From 6260ae3583456808dceb4d78077e6388c49ca6d7 Mon Sep 17 00:00:00 2001 From: Sergey Senozhatsky Date: Wed, 18 Jan 2023 09:52:07 +0900 Subject: [PATCH] zsmalloc: rework zspage chain size selection Patch series "zsmalloc: make zspage chain size configurable". 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/20230118005210.2814763-1-senozhatsky@chromium.org Link: https://lkml.kernel.org/r/20230118005210.2814763-2-senozhatsky@chromium.org Signed-off-by: Sergey Senozhatsky Acked-by: Minchan Kim Cc: Mike Kravetz Signed-off-by: Andrew Morton --- mm/zsmalloc.c | 56 +++++++++++++++++++------------------------------------- 1 file changed, 19 insertions(+), 37 deletions(-) diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c index 9d27d9b..00ab4cc 100644 --- a/mm/zsmalloc.c +++ b/mm/zsmalloc.c @@ -822,42 +822,6 @@ 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); @@ -2401,6 +2365,24 @@ static int zs_register_shrinker(struct zs_pool *pool) 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 @@ -2445,7 +2427,7 @@ struct zs_pool *zs_create_pool(const char *name) 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; /* -- 2.7.4