[PATCH] zswap: Zero-filled pages handling

[Srividya Desireddy]

On Sat, Mar 4, 2017 at 02:55 AM, Dan Streetman <ddstreet@xxxxxxxx> wrote:
> On Sat, Feb 25, 2017 at 12:18 PM, Sarbojit Ganguly
> <unixman.linuxboy@xxxxxxxxx> wrote:
>> On 25 February 2017 at 20:12, Srividya Desireddy
>> <srividya.dr@xxxxxxxxxxx> wrote:
>>> From: Srividya Desireddy <srividya.dr@xxxxxxxxxxx>
>>> Date: Thu, 23 Feb 2017 15:04:06 +0530
>>> Subject: [PATCH] zswap: Zero-filled pages handling
>
> your email is base64-encoded; please send plain text emails.
>
>>>
>>> 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
>
> 20%?  that's a LOT of zero pages...which seems like applications are
> wasting a lot of memory.  what kind of workload are you testing with?
>

I have tested this patch with different workloaded on different devices.
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 on launching and relaunching 15 applications.

>>> 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.
>>>
>>> The overall overhead caused to check for a zero-filled page is very minimal
>>> when compared to the time saved by avoiding compression and allocation in
>>> case of zero-filled pages. Although, compressed size of a zero-filled page
>>> is very less, with this patch load time of a zero-filled page is reduced by
>>> 80% when compared to baseline.
>>
>> Is it possible to share the benchmark details?
>
> Was there an answer to this?
>

This patch is tested on a ARM Quad Core 32-bit device with 1.5GB RAM by
launching and relaunching different applications. With the patch, an
average of 5000 pages zero pages found in zswap out of the ~50000 pages
stored in zswap and application launch time 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@xxxxxxxxxxx>
>>> ---
>>>  mm/zswap.c |   48 +++++++++++++++++++++++++++++++++++++++++++++---
>>>  1 file changed, 45 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/zswap.c b/mm/zswap.c
>>> index 067a0d6..a574008 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
>>> @@ -140,6 +142,8 @@ struct zswap_pool {
>>>   *          decompression
>>>   * pool - the zswap_pool the entry's data is in
>>>   * handle - zpool allocation handle that stores the compressed page data
>>> + * zeroflag - the flag is set if the content of the page is filled with
>>> + *            zeros
>>>   */
>>>  struct zswap_entry {
>>>         struct rb_node rbnode;
>>> @@ -148,6 +152,7 @@ struct zswap_entry {
>>>         unsigned int length;
>>>         struct zswap_pool *pool;
>>>         unsigned long handle;
>>> +       unsigned char zeroflag;
>
> instead of a flag, we can use length == 0; the length will never be 0
> for any actually compressed page.
>

I tested the patch using entry->length itself as flag for marking zero pages.
It works fine and the functionality is same. Should I resend with this change
updated?

>>>  };
>>>
>>>  struct zswap_header {
>>> @@ -236,6 +241,7 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
>>>         if (!entry)
>>>                 return NULL;
>>>         entry->refcount = 1;
>>> +       entry->zeroflag = 0;
>>>         RB_CLEAR_NODE(&entry->rbnode);
>>>         return entry;
>>>  }
>>> @@ -306,8 +312,12 @@ 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->zeroflag)
>>> +               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();
>>> @@ -877,6 +887,19 @@ 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
>>>  **********************************/
>>> @@ -917,6 +940,15 @@ 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->zeroflag = 1;
>>> +               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) {
>>> @@ -927,7 +959,6 @@ 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);
>>> @@ -961,6 +992,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>>>         entry->handle = handle;
>>>         entry->length = dlen;
>>>
>>> +insert_entry:
>>>         /* map */
>>>         spin_lock(&tree->lock);
>>>         do {
>>> @@ -1013,6 +1045,13 @@ 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->handle,
>>> @@ -1025,6 +1064,7 @@ 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);
>>> @@ -1133,6 +1173,8 @@ 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
>>
>>
>>
>> --
>> Regards,
>> Sarbojit