Re: [RFC PATCH v2 3/3] plugins: cache: Added FIFO and LRU eviction policies.

[Alex Bennée]

Mahmoud Mandour <ma.mandourr@gmail.com> writes:

> 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 <ma.mandourr@gmail.com>
> ---
>  contrib/plugins/cache.c | 159 ++++++++++++++++++++++++++++++++++++----
>  1 file changed, 146 insertions(+), 13 deletions(-)
>
> diff --git a/contrib/plugins/cache.c b/contrib/plugins/cache.c
> index fa0bf1dd40..1e323494bf 100644
> --- a/contrib/plugins/cache.c
> +++ b/contrib/plugins/cache.c
> @@ -18,6 +18,8 @@
>  
>  QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;
>  
> +static bool fifo, lru, rnd;
> +

Ironically this would be a good use for a single variable with an enum,
or alternatively a function pointer which can be set on initialisation.

>  static GRand *rng;
>  static GHashTable *dmiss_ht;
>  static GHashTable *imiss_ht;
> @@ -55,6 +57,8 @@ struct CacheBlock {
>  
>  struct CacheSet {
>      struct CacheBlock *blocks;
> +    uint16_t *priorities;
> +    GQueue *evict_queue;
>  };
>  
>  struct Cache {
> @@ -93,6 +97,84 @@ static inline uint64_t extract_set(struct Cache *cache, 
> uint64_t addr)
>      return (addr & cache->set_mask) >> (pow_of_two(cache->blksize));
>  }

I think it would be useful to summarise the LRU behaviour here in a
comment and explain how the priorities are meant to change as the cache
is used.

>  
> +static void lru_priorities_init(struct Cache *cache)
> +{
> +    int i, j;
> +
> +    for (i = 0; i < cache->num_sets; i++) {
> +        cache->sets[i].priorities = g_new(uint16_t, cache->assoc);
> +        for (j = 0; j < cache->assoc; j++) {
> +            cache->sets[i].priorities[j] = cache->assoc - j - 1;
> +        }
> +    }
> +}
> +
> +static void lru_update_on_miss(struct Cache *cache,
> +                                      int set_idx,
> +                                      int blk_idx)
> +{
> +    int i;
> +
> +    for (i = 0; i < cache->assoc; i++) {
> +        cache->sets[set_idx].priorities[i]++;
> +    }
> +
> +    cache->sets[set_idx].priorities[blk_idx] = 0;

So we increment priority for all non-hit blocks and reset it for the
entry just used? This isn't totally clear to follow however see bellow:

> +}
> +
> +static void lru_update_on_hit(struct Cache *cache,
> +                                         int set_idx,
> +                                         int blk_idx)
> +{
> +    uint16_t blk_priority;
> +    int i;
> +
> +    blk_priority = cache->sets[set_idx].priorities[blk_idx];
> +    for (i = 0; i < cache->assoc; i++) {
> +        if (cache->sets[set_idx].priorities[i] < blk_priority) {
> +            cache->sets[set_idx].priorities[i]++;
> +        }
> +    }
> +    cache->sets[set_idx].priorities[blk_idx] = 0;

This seems pretty expensive depending on the number of blocks. Another
approach would be to have a generation number that is incremented on
each access and stored in the appropriate set. Then...

> +}
> +
> +static int lru_get_lru_block(struct Cache *cache, int set_idx)
> +{
> +    int i;
> +
> +    for (i = 0; i < cache->assoc; i++) {
> +        if (cache->sets[set_idx].priorities[i] == cache->assoc - 1) {
> +            return i;
> +        }
> +    }

when you get to search for a "stale" block you just look for the lowest
generation number. The eviction logic should be being called less than
the update logic right?

> +
> +    g_assert_not_reached();
> +}
> +
> +static void fifo_init(struct Cache *cache)
> +{
> +    int i;
> +
> +    for (i = 0; i < cache->num_sets; i++) {
> +        cache->sets[i].evict_queue = g_queue_new();
> +    }
> +}
> +
> +static int fifo_get_first_in_block(struct Cache *cache, int set)
> +{
> +    GQueue *q = cache->sets[set].evict_queue;
> +    return GPOINTER_TO_INT(g_queue_pop_tail(q));
> +}
> +
> +static void fifo_update_on_miss(struct Cache *cache,
> +                                int set,
> +                                int blk_idx)
> +{
> +    GQueue *q = cache->sets[set].evict_queue;
> +    g_queue_push_head(q, GINT_TO_POINTER(blk_idx));
> +}

Again some commentary would be helpful around above the fifo functions.

> +
> +
>  static struct Cache *cache_init(int blksize, int assoc, int cachesize)
>  {
>      struct Cache *cache;
> @@ -113,6 +195,12 @@ static struct Cache *cache_init(int blksize, int assoc, 
> int cachesize)
>      cache->set_mask = ((cache->num_sets - 1) << 
> (pow_of_two(cache->blksize)));
>      cache->tag_mask = ~(cache->set_mask | cache->blk_mask);
>  
> +    if (lru) {
> +        lru_priorities_init(cache);
> +    } else if (fifo) {
> +        fifo_init(cache);
> +    }
> +
>      return cache;
>  }
>  
> @@ -131,12 +219,20 @@ static int get_invalid_block(struct Cache *cache, 
> uint64_t set)
>      return -1;
>  }
>  
> -static int get_replaced_block(struct Cache *cache)
> +static int get_replaced_block(struct Cache *cache, int set)
>  {
> -    return g_rand_int_range(rng, 0, cache->assoc);
> +    if (rnd) {
> +        return g_rand_int_range(rng, 0, cache->assoc);
> +    } else if (lru) {
> +        return lru_get_lru_block(cache, set);
> +    } else if (fifo) {
> +        return fifo_get_first_in_block(cache, set);
> +    }
> +
> +    g_assert_not_reached();
>  }
>  
> -static bool in_cache(struct Cache *cache, uint64_t addr)
> +static int in_cache(struct Cache *cache, uint64_t addr)
>  {
>      int i;
>      uint64_t tag, set;
> @@ -147,29 +243,39 @@ static bool in_cache(struct 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;
>  }
>  
>  static enum AccessResult access_cache(struct Cache *cache, uint64_t addr)
>  {
>      uint64_t tag, set;
> -    int replaced_blk;
> -
> -    if (in_cache(cache, addr)) {
> -        return HIT;
> -    }
> +    int hit_blk, replaced_blk;
>  
>      tag = extract_tag(cache, addr);
>      set = extract_set(cache, addr);
> +    hit_blk = in_cache(cache, addr);
> +
> +    if (hit_blk != -1) {
> +        if (lru) {
> +            lru_update_on_hit(cache, set, hit_blk);
> +        }
> +        return HIT;
> +    }
>  
>      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 (lru) {
> +        lru_update_on_miss(cache, set, replaced_blk);
> +    } else if (fifo) {
> +        fifo_update_on_miss(cache, set, replaced_blk);
>      }

I wonder if just having a update_hit and update_miss function pointer
would keep things cleaner?

  if (update_hit) {
       update_hit(cache, set, hit, block)
  }

etc...

>  
>      cache->sets[set].blocks[replaced_blk].tag = tag;
> @@ -307,6 +413,11 @@ static void free_cache(struct Cache *cache)
>  {
>      for (int i = 0; i < cache->num_sets; i++) {
>          g_free(cache->sets[i].blocks);
> +        if (lru) {
> +            g_free(cache->sets[i].priorities);

Hmm I've obviously missed something about how priorities are meant to be sued.

> +        } else if (fifo) {
> +            g_queue_free(cache->sets[i].evict_queue);
> +        }
>      }
>  
>      g_free(cache->sets);
> @@ -403,8 +514,6 @@ int qemu_plugin_install(qemu_plugin_id_t id, const 
> qemu_info_t *info,
>      iblksize = 64;
>      icachesize = iblksize * iassoc * 32;
>  
> -    rng = g_rand_new();
> -
>      for (i = 0; i < argc; i++) {
>          char *opt = argv[i];
>          if (g_str_has_prefix(opt, "I=")) {
> @@ -433,6 +542,22 @@ int qemu_plugin_install(qemu_plugin_id_t id, const 
> qemu_info_t *info,
>              if (!tracefile) {
>                  fprintf(stderr, "could not open: %s for writing\n", 
> file_name);
>              }
> +        } else if (g_str_has_prefix(opt, "evict=")) {
> +            if (lru || rnd || fifo) {
> +                fprintf(stderr, "eviction policy specified more than 
> once\n");
> +                return -1;
> +            }

This is one argument for the separate bools although generally QEMU
operates on the basis of last argument wins ;-) 

> +            gchar *policy = opt + 6;
> +            if (g_strcmp0(policy, "rand") == 0) {
> +                rnd = true;
> +            } else if (g_strcmp0(policy, "lru") == 0) {
> +                lru = true;
> +            } else if (g_strcmp0(policy, "fifo") == 0) {
> +                fifo = true;
> +            } else {
> +                fprintf(stderr, "invalid eviction policy: %s\n", opt);
> +                return -1;
> +            }
>          } else {
>              fprintf(stderr, "option parsing failed: %s\n", opt);
>              return -1;
> @@ -449,6 +574,14 @@ int qemu_plugin_install(qemu_plugin_id_t id, const 
> qemu_info_t *info,
>          return -1;
>      }
>  
> +    if (!rnd && !lru && !fifo) {
> +        lru = true;
> +    }
> +
> +    if (rnd) {
> +        rng = g_rand_new();
> +    }
> +
>      dcache = cache_init(dblksize, dassoc, dcachesize);
>      icache = cache_init(iblksize, iassoc, icachesize);


-- 
Alex Bennée