The patch titled Subject: zram: try to avoid worst-case scenario on same element pages has been added to the -mm tree. Its filename is zram-try-to-avoid-worst-case-scenario-on-same-element-pages.patch This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/zram-try-to-avoid-worst-case-scenario-on-same-element-pages.patch and later at http://ozlabs.org/~akpm/mmotm/broken-out/zram-try-to-avoid-worst-case-scenario-on-same-element-pages.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Taejoon Song <taejoon.song@xxxxxxx> Subject: zram: try to avoid worst-case scenario on same element pages The worst-case scenario on finding same element pages is that almost all elements are same at the first glance but only last few elements are different. Since the same element tends to be grouped from the beginning of the pages, if we check the first element with the last element before looping through all elements, we might have some chances to quickly detect non-same element pages. 1. Test is done under LG webOS TV (64-bit arch) 2. Dump the swap-out pages (~819200 pages) 3. Analyze the pages with simple test script which counts the iteration number and measures the speed at off-line Under 64-bit arch, the worst iteration count is PAGE_SIZE / 8 bytes = 512. The speed is based on the time to consume page_same_filled() function only. The result, on average, is listed as below: Num of Iter Speed(MB/s) Looping-Forward (Orig) 38 99265 Looping-Backward 36 102725 Last-element-check (This Patch) 33 125072 The result shows that the average iteration count decreases by 13% and the speed increases by 25% with this patch. This patch does not increase the overall time complexity, though. I also ran simpler version which uses backward loop. Just looping backward also makes some improvement, but less than this patch. Link: http://lkml.kernel.org/r/1575424418-16119-1-git-send-email-taejoon.song@xxxxxxx Signed-off-by: Taejoon Song <taejoon.song@xxxxxxx> Acked-by: Minchan Kim <minchan@xxxxxxxxxx> Cc: Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx> Cc: Jens Axboe <axboe@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- drivers/block/zram/zram_drv.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) --- a/drivers/block/zram/zram_drv.c~zram-try-to-avoid-worst-case-scenario-on-same-element-pages +++ a/drivers/block/zram/zram_drv.c @@ -207,14 +207,17 @@ static inline void zram_fill_page(void * static bool page_same_filled(void *ptr, unsigned long *element) { - unsigned int pos; unsigned long *page; unsigned long val; + unsigned int pos, last_pos = PAGE_SIZE / sizeof(*page) - 1; page = (unsigned long *)ptr; val = page[0]; - for (pos = 1; pos < PAGE_SIZE / sizeof(*page); pos++) { + if (val != page[last_pos]) + return false; + + for (pos = 1; pos < last_pos - 1; pos++) { if (val != page[pos]) return false; } _ Patches currently in -mm which might be from taejoon.song@xxxxxxx are zram-try-to-avoid-worst-case-scenario-on-same-element-pages.patch