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Subject[PATCH v2] zswap: Same-filled pages handling
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From: Srividya Desireddy <srividya.dr@samsung.com>
Date: Sat, 18 Nov 2017 18:29:16 +0530
Subject: [PATCH v2] zswap: Same-filled pages handling

Changes since v1 :

Added memset_l instead of for loop.

Zswap is a cache which compresses the pages that are being swapped out
and stores them into a dynamically allocated RAM-based memory pool.
Experiments have shown that around 10-20% of pages stored in zswap
are same-filled pages (i.e. contents of the page are all same), but
these pages are handled as normal pages by compressing and allocating
memory in the pool.

This patch adds a check in zswap_frontswap_store() to identify same-filled
page before compression of the page. If the page is a same-filled page, set
zswap_entry.length to zero, save the same-filled value and skip the
compression of the page and alloction of memory in zpool.
In zswap_frontswap_load(), check if value of zswap_entry.length is zero
corresponding to the page to be loaded. If zswap_entry.length is zero,
fill the page with same-filled value. This saves the decompression time
during load.

On a ARM Quad Core 32-bit device with 1.5GB RAM by launching and
relaunching different applications, out of ~64000 pages stored in
zswap, ~11000 pages were same-value filled pages (including zero-filled
pages) and ~9000 pages were zero-filled pages.

An average of 17% of pages(including zero-filled pages) in zswap are
same-value filled pages and 14% pages are zero-filled pages.
An average of 3% of pages are same-filled non-zero pages.

The below table shows the execution time profiling with the patch.

Baseline With patch % Improvement
-----------------------------------------------------------------
*Zswap Store Time 26.5ms 18ms 32%
(of same value pages)
*Zswap Load Time
(of same value pages) 25.5ms 13ms 49%
-----------------------------------------------------------------

On Ubuntu PC with 2GB RAM, while executing kernel build and other test
scripts and running multimedia applications, out of 360000 pages
stored in zswap 78000(~22%) of pages were found to be same-value filled
pages (including zero-filled pages) and 64000(~17%) are zero-filled
pages. So an average of %5 of pages are same-filled non-zero pages.

The below table shows the execution time profiling with the patch.

Baseline With patch % Improvement
-----------------------------------------------------------------
*Zswap Store Time 91ms 74ms 19%
(of same value pages)
*Zswap Load Time 50ms 7.5ms 85%
(of same value pages)
-----------------------------------------------------------------

*The execution times may vary with test device used.

Signed-off-by: Srividya Desireddy <srividya.dr@samsung.com>
---
mm/zswap.c | 71 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 66 insertions(+), 5 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index d39581a..1133b4ce 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -49,6 +49,8 @@
static u64 zswap_pool_total_size;
/* The number of compressed pages currently stored in zswap */
static atomic_t zswap_stored_pages = ATOMIC_INIT(0);
+/* The number of same-value filled pages currently stored in zswap */
+static atomic_t zswap_same_filled_pages = ATOMIC_INIT(0);

/*
* The statistics below are not protected from concurrent access for
@@ -116,6 +118,11 @@ module_param_cb(zpool, &zswap_zpool_param_ops, &zswap_zpool_type, 0644);
static unsigned int zswap_max_pool_percent = 20;
module_param_named(max_pool_percent, zswap_max_pool_percent, uint, 0644);

+/* Enable/disable handling same-value filled pages (enabled by default) */
+static bool zswap_same_filled_pages_enabled = true;
+module_param_named(same_filled_pages_enabled, zswap_same_filled_pages_enabled,
+ bool, 0644);
+
/*********************************
* data structures
**********************************/
@@ -145,9 +152,10 @@ struct zswap_pool {
* be held while changing the refcount. Since the lock must
* be held, there is no reason to also make refcount atomic.
* length - the length in bytes of the compressed page data. Needed during
- * decompression
+ * decompression. For a same value filled page length is 0.
* pool - the zswap_pool the entry's data is in
* handle - zpool allocation handle that stores the compressed page data
+ * value - value of the same-value filled pages which have same content
*/
struct zswap_entry {
struct rb_node rbnode;
@@ -155,7 +163,10 @@ struct zswap_entry {
int refcount;
unsigned int length;
struct zswap_pool *pool;
- unsigned long handle;
+ union {
+ unsigned long handle;
+ unsigned long value;
+ };
};

struct zswap_header {
@@ -320,8 +331,12 @@ static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
*/
static void zswap_free_entry(struct zswap_entry *entry)
{
- zpool_free(entry->pool->zpool, entry->handle);
- zswap_pool_put(entry->pool);
+ if (!entry->length)
+ atomic_dec(&zswap_same_filled_pages);
+ else {
+ zpool_free(entry->pool->zpool, entry->handle);
+ zswap_pool_put(entry->pool);
+ }
zswap_entry_cache_free(entry);
atomic_dec(&zswap_stored_pages);
zswap_update_total_size();
@@ -953,6 +968,28 @@ static int zswap_shrink(void)
return ret;
}

+static int zswap_is_page_same_filled(void *ptr, unsigned long *value)
+{
+ unsigned int pos;
+ unsigned long *page;
+
+ page = (unsigned long *)ptr;
+ for (pos = 1; pos < PAGE_SIZE / sizeof(*page); pos++) {
+ if (page[pos] != page[0])
+ return 0;
+ }
+ *value = page[0];
+ return 1;
+}
+
+static void zswap_fill_page(void *ptr, unsigned long value)
+{
+ unsigned long *page;
+
+ page = (unsigned long *)ptr;
+ memset_l(page, value, PAGE_SIZE / sizeof(unsigned long));
+}
+
/*********************************
* frontswap hooks
**********************************/
@@ -965,7 +1002,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
struct crypto_comp *tfm;
int ret;
unsigned int dlen = PAGE_SIZE, len;
- unsigned long handle;
+ unsigned long handle, value;
char *buf;
u8 *src, *dst;
struct zswap_header *zhdr;
@@ -993,6 +1030,19 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
goto reject;
}

+ if (zswap_same_filled_pages_enabled) {
+ src = kmap_atomic(page);
+ if (zswap_is_page_same_filled(src, &value)) {
+ kunmap_atomic(src);
+ entry->offset = offset;
+ entry->length = 0;
+ entry->value = value;
+ atomic_inc(&zswap_same_filled_pages);
+ goto insert_entry;
+ }
+ kunmap_atomic(src);
+ }
+
/* if entry is successfully added, it keeps the reference */
entry->pool = zswap_pool_current_get();
if (!entry->pool) {
@@ -1037,6 +1087,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
entry->handle = handle;
entry->length = dlen;

+insert_entry:
/* map */
spin_lock(&tree->lock);
do {
@@ -1089,6 +1140,13 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
}
spin_unlock(&tree->lock);

+ if (!entry->length) {
+ dst = kmap_atomic(page);
+ zswap_fill_page(dst, entry->value);
+ kunmap_atomic(dst);
+ goto freeentry;
+ }
+
/* decompress */
dlen = PAGE_SIZE;
src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->handle,
@@ -1101,6 +1159,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
zpool_unmap_handle(entry->pool->zpool, entry->handle);
BUG_ON(ret);

+freeentry:
spin_lock(&tree->lock);
zswap_entry_put(tree, entry);
spin_unlock(&tree->lock);
@@ -1209,6 +1268,8 @@ static int __init zswap_debugfs_init(void)
zswap_debugfs_root, &zswap_pool_total_size);
debugfs_create_atomic_t("stored_pages", S_IRUGO,
zswap_debugfs_root, &zswap_stored_pages);
+ debugfs_create_atomic_t("same_filled_pages", 0444,
+ zswap_debugfs_root, &zswap_same_filled_pages);

return 0;
}
--
2.7.4
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