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authorJesse Gross <jesse@nicira.com>2011-05-26 16:25:02 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2011-05-26 17:12:33 -0700
commit704f15ddb5fc2a7f25a12eb0913302d8ad9ffab3 (patch)
treeef17a945288c333c345643325783f374e10a4020 /lib
parent5bf54a9758c230d9e957e7b4f3a41c226660dd49 (diff)
downloadlinux-704f15ddb5fc2a7f25a12eb0913302d8ad9ffab3.tar.gz
flex_array: avoid divisions when accessing elements
On most architectures division is an expensive operation and accessing an element currently requires four of them. This performance penalty effectively precludes flex arrays from being used on any kind of fast path. However, two of these divisions can be handled at creation time and the others can be replaced by a reciprocal divide, completely avoiding real divisions on access. [eparis@redhat.com: rebase on top of changes to support 0 len elements] [eparis@redhat.com: initialize part_nr when array fits entirely in base] Signed-off-by: Jesse Gross <jesse@nicira.com> Signed-off-by: Eric Paris <eparis@redhat.com> Cc: Dave Hansen <dave@linux.vnet.ibm.com> Cc: David Rientjes <rientjes@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'lib')
-rw-r--r--lib/flex_array.c51
1 files changed, 29 insertions, 22 deletions
diff --git a/lib/flex_array.c b/lib/flex_array.c
index cab7621f98a..9b8b89458c4 100644
--- a/lib/flex_array.c
+++ b/lib/flex_array.c
@@ -24,6 +24,7 @@
#include <linux/slab.h>
#include <linux/stddef.h>
#include <linux/module.h>
+#include <linux/reciprocal_div.h>
struct flex_array_part {
char elements[FLEX_ARRAY_PART_SIZE];
@@ -70,15 +71,15 @@ static inline int elements_fit_in_base(struct flex_array *fa)
* Element size | Objects | Objects |
* PAGE_SIZE=4k | 32-bit | 64-bit |
* ---------------------------------|
- * 1 bytes | 4186112 | 2093056 |
- * 2 bytes | 2093056 | 1046528 |
- * 3 bytes | 1395030 | 697515 |
- * 4 bytes | 1046528 | 523264 |
- * 32 bytes | 130816 | 65408 |
- * 33 bytes | 126728 | 63364 |
- * 2048 bytes | 2044 | 1022 |
- * 2049 bytes | 1022 | 511 |
- * void * | 1046528 | 261632 |
+ * 1 bytes | 4177920 | 2088960 |
+ * 2 bytes | 2088960 | 1044480 |
+ * 3 bytes | 1392300 | 696150 |
+ * 4 bytes | 1044480 | 522240 |
+ * 32 bytes | 130560 | 65408 |
+ * 33 bytes | 126480 | 63240 |
+ * 2048 bytes | 2040 | 1020 |
+ * 2049 bytes | 1020 | 510 |
+ * void * | 1044480 | 261120 |
*
* Since 64-bit pointers are twice the size, we lose half the
* capacity in the base structure. Also note that no effort is made
@@ -88,11 +89,15 @@ struct flex_array *flex_array_alloc(int element_size, unsigned int total,
gfp_t flags)
{
struct flex_array *ret;
+ int elems_per_part = 0;
+ int reciprocal_elems = 0;
int max_size = 0;
- if (element_size)
- max_size = FLEX_ARRAY_NR_BASE_PTRS *
- FLEX_ARRAY_ELEMENTS_PER_PART(element_size);
+ if (element_size) {
+ elems_per_part = FLEX_ARRAY_ELEMENTS_PER_PART(element_size);
+ reciprocal_elems = reciprocal_value(elems_per_part);
+ max_size = FLEX_ARRAY_NR_BASE_PTRS * elems_per_part;
+ }
/* max_size will end up 0 if element_size > PAGE_SIZE */
if (total > max_size)
@@ -102,6 +107,8 @@ struct flex_array *flex_array_alloc(int element_size, unsigned int total,
return NULL;
ret->element_size = element_size;
ret->total_nr_elements = total;
+ ret->elems_per_part = elems_per_part;
+ ret->reciprocal_elems = reciprocal_elems;
if (elements_fit_in_base(ret) && !(flags & __GFP_ZERO))
memset(&ret->parts[0], FLEX_ARRAY_FREE,
FLEX_ARRAY_BASE_BYTES_LEFT);
@@ -112,7 +119,7 @@ EXPORT_SYMBOL(flex_array_alloc);
static int fa_element_to_part_nr(struct flex_array *fa,
unsigned int element_nr)
{
- return element_nr / FLEX_ARRAY_ELEMENTS_PER_PART(fa->element_size);
+ return reciprocal_divide(element_nr, fa->reciprocal_elems);
}
/**
@@ -141,12 +148,12 @@ void flex_array_free(struct flex_array *fa)
EXPORT_SYMBOL(flex_array_free);
static unsigned int index_inside_part(struct flex_array *fa,
- unsigned int element_nr)
+ unsigned int element_nr,
+ unsigned int part_nr)
{
unsigned int part_offset;
- part_offset = element_nr %
- FLEX_ARRAY_ELEMENTS_PER_PART(fa->element_size);
+ part_offset = element_nr - part_nr * fa->elems_per_part;
return part_offset * fa->element_size;
}
@@ -186,7 +193,7 @@ __fa_get_part(struct flex_array *fa, int part_nr, gfp_t flags)
int flex_array_put(struct flex_array *fa, unsigned int element_nr, void *src,
gfp_t flags)
{
- int part_nr;
+ int part_nr = 0;
struct flex_array_part *part;
void *dst;
@@ -202,7 +209,7 @@ int flex_array_put(struct flex_array *fa, unsigned int element_nr, void *src,
if (!part)
return -ENOMEM;
}
- dst = &part->elements[index_inside_part(fa, element_nr)];
+ dst = &part->elements[index_inside_part(fa, element_nr, part_nr)];
memcpy(dst, src, fa->element_size);
return 0;
}
@@ -217,7 +224,7 @@ EXPORT_SYMBOL(flex_array_put);
*/
int flex_array_clear(struct flex_array *fa, unsigned int element_nr)
{
- int part_nr;
+ int part_nr = 0;
struct flex_array_part *part;
void *dst;
@@ -233,7 +240,7 @@ int flex_array_clear(struct flex_array *fa, unsigned int element_nr)
if (!part)
return -EINVAL;
}
- dst = &part->elements[index_inside_part(fa, element_nr)];
+ dst = &part->elements[index_inside_part(fa, element_nr, part_nr)];
memset(dst, FLEX_ARRAY_FREE, fa->element_size);
return 0;
}
@@ -302,7 +309,7 @@ EXPORT_SYMBOL(flex_array_prealloc);
*/
void *flex_array_get(struct flex_array *fa, unsigned int element_nr)
{
- int part_nr;
+ int part_nr = 0;
struct flex_array_part *part;
if (!fa->element_size)
@@ -317,7 +324,7 @@ void *flex_array_get(struct flex_array *fa, unsigned int element_nr)
if (!part)
return NULL;
}
- return &part->elements[index_inside_part(fa, element_nr)];
+ return &part->elements[index_inside_part(fa, element_nr, part_nr)];
}
EXPORT_SYMBOL(flex_array_get);