B) is precalculated, we change a divide by a multiply
+ */
+#define RECIPROCAL_VALUE(B) (u32)(((1LL << 32) + ((B) - 1))/ (B))
+
+static inline u32 reciprocal_value(unsigned int k)
+{
+	if (__builtin_constant_p(k))
+		return RECIPROCAL_VALUE(k);
+	else {
+		u64 val = (1LL << 32) + (k - 1);
+		do_div(val, k);
+		return (u32)val;
+	}
+}
+
+/*
+ * We want to avoid an expensive divide : (A / B)
+ * If B is known in advance, its reciprocal R can be precalculated/stored.
+ * then  (A / B)  =  (u32)(((u64)(A) * (R)) >> 32)
+ */
+#define RECIPROCAL_DIVIDE(A, R) (u32)(((u64)(A) * (R)) >> 32)
+
+#endif
--- linux-2.6.19/mm/slab.c	2006-12-04 11:50:19.000000000 +0100
+++ linux-2.6.19-ed/mm/slab.c	2006-12-04 19:03:42.000000000 +0100
@@ -107,6 +107,7 @@
 #include	<linux/mempolicy.h>
 #include	<linux/mutex.h>
 #include	<linux/rtmutex.h>
+#include	<linux/reciprocal_div.h>
 
 #include	<asm/uaccess.h>
 #include	<asm/cacheflush.h>
@@ -385,6 +386,7 @@ struct kmem_cache {
 	unsigned int shared;
 
 	unsigned int buffer_size;
+	unsigned int reciprocal_buffer_size;
 /* 3) touched by every alloc & free from the backend */
 	struct kmem_list3 *nodelists[MAX_NUMNODES];
 
@@ -626,10 +628,17 @@ static inline void *index_to_obj(struct 
 	return slab->s_mem + cache->buffer_size * idx;
 }
 
-static inline unsigned int obj_to_index(struct kmem_cache *cache,
-					struct slab *slab, void *obj)
+/*
+ * We want to avoid an expensive divide : (offset / cache->buffer_size)
+ *   Using the fact that buffer_size is a constant for a particular cache,
+ *   we can replace (offset / cache->buffer_size) by
+ *   RECIPROCAL_DIVIDE(offset, cache->reciprocal_buffer_size)
+ */
+static inline unsigned int obj_to_index(const struct kmem_cache *cache,
+					const struct slab *slab, void *obj)
 {
-	return (unsigned)(obj - slab->s_mem) / cache->buffer_size;
+	unsigned int offset = (obj - slab->s_mem);
+	return RECIPROCAL_DIVIDE(offset, cache->reciprocal_buffer_size);
 }
 
 /*
@@ -1400,6 +1409,8 @@ void __init kmem_cache_init(void)
 
 	cache_cache.buffer_size = ALIGN(cache_cache.buffer_size,
 					cache_line_size());
+	cache_cache.reciprocal_buffer_size =
+		reciprocal_value(cache_cache.buffer_size);
 
 	for (order = 0; order < MAX_ORDER; order++) {
 		cache_estimate(order, cache_cache.buffer_size,
@@ -2297,6 +2308,7 @@ kmem_cache_create (const char *name, siz
 	if (flags & SLAB_CACHE_DMA)
 		cachep->gfpflags |= GFP_DMA;
 	cachep->buffer_size = size;
+	cachep->reciprocal_buffer_size = reciprocal_value(size);
 
 	if (flags & CFLGS_OFF_SLAB) {
 		cachep->slabp_cache = kmem_find_general_cachep(slab_size, 0u);