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.

Dan said:

: I did test this patch out this week, and I added some instrumentation to
: check the performance impact, and tested with a small program to try to
: check the best and worst cases.
:
: When doing a lot of swap where all (or almost all) pages are same-value, I
: found this patch does save both time and space, significantly.  The exact
: improvement in time and space depends on which compressor is being used,
: but roughly agrees with the numbers you listed.
:
: In the worst case situation, where all (or almost all) pages have the
: same-value *except* the final long (meaning, zswap will check each long on
: the entire page but then still have to pass the page to the compressor),
: the same-value check is around 10-15% of the total time spent in
: zswap_frontswap_store().  That's a not-insignificant amount of time, but
: it's not huge.  Considering that most systems will probably be swapping
: pages that aren't similar to the worst case (although I don't have any
: data to know that), I'd say the improvement is worth the possible
: worst-case performance impact.

[srividya.dr@samsung.com: add memset_l instead of for loop]
Link: http://lkml.kernel.org/r/20171018104832epcms5p1b2232e2236258de3d03d1344dde9fce0@epcms5p1


Signed-off-by: Srividya Desireddy <srividya.dr@samsung.com>
Acked-by: Dan Streetman <ddstreet@ieee.org>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Dinakar Reddy Pathireddy <dinakar.p@samsung.com>
Cc: SHARAN ALLUR <sharan.allur@samsung.com>
Cc: RAJIB BASU <rajib.basu@samsung.com>
Cc: JUHUN KIM <juhunkim@samsung.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Timofey Titovets <nefelim4ag@gmail.com>
Cc: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>