From 90f82cbfe502e96e602614ebbaf7725470c5208a Mon Sep 17 00:00:00 2001
From: Taejoon Song <taejoon.song@lge.com>
Date: Thu, 30 Jan 2020 22:15:22 -0800
Subject: [PATCH] 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.
[taejoon.song@lge.com: fix off-by-one]
Link: http://lkml.kernel.org/r/1578642001-11765-1-git-send-email-taejoon.song@lge.com
Link: http://lkml.kernel.org/r/1575424418-16119-1-git-send-email-taejoon.song@lge.com
Signed-off-by: Taejoon Song <taejoon.song@lge.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
drivers/block/zram/zram_drv.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 4285e75e52c34..71d594609b78e 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -207,14 +207,17 @@ static inline void zram_fill_page(void *ptr, unsigned long len,
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; pos++) {
if (val != page[pos])
return false;
}
--
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