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Subject[PATCH v2 2/2] KSM: Replace jhash2 with xxhash
Date
jhash2 used for calculating checksum
for in memory pages, for detect fact of
changes in page.

xxhash much faster then jhash2, some tests:
x86_64 host:
CPU: Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz
PAGE_SIZE: 4096, loop count: 1048576
jhash2: 0xacbc7a5b time: 1907 ms, th: 2251.9 MiB/s
xxhash32: 0x570da981 time: 739 ms, th: 5809.4 MiB/s
xxhash64: 0xa1fa032ab85bbb62 time: 371 ms, th: 11556.6 MiB/s

CPU: Intel(R) Xeon(R) CPU E5-2420 0 @ 1.90GHz
PAGE_SIZE: 4096, loop count: 1048576
jhash2: 0xe680b382 time: 3722 ms, th: 1153.896680 MiB/s
xxhash32: 0x56d00be4 time: 1183 ms, th: 3629.130689 MiB/s
xxhash64: 0x8c194cff29cc4dee time: 725 ms, th: 5918.003401 MiB/s

xxhash64 on x86_32 work with ~ same speed as jhash2.
xxhash32 on x86_32 work with ~ same speed as for x86_64
jhash2 are faster than xxhash on input data smaller than 32 byte

So use xxhash() which will take appropriate hash version
for target arch

I did some benchmarks (i get cpu load of ksmd from htop):
CPU: Intel(R) Xeon(R) CPU E5-2420 0 @ 1.90GHz
ksm: sleep_millisecs = 1
jhash2: ~18%
xxhash64: ~11%
ksm: sleep_millisecs = 20 - default
jhash2: ~4.7%
xxhash64: ~3.3%

- 11 / 18 ~= 0.6 -> Profit: ~40%
- 3.3/4.7 ~= 0.7 -> Profit: ~30%

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
Acked-by: Andi Kleen <ak@linux.intel.com>
---
mm/Kconfig | 1 +
mm/ksm.c | 14 +++++++-------
2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index 9c4bdddd80c2..252ab266ac23 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -305,6 +305,7 @@ config MMU_NOTIFIER
config KSM
bool "Enable KSM for page merging"
depends on MMU
+ select XXHASH
help
Enable Kernel Samepage Merging: KSM periodically scans those areas
of an application's address space that an app has advised may be
diff --git a/mm/ksm.c b/mm/ksm.c
index 15dd7415f7b3..6527fe21aaa3 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -25,7 +25,7 @@
#include <linux/pagemap.h>
#include <linux/rmap.h>
#include <linux/spinlock.h>
-#include <linux/jhash.h>
+#include <linux/xxhash.h>
#include <linux/delay.h>
#include <linux/kthread.h>
#include <linux/wait.h>
@@ -186,7 +186,7 @@ struct rmap_item {
};
struct mm_struct *mm;
unsigned long address; /* + low bits used for flags below */
- unsigned int oldchecksum; /* when unstable */
+ xxhash_t oldchecksum; /* when unstable */
union {
struct rb_node node; /* when node of unstable tree */
struct { /* when listed from stable tree */
@@ -255,7 +255,7 @@ static unsigned int ksm_thread_pages_to_scan = 100;
static unsigned int ksm_thread_sleep_millisecs = 20;

/* Checksum of an empty (zeroed) page */
-static unsigned int zero_checksum __read_mostly;
+static xxhash_t zero_checksum __read_mostly;

/* Whether to merge empty (zeroed) pages with actual zero pages */
static bool ksm_use_zero_pages __read_mostly;
@@ -982,11 +982,11 @@ static int unmerge_and_remove_all_rmap_items(void)
}
#endif /* CONFIG_SYSFS */

-static u32 calc_checksum(struct page *page)
+static xxhash_t calc_checksum(struct page *page)
{
- u32 checksum;
+ xxhash_t checksum;
void *addr = kmap_atomic(page);
- checksum = jhash2(addr, PAGE_SIZE / 4, 17);
+ checksum = xxhash(addr, PAGE_SIZE, 0);
kunmap_atomic(addr);
return checksum;
}
@@ -1994,7 +1994,7 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
struct page *tree_page = NULL;
struct stable_node *stable_node;
struct page *kpage;
- unsigned int checksum;
+ xxhash_t checksum;
int err;
bool max_page_sharing_bypass = false;

--
2.14.1
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