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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 <senozhatsky@chromium.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Sergey Senozhatsky 2023-01-18 09:52:07 +09:00 committed by Andrew Morton
parent 076cf7ea67
commit 6260ae3583
1 changed files with 19 additions and 37 deletions
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56
mm/zsmalloc.c
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@ -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;
/*