The patch titled
Subject: mm/zswap: try to avoid worst-case scenario on same element pages
has been added to the -mm mm-unstable branch. Its filename is
mm-zswap-try-to-avoid-worst-case-scenario-on-same-element-pages.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-zswap-try-to-avoid-worst-case-scenario-on-same-element-pages.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Taejoon Song <taejoon.song@xxxxxxx>
Subject: mm/zswap: try to avoid worst-case scenario on same element pages
Date: Mon, 6 Feb 2023 04:00:36 +0900
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.
A similar change has already been made to zram in 90f82cbfe502 ("zram: try
to avoid worst-case scenario on same element pages").
Link: https://lkml.kernel.org/r/20230205190036.1730134-1-taejoon.song@xxxxxxx
Signed-off-by: Taejoon Song <taejoon.song@xxxxxxx>
Cc: Dan Streetman <ddstreet@xxxxxxxx>
Cc: Seth Jennings <sjenning@xxxxxxxxxx>
Cc: Taejoon Song <taejoon.song@xxxxxxx>
Cc: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Cc: <yjay.kim@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
diff --git a/mm/zswap.c~mm-zswap-try-to-avoid-worst-case-scenario-on-same-element-pages b/mm/zswap.c
--- a/mm/zswap.c~mm-zswap-try-to-avoid-worst-case-scenario-on-same-element-pages
+++ b/mm/zswap.c
@@ -1073,15 +1073,23 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)
static int zswap_is_page_same_filled(void *ptr, unsigned long *value)
{
- unsigned int pos;
unsigned long *page;
+ unsigned long val;
+ unsigned int pos, last_pos = PAGE_SIZE / sizeof(*page) - 1;
page = (unsigned long *)ptr;
- for (pos = 1; pos < PAGE_SIZE / sizeof(*page); pos++) {
- if (page[pos] != page[0])
+ val = page[0];
+
+ if (val != page[last_pos])
+ return 0;
+
+ for (pos = 1; pos < last_pos; pos++) {
+ if (val != page[pos])
return 0;
}
- *value = page[0];
+
+ *value = val;
+
return 1;
}
_
Patches currently in -mm which might be from taejoon.song@xxxxxxx are
mm-zswap-try-to-avoid-worst-case-scenario-on-same-element-pages.patch
