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Date:   Wed, 8 Mar 2017 09:43:21 -0500
From:   Dan Streetman <ddstreet@...e.org>
To:     srividya.dr@...sung.com
Cc:     Sarbojit Ganguly <unixman.linuxboy@...il.com>,
        "sjenning@...hat.com" <sjenning@...hat.com>,
        "penberg@...nel.org" <penberg@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dinakar Reddy Pathireddy <dinakar.p@...sung.com>,
        SHARAN ALLUR <sharan.allur@...sung.com>,
        SUNEEL KUMAR SURIMANI <suneel@...sung.com>,
        JUHUN KIM <juhunkim@...sung.com>,
        Sarbojit Ganguly <ganguly.s@...sung.com>
Subject: Re: [PATCH] zswap: Zero-filled pages handling

On Wed, Mar 8, 2017 at 6:47 AM, Srividya Desireddy
<srividya.dr@...sung.com> wrote:
>
> On Sat, Mar 4, 2017 at 02:55 AM, Dan Streetman <ddstreet@...e.org> wrote:
>> On Sat, Feb 25, 2017 at 12:18 PM, Sarbojit Ganguly
>> <unixman.linuxboy@...il.com> wrote:
>>> On 25 February 2017 at 20:12, Srividya Desireddy
>>> <srividya.dr@...sung.com> wrote:
>>>> From: Srividya Desireddy <srividya.dr@...sung.com>
>>>> 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@...sung.com>
>>>> ---
>>>>  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?

yes, the patch looks good otherwise, please resend with the
entry->length change.  Feel free to include the testing/benchmark
details in the commit message, it's interesting data.

thanks!

>
>>>>  };
>>>>
>>>>  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

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