The patch titled
Subject: zsmalloc: rework zspage chain size selection
has been added to the -mm mm-unstable branch. Its filename is
zsmalloc-rework-zspage-chain-size-selection.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/zsmalloc-rework-zspage-chain-size-selection.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: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Subject: zsmalloc: rework zspage chain size selection
Date: Mon, 9 Jan 2023 12:38:35 +0900
Patch series "zsmalloc: make zspage chain size configurable", v2.
This turns the hard coded limit on maximum number of physical pages
per-zspage into a config option. It also increases the default limit from
4 to 8.
This patch (of 4):
Computers are bad at division. We currently decide the best zspage chain
size (max number of physical pages per-zspage) by looking at a `used
percentage` value. This is not enough as we lose precision during usage
percentage calculations For example, let's look at size class 208:
pages per zspage wasted bytes used%
1 144 96
2 80 99
3 16 99
4 160 99
Current algorithm will select 2 page per zspage configuration, as it's the
first one to reach 99%. However, 3 pages per zspage waste less memory.
Change algorithm and select zspage configuration that has lowest wasted
value.
Link: https://lkml.kernel.org/r/20230109033838.2779902-1-senozhatsky@xxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20230109033838.2779902-2-senozhatsky@xxxxxxxxxxxx
Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/zsmalloc.c | 56 ++++++++++++++++--------------------------------
1 file changed, 19 insertions(+), 37 deletions(-)
--- a/mm/zsmalloc.c~zsmalloc-rework-zspage-chain-size-selection
+++ a/mm/zsmalloc.c
@@ -802,42 +802,6 @@ out:
return newfg;
}
-/*
- * We have to decide on how many pages to link together
- * to form a zspage for each size class. This is important
- * to reduce wastage due to unusable space left at end of
- * each zspage which is given as:
- * wastage = Zp % class_size
- * usage = Zp - wastage
- * where Zp = zspage size = k * PAGE_SIZE where k = 1, 2, ...
- *
- * For example, for size class of 3/8 * PAGE_SIZE, we should
- * link together 3 PAGE_SIZE sized pages to form a zspage
- * since then we can perfectly fit in 8 such objects.
- */
-static int get_pages_per_zspage(int class_size)
-{
- int i, max_usedpc = 0;
- /* zspage order which gives maximum used size per KB */
- int max_usedpc_order = 1;
-
- for (i = 1; i <= ZS_MAX_PAGES_PER_ZSPAGE; i++) {
- int zspage_size;
- int waste, usedpc;
-
- zspage_size = i * PAGE_SIZE;
- waste = zspage_size % class_size;
- usedpc = (zspage_size - waste) * 100 / zspage_size;
-
- if (usedpc > max_usedpc) {
- max_usedpc = usedpc;
- max_usedpc_order = i;
- }
- }
-
- return max_usedpc_order;
-}
-
static struct zspage *get_zspage(struct page *page)
{
struct zspage *zspage = (struct zspage *)page_private(page);
@@ -2318,6 +2282,24 @@ static int zs_register_shrinker(struct z
pool->name);
}
+static int calculate_zspage_chain_size(int class_size)
+{
+ int i, min_waste = INT_MAX;
+ int chain_size = 1;
+
+ for (i = 1; i <= ZS_MAX_PAGES_PER_ZSPAGE; i++) {
+ int waste;
+
+ waste = (i * PAGE_SIZE) % class_size;
+ if (waste < min_waste) {
+ min_waste = waste;
+ chain_size = i;
+ }
+ }
+
+ return chain_size;
+}
+
/**
* zs_create_pool - Creates an allocation pool to work from.
* @name: pool name to be created
@@ -2362,7 +2344,7 @@ struct zs_pool *zs_create_pool(const cha
size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
if (size > ZS_MAX_ALLOC_SIZE)
size = ZS_MAX_ALLOC_SIZE;
- pages_per_zspage = get_pages_per_zspage(size);
+ pages_per_zspage = calculate_zspage_chain_size(size);
objs_per_zspage = pages_per_zspage * PAGE_SIZE / size;
/*
_
Patches currently in -mm which might be from senozhatsky@xxxxxxxxxxxx are
zram-correctly-handle-all-next_arg-cases.patch
zsmalloc-rework-zspage-chain-size-selection.patch
zsmalloc-skip-chain-size-calculation-for-pow_of_2-classes.patch
zsmalloc-make-zspage-chain-size-configurable.patch
zsmalloc-set-default-zspage-chain-size-to-8.patch
