zswap: same-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 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>
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