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plugins/cache: Added FIFO and LRU eviction policies
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Implemented FIFO and LRU eviction policies. Now one of the three
eviction policies can be chosen as an argument. On not specifying an
argument, LRU is used by default.

Signed-off-by: Mahmoud Mandour <[email protected]>
Signed-off-by: Alex Bennée <[email protected]>
Message-Id: <[email protected]>
Message-Id: <[email protected]>
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@i3abghany @stsquad
i3abghany authored and stsquad committed Jul 14, 2021
1 parent 86ae3a1 commit 40c4a55
Showing 1 changed file with 190 additions and 13 deletions.
203 changes: 190 additions & 13 deletions contrib/plugins/cache.c
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,14 @@ static uint64_t dmisses;
static uint64_t imem_accesses;
static uint64_t imisses;

enum EvictionPolicy {
LRU,
FIFO,
RAND,
};

enum EvictionPolicy policy;

/*
* A CacheSet is a set of cache blocks. A memory block that maps to a set can be
* put in any of the blocks inside the set. The number of block per set is
Expand All @@ -48,6 +56,8 @@ static uint64_t imisses;
* The set number is used to identify the set in which the block may exist.
* The tag is compared against all the tags of a set to search for a match. If a
* match is found, then the access is a hit.
*
* The CacheSet also contains bookkeaping information about eviction details.
*/

typedef struct {
Expand All @@ -57,6 +67,9 @@ typedef struct {

typedef struct {
CacheBlock *blocks;
uint64_t *lru_priorities;
uint64_t lru_gen_counter;
GQueue *fifo_queue;
} CacheSet;

typedef struct {
Expand All @@ -77,6 +90,12 @@ typedef struct {
uint64_t imisses;
} InsnData;

void (*update_hit)(Cache *cache, int set, int blk);
void (*update_miss)(Cache *cache, int set, int blk);

void (*metadata_init)(Cache *cache);
void (*metadata_destroy)(Cache *cache);

Cache *dcache, *icache;

static int pow_of_two(int num)
Expand All @@ -89,6 +108,103 @@ static int pow_of_two(int num)
return ret;
}

/*
* LRU evection policy: For each set, a generation counter is maintained
* alongside a priority array.
*
* On each set access, the generation counter is incremented.
*
* On a cache hit: The hit-block is assigned the current generation counter,
* indicating that it is the most recently used block.
*
* On a cache miss: The block with the least priority is searched and replaced
* with the newly-cached block, of which the priority is set to the current
* generation number.
*/

static void lru_priorities_init(Cache *cache)
{
int i;

for (i = 0; i < cache->num_sets; i++) {
cache->sets[i].lru_priorities = g_new0(uint64_t, cache->assoc);
cache->sets[i].lru_gen_counter = 0;
}
}

static void lru_update_blk(Cache *cache, int set_idx, int blk_idx)
{
CacheSet *set = &cache->sets[set_idx];
set->lru_priorities[blk_idx] = cache->sets[set_idx].lru_gen_counter;
set->lru_gen_counter++;
}

static int lru_get_lru_block(Cache *cache, int set_idx)
{
int i, min_idx, min_priority;

min_priority = cache->sets[set_idx].lru_priorities[0];
min_idx = 0;

for (i = 1; i < cache->assoc; i++) {
if (cache->sets[set_idx].lru_priorities[i] < min_priority) {
min_priority = cache->sets[set_idx].lru_priorities[i];
min_idx = i;
}
}
return min_idx;
}

static void lru_priorities_destroy(Cache *cache)
{
int i;

for (i = 0; i < cache->num_sets; i++) {
g_free(cache->sets[i].lru_priorities);
}
}

/*
* FIFO eviction policy: a FIFO queue is maintained for each CacheSet that
* stores accesses to the cache.
*
* On a compulsory miss: The block index is enqueued to the fifo_queue to
* indicate that it's the latest cached block.
*
* On a conflict miss: The first-in block is removed from the cache and the new
* block is put in its place and enqueued to the FIFO queue.
*/

static void fifo_init(Cache *cache)
{
int i;

for (i = 0; i < cache->num_sets; i++) {
cache->sets[i].fifo_queue = g_queue_new();
}
}

static int fifo_get_first_block(Cache *cache, int set)
{
GQueue *q = cache->sets[set].fifo_queue;
return GPOINTER_TO_INT(g_queue_pop_tail(q));
}

static void fifo_update_on_miss(Cache *cache, int set, int blk_idx)
{
GQueue *q = cache->sets[set].fifo_queue;
g_queue_push_head(q, GINT_TO_POINTER(blk_idx));
}

static void fifo_destroy(Cache *cache)
{
int i;

for (i = 0; i < cache->assoc; i++) {
g_queue_free(cache->sets[i].fifo_queue);
}
}

static inline uint64_t extract_tag(Cache *cache, uint64_t addr)
{
return addr & cache->tag_mask;
Expand Down Expand Up @@ -139,6 +255,11 @@ static Cache *cache_init(int blksize, int assoc, int cachesize)
blk_mask = blksize - 1;
cache->set_mask = ((cache->num_sets - 1) << cache->blksize_shift);
cache->tag_mask = ~(cache->set_mask | blk_mask);

if (metadata_init) {
metadata_init(cache);
}

return cache;
}

Expand All @@ -155,12 +276,21 @@ static int get_invalid_block(Cache *cache, uint64_t set)
return -1;
}

static int get_replaced_block(Cache *cache)
static int get_replaced_block(Cache *cache, int set)
{
return g_rand_int_range(rng, 0, cache->assoc);
switch (policy) {
case RAND:
return g_rand_int_range(rng, 0, cache->assoc);
case LRU:
return lru_get_lru_block(cache, set);
case FIFO:
return fifo_get_first_block(cache, set);
default:
g_assert_not_reached();
}
}

static bool in_cache(Cache *cache, uint64_t addr)
static int in_cache(Cache *cache, uint64_t addr)
{
int i;
uint64_t tag, set;
Expand All @@ -171,11 +301,11 @@ static bool in_cache(Cache *cache, uint64_t addr)
for (i = 0; i < cache->assoc; i++) {
if (cache->sets[set].blocks[i].tag == tag &&
cache->sets[set].blocks[i].valid) {
return true;
return i;
}
}

return false;
return -1;
}

/**
Expand All @@ -188,20 +318,28 @@ static bool in_cache(Cache *cache, uint64_t addr)
*/
static bool access_cache(Cache *cache, uint64_t addr)
{
int hit_blk, replaced_blk;
uint64_t tag, set;
int replaced_blk;

if (in_cache(cache, addr)) {
return true;
}

tag = extract_tag(cache, addr);
set = extract_set(cache, addr);

hit_blk = in_cache(cache, addr);
if (hit_blk != -1) {
if (update_hit) {
update_hit(cache, set, hit_blk);
}
return true;
}

replaced_blk = get_invalid_block(cache, set);

if (replaced_blk == -1) {
replaced_blk = get_replaced_block(cache);
replaced_blk = get_replaced_block(cache, set);
}

if (update_miss) {
update_miss(cache, set, replaced_blk);
}

cache->sets[set].blocks[replaced_blk].tag = tag;
Expand Down Expand Up @@ -308,6 +446,10 @@ static void cache_free(Cache *cache)
g_free(cache->sets[i].blocks);
}

if (metadata_destroy) {
metadata_destroy(cache);
}

g_free(cache->sets);
g_free(cache);
}
Expand Down Expand Up @@ -395,6 +537,28 @@ static void plugin_exit(qemu_plugin_id_t id, void *p)
g_hash_table_destroy(miss_ht);
}

static void policy_init()
{
switch (policy) {
case LRU:
update_hit = lru_update_blk;
update_miss = lru_update_blk;
metadata_init = lru_priorities_init;
metadata_destroy = lru_priorities_destroy;
break;
case FIFO:
update_miss = fifo_update_on_miss;
metadata_init = fifo_init;
metadata_destroy = fifo_destroy;
break;
case RAND:
rng = g_rand_new();
break;
default:
g_assert_not_reached();
}
}

QEMU_PLUGIN_EXPORT
int qemu_plugin_install(qemu_plugin_id_t id, const qemu_info_t *info,
int argc, char **argv)
Expand All @@ -414,6 +578,7 @@ int qemu_plugin_install(qemu_plugin_id_t id, const qemu_info_t *info,
iblksize = 64;
icachesize = iblksize * iassoc * 32;

policy = LRU;

for (i = 0; i < argc; i++) {
char *opt = argv[i];
Expand All @@ -431,12 +596,26 @@ int qemu_plugin_install(qemu_plugin_id_t id, const qemu_info_t *info,
dcachesize = g_ascii_strtoll(opt + 11, NULL, 10);
} else if (g_str_has_prefix(opt, "limit=")) {
limit = g_ascii_strtoll(opt + 6, NULL, 10);
} else if (g_str_has_prefix(opt, "evict=")) {
gchar *p = opt + 6;
if (g_strcmp0(p, "rand") == 0) {
policy = RAND;
} else if (g_strcmp0(p, "lru") == 0) {
policy = LRU;
} else if (g_strcmp0(p, "fifo") == 0) {
policy = FIFO;
} else {
fprintf(stderr, "invalid eviction policy: %s\n", opt);
return -1;
}
} else {
fprintf(stderr, "option parsing failed: %s\n", opt);
return -1;
}
}

policy_init();

dcache = cache_init(dblksize, dassoc, dcachesize);
if (!dcache) {
const char *err = cache_config_error(dblksize, dassoc, dcachesize);
Expand All @@ -453,8 +632,6 @@ int qemu_plugin_install(qemu_plugin_id_t id, const qemu_info_t *info,
return -1;
}

rng = g_rand_new();

qemu_plugin_register_vcpu_tb_trans_cb(id, vcpu_tb_trans);
qemu_plugin_register_atexit_cb(id, plugin_exit, NULL);

Expand Down

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