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Date: Sun, 02 Jul 2017 14:19:59 +0000
From: Srividya Desireddy <srividya.dr@...sung.com>
To: "sjenning@...hat.com" <sjenning@...hat.com>,
"ddstreet@...e.org" <ddstreet@...e.org>,
"penberg@...nel.org" <penberg@...nel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC: Dinakar Reddy Pathireddy <dinakar.p@...sung.com>,
SHARAN ALLUR <sharan.allur@...sung.com>,
SUNEEL KUMAR SURIMANI <suneel@...sung.com>,
JUHUN KIM <juhunkim@...sung.com>,
"srividya.desireddy@...il.com" <srividya.desireddy@...il.com>
Subject: [PATCH v2] zswap: Zero-filled pages handling
From: Srividya Desireddy <srividya.dr@...sung.com>
Date: Sun, 2 Jul 2017 19:15:37 +0530
Subject: [PATCH v2] zswap: Zero-filled pages handling
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 zero-filled pages (i.e. contents of the page are all zeros), 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 zero-filled
page before compression of the page. If the page is a zero-filled page, set
zswap_entry.zeroflag and skip the compression of the page and alloction
of memory in zpool. In zswap_frontswap_load(), check if the zeroflag is
set for the page in zswap_entry. If the flag is set, memset the page with
zero. This saves the decompression time during load.
On Ubuntu PC with 2GB RAM, while executing kernel build and other test
scripts ~15% of pages in zswap were zero pages. With multimedia workload
more than 20% of zswap pages were found to be zero pages.
On a ARM Quad Core 32-bit device with 1.5GB RAM an average 10% of zero
pages were found in zswap (an average of 5000 zero pages found out of
~50000 pages stored in zswap) on launching and relaunching 15 applications.
The launch time of the applications improved by ~3%.
Test Parameters Baseline With patch Improvement
-----------------------------------------------------------
Total RAM 1343MB 1343MB
Available RAM 451MB 445MB -6MB
Avg. Memfree 69MB 70MB 1MB
Avg. Swap Used 226MB 215MB -11MB
Avg. App entry time 644msec 623msec 3%
With patch, every page swapped to zswap is checked if it is a zero
page or not and for all the zero pages compression and memory allocation
operations are skipped. Overall there is an improvement of 30% in zswap
store time.
In case of non-zero pages there is no overhead during zswap page load. For
zero pages there is a improvement of more than 60% in the zswap load time
as the zero page decompression is avoided.
The below table shows the execution time profiling of the patch.
Zswap Store Operation Baseline With patch % Improvement
--------------------------------------------------------------
* Zero page check -- 22.5ms
(for non-zero pages)
* Zero page check -- 24ms
(for zero pages)
* Compression time 55ms --
(of zero pages)
* Allocation time 14ms --
(to store compressed
zero pages)
-------------------------------------------------------------
Total 69ms 46.5ms 32%
Zswap Load Operation Baseline With patch % Improvement
-------------------------------------------------------------
* Decompression time 30.4ms --
(of zero pages)
* Zero page check + -- 10.04ms
memset operation
(of zero pages)
-------------------------------------------------------------
Total 30.4ms 10.04ms 66%
*The execution times may vary with test device used.
Signed-off-by: Srividya Desireddy <srividya.dr@...sung.com>
---
mm/zswap.c | 46 ++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 42 insertions(+), 4 deletions(-)
diff --git a/mm/zswap.c b/mm/zswap.c
index eedc278..edc584b 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 zero filled pages swapped out to zswap */
+static atomic_t zswap_zero_pages = ATOMIC_INIT(0);
/*
* The statistics below are not protected from concurrent access for
@@ -145,7 +147,7 @@ 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 zero page length is 0.
* pool - the zswap_pool the entry's data is in
* handle - zpool allocation handle that stores the compressed page data
*/
@@ -320,8 +322,12 @@ 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_zero_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();
@@ -956,6 +962,19 @@ static int zswap_shrink(void)
return ret;
}
+static int zswap_is_page_zero_filled(void *ptr)
+{
+ unsigned int pos;
+ unsigned long *page;
+
+ page = (unsigned long *)ptr;
+ for (pos = 0; pos != PAGE_SIZE / sizeof(*page); pos++) {
+ if (page[pos])
+ return 0;
+ }
+ return 1;
+}
+
/*********************************
* frontswap hooks
**********************************/
@@ -996,6 +1015,15 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
goto reject;
}
+ src = kmap_atomic(page);
+ if (zswap_is_page_zero_filled(src)) {
+ kunmap_atomic(src);
+ entry->offset = offset;
+ entry->length = 0;
+ atomic_inc(&zswap_zero_pages);
+ goto insert_entry;
+ }
+
/* if entry is successfully added, it keeps the reference */
entry->pool = zswap_pool_current_get();
if (!entry->pool) {
@@ -1006,7 +1034,6 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
/* compress */
dst = get_cpu_var(zswap_dstmem);
tfm = *get_cpu_ptr(entry->pool->tfm);
- src = kmap_atomic(page);
ret = crypto_comp_compress(tfm, src, PAGE_SIZE, dst, &dlen);
kunmap_atomic(src);
put_cpu_ptr(entry->pool->tfm);
@@ -1040,6 +1067,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
entry->handle = handle;
entry->length = dlen;
+insert_entry:
/* map */
spin_lock(&tree->lock);
do {
@@ -1092,6 +1120,13 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
}
spin_unlock(&tree->lock);
+ if (!entry->length) {
+ dst = kmap_atomic(page);
+ memset(dst, 0, PAGE_SIZE);
+ kunmap_atomic(dst);
+ goto freeentry;
+ }
+
/* decompress */
dlen = PAGE_SIZE;
src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->handle,
@@ -1104,6 +1139,7 @@ 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);
@@ -1212,6 +1248,8 @@ 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("zero_pages", 0444,
+ zswap_debugfs_root, &zswap_zero_pages);
return 0;
}
--
1.7.9.5
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