Re: [PATCH 2/2] zsmalloc: fine-grained inuse ratio based fullness grouping

Yosry Ahmed
2023-02-07T01:02:59Z

On Mon, Feb 6, 2023 at 1:26 AM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> Each zspage maintains ->inuse counter which keeps track of the
> number of objects stored in the page. The ->inuse counter also
> determines the page's "fullness group" which is calculated as
> the ratio of the "inuse" objects to the total number of objects
> the page can hold (objs_per_zspage). The closer the ->inuse
> counter is to objs_per_zspage, the better.
>
> Each size class maintains several fullness lists, that keep
> track of zspages of particular "fullness". There are four lists
> at the moment:
>
> ZS_EMPTY for pages with zero "inuse" counter
> ZS_FULL for pages with "inuse" equal to objs_per_zspage
> ZS_ALMOST_EMPTY for pages with "inuse" less than or equal to
> 3 * objs_per_zspage / 4
> ZS_ALMOST_FULL for pages with "inuse" greater than
> 3 * objs_per_zspage / 4.
>
> First or all, this makes ZS_ALMOST_EMPTY fullness list pretty
> busy for certain size classes. For example, the ZS_ALMOST_EMPTY
> list for class-112 (which can store 256 objects per zspage) will
> contain pages with ->inuse counters in the range from 1 to 192.
>
> Second, pages within each fullness list are stored in random
> order with regard to the ->inuse counter. This is because
> sorting the pages by ->inuse counter each time obj_malloc() or
> obj_free() is called would be too expensive.
>
> However, the ->inuse counter is still a crucial factor in many
> situations.
>
> In a busy system with many obj_malloc() and obj_free() calls,
> fullness lists become inefficient. For instance, the ->inuse
> counters for the first 7 zspages of some random classes are:
>
> class-1840 objs_per_zspage 20:
> ZS_ALMOST_EMPTY: 3 13 8 2 11 14 3
> ZS_ALMOST_FULL : empty
>
> class-688 objs_per_zspage 59:
> ZS_ALMOST_EMPTY: 1 3 5 1 18 13 15
> ZS_ALMOST_FULL : empty
>
> For the two major zsmalloc operations, zs_malloc() and zs_compact(),
> we typically select the head page from the corresponding fullness
> list as the best candidate page. However, this assumption is not
> always accurate.
>
> For the zs_malloc() operation, the optimal candidate page should
> have the highest ->inuse counter. This is because the goal is to
> maximize the number of ZS_FULL pages and make full use of all
> allocated memory.
>
> For the zs_compact() operation, the optimal candidate page should
> have the lowest ->inuse counter. This is because compaction needs
> to move objects in use to another page before it can release the
> zspage and return its physical pages to the buddy allocator. The
> fewer objects in use, the quicker compaction can release the page.
> Additionally, compaction is measured by the number of pages it
> releases. For example, assume the following case:
>
> - size class stores 8 objects per zspage
> - ALMOST_FULL list contains one page that has ->inuse equal to 6
> - ALMOST_EMPTY list contains 3 pages: one pages has ->inuse
> equal to 2, and two pages have ->inuse equal to 1.
>
> The current compaction algorithm selects the head page of the
> ALMOST_EMPTY list (the source page), which has inuse equals 2,
> moves its objects to the ALMOST_FULL list page (the destination
> page), and then releases the source page. The ALMOST_FULL page
> (destination page) becomes FULL, so further compaction is not
> possible.
>
> At the same time, if compaction were to choose ALMOST_EMPTY pages
> with ->inuse equal to 1, it would be able to release two zspages
> while still performing the same number of memcpy() operations.
>
> This patch reworks the fullness grouping mechanism. Instead of
> relying on a threshold that results in too many pages being
> included in the ALMOST_EMPTY group for specific classes, size
> classes maintain a larger number of fullness lists that give
> strict guarantees on the minimum and maximum ->inuse values
> within each group. Each group represents a 10% change in the
> ->inuse ratio compared to neighboring groups. In essence, there
> are groups for pages with 0%, 10%, 20% usage ratios, and so on,
> up to 100%.
>
> This enhances the selection of candidate pages for both zs_malloc()
> and zs_compact(). A printout of the ->inuse counters of the first 7
> pages per (random) class fullness group:
>
> class-768 objs_per_zspage 16:
> fullness 100%: empty
> fullness 99%: empty
> fullness 90%: empty
> fullness 80%: empty
> fullness 70%: empty
> fullness 60%: 8 8 9 9 8 8 8
> fullness 50%: empty
> fullness 40%: 5 5 6 5 5 5 5
> fullness 30%: 4 4 4 4 4 4 4
> fullness 20%: 2 3 2 3 3 2 2
> fullness 10%: 1 1 1 1 1 1 1
> fullness 0%: empty
>
> The zs_malloc() function searches through the groups of pages
> starting with the one having the highest usage ratio. This means
> that it always selects a page from the group with the least
> internal fragmentation (highest usage ratio) and makes it even
> less fragmented by increasing its usage ratio.
>
> The zs_compact() function, on the other hand, begins by scanning
> the group with the highest fragmentation (lowest usage ratio) to
> locate the source page. The first available page is selected, and
> then the function moves downward to find a destination page in
> the group with the lowest internal fragmentation (highest usage
> ratio).
>
> The example demonstrates that zs_malloc() would choose a page
> with ->inuse of 8 as the candidate page, while zs_compact()
> would pick a page with ->inuse of 1 as the source page and
> another page with ->inuse of 8 as the destination page.
>
> A 1/10 difference in ratio between fullness groups is intentional
> and critical for classes that have a high number of objs_per_zspage.
> For instance, class-624 stores 59 objects per zspage. With a 1/10
> ratio grouping, the difference in inuse values between the page
> with the lowest and highest inuse in a single fullness group is
> only 4 objects (2469 bytes), whereas a 1/5 ratio grouping would
> result in a difference of 10 objects (6240 bytes).

The memory extra overhead would be sizeof(list_head) * nr of classes *
extra fullness groups = 16 * 255 * 6 = 24480 bytes ~= 24KB on a
machine with 4096 page size. Sounds reasonable (although I wonder how
it scales with PAGE_SIZE).

>
>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
>
> ---
> mm/zsmalloc.c | 224 +++++++++++++++++++++++++++++++++-----------------
> 1 file changed, 148 insertions(+), 76 deletions(-)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index b57a89ed6f30..1901edd01e38 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -127,7 +127,7 @@
> #define OBJ_INDEX_MASK ((_AC(1, UL) << OBJ_INDEX_BITS) - 1)
>
> #define HUGE_BITS 1
> -#define FULLNESS_BITS 2
> +#define FULLNESS_BITS 4
> #define CLASS_BITS 8
> #define ISOLATED_BITS 5
> #define MAGIC_VAL_BITS 8
> @@ -159,24 +159,88 @@
> #define ZS_SIZE_CLASSES (DIV_ROUND_UP(ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE, \
> ZS_SIZE_CLASS_DELTA) + 1)
>
> +/*
> + * Pages are distinguished by the ratio of used memory (that is the ratio
> + * of ->inuse objects to all objects that page can store). For example,
> + * USAGE_30 means that the ratio of used objects is > 20% and <= 30%.
> + *
> + * The number of fullness groups is not random. It allows us to keep
> + * diffeence between the least busy page in the group (minimum permitted
> + * number of ->inuse objects) and the most busy page (maximum permitted
> + * number of ->inuse objects) at a reasonable value.
> + */
> enum fullness_group {
> - ZS_EMPTY,
> - ZS_ALMOST_EMPTY,
> - ZS_ALMOST_FULL,
> - ZS_FULL,
> + ZS_USAGE_0,
> + ZS_USAGE_10,
> + ZS_USAGE_20,
> + ZS_USAGE_30,
> + ZS_USAGE_40,
> + ZS_USAGE_50,
> + ZS_USAGE_60,
> + ZS_USAGE_70,
> + ZS_USAGE_80,
> + ZS_USAGE_90,
> + ZS_USAGE_99,
> + ZS_USAGE_100,
> NR_ZS_FULLNESS,
> };
>

Is there a reason why this can't be done with something like #define
FULLNESS_GROUPS 10? We can make sure during build that (100 %
FULLNESS_GROUPS == 0) to make our lives easier. I feel like the code
will be much more concise and easier to navigate, instead of multiple
enums and static arrays.

>
> enum class_stat_type {
> - CLASS_EMPTY,
> - CLASS_ALMOST_EMPTY,
> - CLASS_ALMOST_FULL,
> - CLASS_FULL,
> + CLASS_USAGE_0,
> + CLASS_USAGE_10,
> + CLASS_USAGE_20,
> + CLASS_USAGE_30,
> + CLASS_USAGE_40,
> + CLASS_USAGE_50,
> + CLASS_USAGE_60,
> + CLASS_USAGE_70,
> + CLASS_USAGE_80,
> + CLASS_USAGE_90,
> + CLASS_USAGE_99,
> + CLASS_USAGE_100,
> OBJ_ALLOCATED,
> OBJ_USED,
> NR_ZS_STAT_TYPE,
> };
>
> +#define NUM_FULLNESS_GROUPS 10
> +
> +/*
> + * Lookup pages in increasing (from lowest to highest) order of usage ratio.
> + * This is useful, for instance, during compaction, when we want to migrate
> + * as few objects as possible in order to free zspage.
> + */
> +static const enum fullness_group fullness_asc[NUM_FULLNESS_GROUPS] = {
> + ZS_USAGE_10,
> + ZS_USAGE_20,
> + ZS_USAGE_30,
> + ZS_USAGE_40,
> + ZS_USAGE_50,
> + ZS_USAGE_60,
> + ZS_USAGE_70,
> + ZS_USAGE_80,
> + ZS_USAGE_90,
> + ZS_USAGE_99
> +};
> +
> +/*
> + * Lookup pages in decreasing (from highest to lowest) order of usage ratio.
> + * This is useful in zs_malloc() and compaction, when we want to have as
> + * many full pages as possible for more efficient memory usage.
> + */
> +static const enum fullness_group fullness_desc[NUM_FULLNESS_GROUPS] = {
> + ZS_USAGE_99,
> + ZS_USAGE_90,
> + ZS_USAGE_80,
> + ZS_USAGE_70,
> + ZS_USAGE_60,
> + ZS_USAGE_50,
> + ZS_USAGE_40,
> + ZS_USAGE_30,
> + ZS_USAGE_20,
> + ZS_USAGE_10,
> +};
> +
> struct zs_size_stat {
> unsigned long objs[NR_ZS_STAT_TYPE];
> };
> @@ -185,21 +249,6 @@ struct zs_size_stat {
> static struct dentry *zs_stat_root;
> #endif
>
> -/*
> - * We assign a page to ZS_ALMOST_EMPTY fullness group when:
> - * n <= N / f, where
> - * n = number of allocated objects
> - * N = total number of objects zspage can store
> - * f = fullness_threshold_frac
> - *
> - * Similarly, we assign zspage to:
> - * ZS_ALMOST_FULL when n > N / f
> - * ZS_EMPTY when n == 0
> - * ZS_FULL when n == N
> - *
> - * (see: fix_fullness_group())
> - */
> -static const int fullness_threshold_frac = 4;
> static size_t huge_class_size;
>
> struct size_class {
> @@ -652,8 +701,23 @@ static int zs_stats_size_show(struct seq_file *s, void *v)
> continue;
>
> spin_lock(&pool->lock);
> - class_almost_full = zs_stat_get(class, CLASS_ALMOST_FULL);
> - class_almost_empty = zs_stat_get(class, CLASS_ALMOST_EMPTY);
> +
> + /*
> + * Replecate old behaviour for almost_full and almost_empty
> + * stats.
> + */
> + class_almost_full = zs_stat_get(class, CLASS_USAGE_99);
> + class_almost_full += zs_stat_get(class, CLASS_USAGE_90);
> + class_almost_full += zs_stat_get(class, CLASS_USAGE_80);
> + class_almost_full += zs_stat_get(class, CLASS_USAGE_70);
> +
> + class_almost_empty = zs_stat_get(class, CLASS_USAGE_60);
> + class_almost_empty += zs_stat_get(class, CLASS_USAGE_50);
> + class_almost_empty += zs_stat_get(class, CLASS_USAGE_40);
> + class_almost_empty += zs_stat_get(class, CLASS_USAGE_30);
> + class_almost_empty += zs_stat_get(class, CLASS_USAGE_20);
> + class_almost_empty += zs_stat_get(class, CLASS_USAGE_10);
> +
> obj_allocated = zs_stat_get(class, OBJ_ALLOCATED);
> obj_used = zs_stat_get(class, OBJ_USED);
> freeable = zs_can_compact(class);
> @@ -723,33 +787,39 @@ static inline void zs_pool_stat_destroy(struct zs_pool *pool)
> }
> #endif
>
> -
> /*
> * For each size class, zspages are divided into different groups
> - * depending on how "full" they are. This was done so that we could
> - * easily find empty or nearly empty zspages when we try to shrink
> - * the pool (not yet implemented). This function returns fullness
> + * depending on their usage ratio. This function returns fullness
> * status of the given page.
> */
> static enum fullness_group get_fullness_group(struct size_class *class,
> - struct zspage *zspage)
> -{
> + struct zspage *zspage)
> +{
> + static const enum fullness_group groups[] = {
> + ZS_USAGE_10,
> + ZS_USAGE_20,
> + ZS_USAGE_30,
> + ZS_USAGE_40,
> + ZS_USAGE_50,
> + ZS_USAGE_60,
> + ZS_USAGE_70,
> + ZS_USAGE_80,
> + ZS_USAGE_90,
> + ZS_USAGE_99,
> + };
> int inuse, objs_per_zspage;
> - enum fullness_group fg;
> + int ratio;
>
> inuse = get_zspage_inuse(zspage);
> objs_per_zspage = class->objs_per_zspage;
>
> if (inuse == 0)
> - fg = ZS_EMPTY;
> - else if (inuse == objs_per_zspage)
> - fg = ZS_FULL;
> - else if (inuse <= 3 * objs_per_zspage / fullness_threshold_frac)
> - fg = ZS_ALMOST_EMPTY;
> - else
> - fg = ZS_ALMOST_FULL;
> + return ZS_USAGE_0;
> + if (inuse == objs_per_zspage)
> + return ZS_USAGE_100;
>
> - return fg;
> + ratio = 100 * inuse / objs_per_zspage;
> + return groups[ratio / 10];
> }
>
> /*
> @@ -781,14 +851,13 @@ static void remove_zspage(struct size_class *class,
> /*
> * Each size class maintains zspages in different fullness groups depending
> * on the number of live objects they contain. When allocating or freeing
> - * objects, the fullness status of the page can change, say, from ALMOST_FULL
> - * to ALMOST_EMPTY when freeing an object. This function checks if such
> - * a status change has occurred for the given page and accordingly moves the
> - * page from the freelist of the old fullness group to that of the new
> - * fullness group.
> + * objects, the fullness status of the page can change, say, from USAGE_80
> + * to USAGE_70 when freeing an object. This function checks if such a status
> + * change has occurred for the given page and accordingly moves the page from
> + * the list of the old fullness group to that of the new fullness group.
> */
> static enum fullness_group fix_fullness_group(struct size_class *class,
> - struct zspage *zspage)
> + struct zspage *zspage)
> {
> int class_idx;
> enum fullness_group currfg, newfg;
> @@ -972,7 +1041,7 @@ static void __free_zspage(struct zs_pool *pool, struct size_class *class,
> assert_spin_locked(&pool->lock);
>
> VM_BUG_ON(get_zspage_inuse(zspage));
> - VM_BUG_ON(fg != ZS_EMPTY);
> + VM_BUG_ON(fg != ZS_USAGE_0);
>
> /* Free all deferred handles from zs_free */
> free_handles(pool, class, zspage);
> @@ -1011,7 +1080,7 @@ static void free_zspage(struct zs_pool *pool, struct size_class *class,
> return;
> }
>
> - remove_zspage(class, zspage, ZS_EMPTY);
> + remove_zspage(class, zspage, ZS_USAGE_0);
> #ifdef CONFIG_ZPOOL
> list_del(&zspage->lru);
> #endif
> @@ -1142,14 +1211,15 @@ static struct zspage *alloc_zspage(struct zs_pool *pool,
> return zspage;
> }
>
> -static struct zspage *find_get_zspage(struct size_class *class)
> +static struct zspage *find_get_zspage(struct size_class *class,
> + const enum fullness_group *groups)
> {
> - int i;
> struct zspage *zspage;
> + int i;
>
> - for (i = ZS_ALMOST_FULL; i >= ZS_EMPTY; i--) {
> + for (i = 0; i < NUM_FULLNESS_GROUPS; i++) {

I think you missed using the passed groups arg here.

>
> zspage = list_first_entry_or_null(&class->fullness_list[i],
> - struct zspage, list);
> + struct zspage, list);
> if (zspage)
> break;
> }
> @@ -1524,7 +1594,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
>
> /* pool->lock effectively protects the zpage migration */
> spin_lock(&pool->lock);
> - zspage = find_get_zspage(class);
> + zspage = find_get_zspage(class, fullness_desc);
> if (likely(zspage)) {
> obj = obj_malloc(pool, zspage, handle);
> /* Now move the zspage to another fullness group, if required */
> @@ -1642,7 +1712,7 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
> obj_free(class->size, obj, NULL);
>
> fullness = fix_fullness_group(class, zspage);
> - if (fullness == ZS_EMPTY)
> + if (fullness == ZS_USAGE_0)
> free_zspage(pool, class, zspage);
>
> spin_unlock(&pool->lock);
> @@ -1824,22 +1894,19 @@ static int migrate_zspage(struct zs_pool *pool, struct size_class *class,
> return ret;
> }
>
> -static struct zspage *isolate_zspage(struct size_class *class, bool source)
> +static struct zspage *isolate_zspage(struct size_class *class,
> + const enum fullness_group *groups)
> {
> - int i;
> struct zspage *zspage;
> - enum fullness_group fg[2] = {ZS_ALMOST_EMPTY, ZS_ALMOST_FULL};
> + int i;
>
> - if (!source) {
> - fg[0] = ZS_ALMOST_FULL;
> - fg[1] = ZS_ALMOST_EMPTY;
> - }
> + for (i = 0; i < NUM_FULLNESS_GROUPS; i++) {
> + enum fullness_group fg = groups[i];
>
> - for (i = 0; i < 2; i++) {
> - zspage = list_first_entry_or_null(&class->fullness_list[fg[i]],
> - struct zspage, list);
> + zspage = list_first_entry_or_null(&class->fullness_list[fg],
> + struct zspage, list);
> if (zspage) {
> - remove_zspage(class, zspage, fg[i]);
> + remove_zspage(class, zspage, fg);
> return zspage;
> }
> }
> @@ -2133,7 +2200,8 @@ static void async_free_zspage(struct work_struct *work)
> continue;
>
> spin_lock(&pool->lock);
> - list_splice_init(&class->fullness_list[ZS_EMPTY], &free_pages);
> + list_splice_init(&class->fullness_list[ZS_USAGE_0],
> + &free_pages);
> spin_unlock(&pool->lock);
> }
>
> @@ -2142,7 +2210,7 @@ static void async_free_zspage(struct work_struct *work)
> lock_zspage(zspage);
>
> get_zspage_mapping(zspage, &class_idx, &fullness);
> - VM_BUG_ON(fullness != ZS_EMPTY);
> + VM_BUG_ON(fullness != ZS_USAGE_0);
> class = pool->size_class[class_idx];
> spin_lock(&pool->lock);
> #ifdef CONFIG_ZPOOL
> @@ -2215,7 +2283,7 @@ static unsigned long __zs_compact(struct zs_pool *pool,
> * as well as zpage allocation/free
> */
> spin_lock(&pool->lock);
> - while ((src_zspage = isolate_zspage(class, true))) {
> + while ((src_zspage = isolate_zspage(class, fullness_asc))) {
> /* protect someone accessing the zspage(i.e., zs_map_object) */
> migrate_write_lock(src_zspage);
>
> @@ -2225,10 +2293,11 @@ static unsigned long __zs_compact(struct zs_pool *pool,
> cc.obj_idx = 0;
> cc.s_page = get_first_page(src_zspage);
>
> - while ((dst_zspage = isolate_zspage(class, false))) {
> + while ((dst_zspage = isolate_zspage(class, fullness_desc))) {
> migrate_write_lock_nested(dst_zspage);
>
> cc.d_page = get_first_page(dst_zspage);
> +
> /*
> * If there is no more space in dst_page, resched
> * and see if anyone had allocated another zspage.
> @@ -2250,7 +2319,7 @@ static unsigned long __zs_compact(struct zs_pool *pool,
> putback_zspage(class, dst_zspage);
> migrate_write_unlock(dst_zspage);
>
> - if (putback_zspage(class, src_zspage) == ZS_EMPTY) {
> + if (putback_zspage(class, src_zspage) == ZS_USAGE_0) {
> migrate_write_unlock(src_zspage);
> free_zspage(pool, class, src_zspage);
> pages_freed += class->pages_per_zspage;
> @@ -2408,7 +2477,7 @@ struct zs_pool *zs_create_pool(const char *name)
> int pages_per_zspage;
> int objs_per_zspage;
> struct size_class *class;
> - int fullness = 0;
> + int fullness;
>
> size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
> if (size > ZS_MAX_ALLOC_SIZE)
> @@ -2462,9 +2531,12 @@ struct zs_pool *zs_create_pool(const char *name)
> class->pages_per_zspage = pages_per_zspage;
> class->objs_per_zspage = objs_per_zspage;
> pool->size_class[i] = class;
> - for (fullness = ZS_EMPTY; fullness < NR_ZS_FULLNESS;
> - fullness++)
> +
> + fullness = ZS_USAGE_0;
> + while (fullness < NR_ZS_FULLNESS) {
> INIT_LIST_HEAD(&class->fullness_list[fullness]);
> + fullness++;
> + }
>
> prev_class = class;
> }
> @@ -2510,7 +2582,7 @@ void zs_destroy_pool(struct zs_pool *pool)
> if (class->index != i)
> continue;
>
> - for (fg = ZS_EMPTY; fg < NR_ZS_FULLNESS; fg++) {
> + for (fg = ZS_USAGE_0; fg < NR_ZS_FULLNESS; fg++) {
> if (!list_empty(&class->fullness_list[fg])) {
> pr_info("Freeing non-empty class with size %db, fullness group %d\n",
> class->size, fg);
> @@ -2686,7 +2758,7 @@ static int zs_reclaim_page(struct zs_pool *pool, unsigned int retries)
> * while the page is removed from the pool. Fix it
> * up for the check in __free_zspage().
> */
> - zspage->fullness = ZS_EMPTY;
> + zspage->fullness = ZS_USAGE_0;
>
> __free_zspage(pool, class, zspage);
> spin_unlock(&pool->lock);
> --
> 2.39.1.519.gcb327c4b5f-goog
>
>