[PATCH 3/4] zswap: Zero-filled pages handling

[PATCH 3/4] zswap: Zero-filled pages handling

[Srividya Desireddy]

[-- Attachment #1: Type: text/plain, Size: 5024 bytes --]

From: Srividya Desireddy <srividya.dr@samsung.com>
Date: Wed, 17 Aug 2016 14:34:14 +0530
Subject: [PATCH 3/4] zswap: Zero-filled pages handling

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.

The overall overhead caused due to zero-filled page check is very minimal
when compared to the time saved by avoiding compression and allocation in
case of zero-filled pages. The load time of a zero-filled page is reduced
by 80% when compared to baseline.

Signed-off-by: Srividya Desireddy <srividya.dr@samsung.com>
---
 mm/zswap.c |   58 ++++++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 50 insertions(+), 8 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index ae39c77..d0c3f96 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -58,6 +58,9 @@ static atomic_t zswap_stored_pages = ATOMIC_INIT(0);
  */
 static atomic_t zswap_duplicate_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
  * performance reasons so they may not be a 100% accurate.  However,
@@ -172,6 +175,8 @@ struct zswap_handle {
  *            be held, there is no reason to also make refcount atomic.
  * pool - the zswap_pool the entry's data is in
  * zhandle - pointer to struct zswap_handle
+ * zeroflag - the flag is set if the content of the page is filled with
+ *            zeros
  */
 struct zswap_entry {
 	struct rb_node rbnode;
@@ -179,6 +184,7 @@ struct zswap_entry {
 	int refcount;
 	struct zswap_pool *pool;
 	struct zswap_handle *zhandle;
+	unsigned char zeroflag;
 };
 
 struct zswap_header {
@@ -269,6 +275,7 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
 	if (!entry)
 		return NULL;
 	entry->refcount = 1;
+	entry->zeroflag = 0;
 	entry->zhandle = NULL;
 	RB_CLEAR_NODE(&entry->rbnode);
 	return entry;
@@ -477,13 +484,17 @@ static bool zswap_handle_is_unique(struct zswap_handle *zhandle)
  */
 static void zswap_free_entry(struct zswap_entry *entry)
 {
-	if (zswap_handle_is_unique(entry->zhandle)) {
-		zpool_free(entry->pool->zpool, entry->zhandle->handle);
-		zswap_handle_cache_free(entry->zhandle);
-		zswap_pool_put(entry->pool);
-	} else {
-		entry->zhandle->ref_count--;
-		atomic_dec(&zswap_duplicate_pages);
+	if (entry->zeroflag)
+		atomic_dec(&zswap_zero_pages);
+	else {
+		if (zswap_handle_is_unique(entry->zhandle)) {
+			zpool_free(entry->pool->zpool, entry->zhandle->handle);
+			zswap_handle_cache_free(entry->zhandle);
+			zswap_pool_put(entry->pool);
+		} else {
+			entry->zhandle->ref_count--;
+			atomic_dec(&zswap_duplicate_pages);
+		}
 	}
 	zswap_entry_cache_free(entry);
 	atomic_dec(&zswap_stored_pages);
@@ -1140,6 +1151,21 @@ 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
 **********************************/
@@ -1183,6 +1209,13 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
 	}
 
 	src = kmap_atomic(page);
+	if (zswap_is_page_zero_filled(src)) {
+		kunmap_atomic(src);
+		entry->offset = offset;
+		entry->zeroflag = 1;
+		atomic_inc(&zswap_zero_pages);
+		goto insert_entry;
+	}
 
 	if (zswap_same_page_sharing) {
 		checksum = jhash2((const u32 *)src, PAGE_SIZE / 4, 17);
@@ -1314,6 +1347,13 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 	}
 	spin_unlock(&tree->lock);
 
+	if (entry->zeroflag) {
+		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->zhandle->handle,
@@ -1327,6 +1367,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 	zpool_unmap_handle(entry->pool->zpool, entry->zhandle->handle);
 	BUG_ON(ret);
 
+freeentry:
 	spin_lock(&tree->lock);
 	zswap_entry_put(tree, entry);
 	spin_unlock(&tree->lock);
@@ -1446,7 +1487,8 @@ static int __init zswap_debugfs_init(void)
 			zswap_debugfs_root, &zswap_stored_pages);
 	debugfs_create_atomic_t("duplicate_pages", S_IRUGO,
 			zswap_debugfs_root, &zswap_duplicate_pages);
-
+	debugfs_create_atomic_t("zero_pages", S_IRUGO,
+			zswap_debugfs_root, &zswap_zero_pages);
 
 	return 0;
 }
-- 
1.7.9.5

Re: [PATCH 3/4] zswap: Zero-filled pages handling

[Pekka Enberg]

On Wed, Aug 17, 2016 at 1:18 PM, Srividya Desireddy
<srividya.dr@samsung.com> wrote:
> 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.
>
> The overall overhead caused due to zero-filled page check is very minimal
> when compared to the time saved by avoiding compression and allocation in
> case of zero-filled pages. The load time of a zero-filled page is reduced
> by 80% when compared to baseline.

AFAICT, that's an overall improvement only if there are a lot of
zero-filled pages because it's just overhead for pages that we *need*
to compress, no? So I suppose the question is, are there a lot of
zero-filled pages that we need to swap and why is that the case?

> @@ -1314,6 +1347,13 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>         }
>         spin_unlock(&tree->lock);
>
> +       if (entry->zeroflag) {
> +               dst = kmap_atomic(page);
> +               memset(dst, 0, PAGE_SIZE);
> +               kunmap_atomic(dst);
> +               goto freeentry;
> +       }

Don't we need the same thing in zswap_writeback_entry() for the
ZSWAP_SWAPCACHE_NEW case?

> +
>         /* decompress */
>         dlen = PAGE_SIZE;
>         src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->zhandle->handle,
> @@ -1327,6 +1367,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>         zpool_unmap_handle(entry->pool->zpool, entry->zhandle->handle);
>         BUG_ON(ret);

- Pekka

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: [PATCH 3/4] zswap: Zero-filled pages handling

[Pekka Enberg]

On Wed, Aug 17, 2016 at 1:18 PM, Srividya Desireddy
<srividya.dr@samsung.com> wrote:
>> 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.
>>
>> The overall overhead caused due to zero-filled page check is very minimal
>> when compared to the time saved by avoiding compression and allocation in
>> case of zero-filled pages. The load time of a zero-filled page is reduced
>> by 80% when compared to baseline.

On Wed, Aug 17, 2016 at 3:25 PM, Pekka Enberg <penberg@kernel.org> wrote:
> AFAICT, that's an overall improvement only if there are a lot of
> zero-filled pages because it's just overhead for pages that we *need*
> to compress, no? So I suppose the question is, are there a lot of
> zero-filled pages that we need to swap and why is that the case?

I suppose reading your cover letter would have been helpful before
sending out my email:

"Experiments have shown that around 10-15% of pages stored in zswap are
duplicates which results in 10-12% more RAM required to store these
duplicate compressed pages."

But I still don't understand why we have zero-filled pages that we are
swapping out.

- Pekka

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

[PATCH 3/4] zswap: Zero-filled pages handling

[Srividya Desireddy]

[-- Attachment #1: Type: text/plain, Size: 1196 bytes --]

On 17 August 2016 at 17:55, Pekka Enberg <penberg@kernel.org> wrote:
> On Wed, Aug 17, 2016 at 1:18 PM, Srividya Desireddy
> <srividya.dr@samsung.com> wrote:
>> @@ -1314,6 +1347,13 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>>         }
>>         spin_unlock(&tree->lock);
>>
>> +       if (entry->zeroflag) {
>> +               dst = kmap_atomic(page);
>> +               memset(dst, 0, PAGE_SIZE);
>> +               kunmap_atomic(dst);
>> +               goto freeentry;
>> +       }
>
> Don't we need the same thing in zswap_writeback_entry() for the
> ZSWAP_SWAPCACHE_NEW case?

Zero-filled pages are not compressed and stored in the zpool memory.
Zpool handle will not be created for zero-filled pages, hence they
can not be picked for eviction/writeback to the swap device.

- Srividya
>
>> +
>>         /* decompress */
>>         dlen = PAGE_SIZE;
>>         src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->zhandle->handle,
>> @@ -1327,6 +1367,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>>         zpool_unmap_handle(entry->pool->zpool, entry->zhandle->handle);
>>         BUG_ON(ret);
>
> - Pekka

[PATCH 3/4] zswap: Zero-filled pages handling

[Srividya Desireddy]

[-- Attachment #1: Type: text/plain, Size: 2007 bytes --]


On 17 August 2016 at 18:02, Pekka Enberg <penberg@kernel.org> wrote:
> On Wed, Aug 17, 2016 at 1:18 PM, Srividya Desireddy
> <srividya.dr@samsung.com> wrote:
>>> 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.
>>>
>>> The overall overhead caused due to zero-filled page check is very minimal
>>> when compared to the time saved by avoiding compression and allocation in
>>> case of zero-filled pages. The load time of a zero-filled page is reduced
>>> by 80% when compared to baseline.
>
> On Wed, Aug 17, 2016 at 3:25 PM, Pekka Enberg <penberg@kernel.org> wrote:
>> AFAICT, that's an overall improvement only if there are a lot of
>> zero-filled pages because it's just overhead for pages that we *need*
>> to compress, no? So I suppose the question is, are there a lot of
>> zero-filled pages that we need to swap and why is that the case?
>
> I suppose reading your cover letter would have been helpful before
> sending out my email:
>
> "Experiments have shown that around 10-15% of pages stored in zswap are
> duplicates which results in 10-12% more RAM required to store these
> duplicate compressed pages."
>
> But I still don't understand why we have zero-filled pages that we are
> swapping out.
>
> - Pekka

Zero-filled pages exists in memory because applications may be
initializing the allocated pages with zeros and not using them; or
the actual content written to the memory pages during execution 
itself is zeros.
The existing page reclamation path in kernel does not check for
zero-filled pages in the anonymous LRU lists before swapping out.

- Srividya

Re: [PATCH 3/4] zswap: Zero-filled pages handling

[Dan Streetman]

On Wed, Aug 17, 2016 at 6:18 AM, Srividya Desireddy
<srividya.dr@samsung.com> wrote:
> From: Srividya Desireddy <srividya.dr@samsung.com>
> Date: Wed, 17 Aug 2016 14:34:14 +0530
> Subject: [PATCH 3/4] zswap: Zero-filled pages handling
>
> 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.
>
> The overall overhead caused due to zero-filled page check is very minimal
> when compared to the time saved by avoiding compression and allocation in
> case of zero-filled pages. The load time of a zero-filled page is reduced
> by 80% when compared to baseline.

this is unrelated to the same-page patches.  send this patch by itself.

>
> Signed-off-by: Srividya Desireddy <srividya.dr@samsung.com>
> ---
>  mm/zswap.c |   58 ++++++++++++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 50 insertions(+), 8 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index ae39c77..d0c3f96 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -58,6 +58,9 @@ static atomic_t zswap_stored_pages = ATOMIC_INIT(0);
>   */
>  static atomic_t zswap_duplicate_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
>   * performance reasons so they may not be a 100% accurate.  However,
> @@ -172,6 +175,8 @@ struct zswap_handle {
>   *            be held, there is no reason to also make refcount atomic.
>   * pool - the zswap_pool the entry's data is in
>   * zhandle - pointer to struct zswap_handle
> + * zeroflag - the flag is set if the content of the page is filled with
> + *            zeros
>   */
>  struct zswap_entry {
>         struct rb_node rbnode;
> @@ -179,6 +184,7 @@ struct zswap_entry {
>         int refcount;
>         struct zswap_pool *pool;
>         struct zswap_handle *zhandle;
> +       unsigned char zeroflag;
>  };
>
>  struct zswap_header {
> @@ -269,6 +275,7 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
>         if (!entry)
>                 return NULL;
>         entry->refcount = 1;
> +       entry->zeroflag = 0;
>         entry->zhandle = NULL;
>         RB_CLEAR_NODE(&entry->rbnode);
>         return entry;
> @@ -477,13 +484,17 @@ static bool zswap_handle_is_unique(struct zswap_handle *zhandle)
>   */
>  static void zswap_free_entry(struct zswap_entry *entry)
>  {
> -       if (zswap_handle_is_unique(entry->zhandle)) {
> -               zpool_free(entry->pool->zpool, entry->zhandle->handle);
> -               zswap_handle_cache_free(entry->zhandle);
> -               zswap_pool_put(entry->pool);
> -       } else {
> -               entry->zhandle->ref_count--;
> -               atomic_dec(&zswap_duplicate_pages);
> +       if (entry->zeroflag)
> +               atomic_dec(&zswap_zero_pages);
> +       else {
> +               if (zswap_handle_is_unique(entry->zhandle)) {
> +                       zpool_free(entry->pool->zpool, entry->zhandle->handle);
> +                       zswap_handle_cache_free(entry->zhandle);
> +                       zswap_pool_put(entry->pool);
> +               } else {
> +                       entry->zhandle->ref_count--;
> +                       atomic_dec(&zswap_duplicate_pages);
> +               }
>         }
>         zswap_entry_cache_free(entry);
>         atomic_dec(&zswap_stored_pages);
> @@ -1140,6 +1151,21 @@ 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
>  **********************************/
> @@ -1183,6 +1209,13 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>         }
>
>         src = kmap_atomic(page);
> +       if (zswap_is_page_zero_filled(src)) {
> +               kunmap_atomic(src);
> +               entry->offset = offset;
> +               entry->zeroflag = 1;
> +               atomic_inc(&zswap_zero_pages);
> +               goto insert_entry;
> +       }
>
>         if (zswap_same_page_sharing) {
>                 checksum = jhash2((const u32 *)src, PAGE_SIZE / 4, 17);
> @@ -1314,6 +1347,13 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>         }
>         spin_unlock(&tree->lock);
>
> +       if (entry->zeroflag) {
> +               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->zhandle->handle,
> @@ -1327,6 +1367,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>         zpool_unmap_handle(entry->pool->zpool, entry->zhandle->handle);
>         BUG_ON(ret);
>
> +freeentry:
>         spin_lock(&tree->lock);
>         zswap_entry_put(tree, entry);
>         spin_unlock(&tree->lock);
> @@ -1446,7 +1487,8 @@ static int __init zswap_debugfs_init(void)
>                         zswap_debugfs_root, &zswap_stored_pages);
>         debugfs_create_atomic_t("duplicate_pages", S_IRUGO,
>                         zswap_debugfs_root, &zswap_duplicate_pages);
> -
> +       debugfs_create_atomic_t("zero_pages", S_IRUGO,
> +                       zswap_debugfs_root, &zswap_zero_pages);
>
>         return 0;
>  }
> --
> 1.7.9.5
>

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>