use a LRU eviction policy for the delta base cache

This provides a smoother degradation in performance when the cache
gets trashed due to the delta_base_cache_limit being reached.  Limited
testing with really small delta_base_cache_limit values appears to confirm
this.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff --git a/sha1_file.c b/sha1_file.c
index 8a19d7ee29c24b4f3df229b7cffc186d85ae573a..9fe2bd69a1edbaf1103de79d5e9d857939470d4f 100644 (file)
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -1355,11 +1355,18 @@ static void *unpack_compressed_entry(struct packed_git *p,
 #define MAX_DELTA_CACHE (256)
 
 static size_t delta_base_cached;
+
+static struct delta_base_cache_lru_list {
+       struct delta_base_cache_lru_list *prev;
+       struct delta_base_cache_lru_list *next;
+} delta_base_cache_lru = { &delta_base_cache_lru, &delta_base_cache_lru };
+
 static struct delta_base_cache_entry {
+       struct delta_base_cache_lru_list lru;
+       void *data;
        struct packed_git *p;
        off_t base_offset;
        unsigned long size;
-       void *data;
        enum object_type type;
 } delta_base_cache[MAX_DELTA_CACHE];
 
@@ -1387,6 +1394,8 @@ static void *cache_or_unpack_entry(struct packed_git *p, off_t base_offset,
 found_cache_entry:
        if (!keep_cache) {
                ent->data = NULL;
+               ent->lru.next->prev = ent->lru.prev;
+               ent->lru.prev->next = ent->lru.next;
                delta_base_cached -= ent->size;
        }
        else {
@@ -1404,6 +1413,8 @@ static inline void release_delta_base_cache(struct delta_base_cache_entry *ent)
        if (ent->data) {
                free(ent->data);
                ent->data = NULL;
+               ent->lru.next->prev = ent->lru.prev;
+               ent->lru.prev->next = ent->lru.next;
                delta_base_cached -= ent->size;
        }
 }
@@ -1411,26 +1422,38 @@ static inline void release_delta_base_cache(struct delta_base_cache_entry *ent)
 static void add_delta_base_cache(struct packed_git *p, off_t base_offset,
        void *base, unsigned long base_size, enum object_type type)
 {
-       unsigned long i, hash = pack_entry_hash(p, base_offset);
+       unsigned long hash = pack_entry_hash(p, base_offset);
        struct delta_base_cache_entry *ent = delta_base_cache + hash;
+       struct delta_base_cache_lru_list *lru;
 
        release_delta_base_cache(ent);
        delta_base_cached += base_size;
-       for (i = 0; delta_base_cached > delta_base_cache_limit
-                   && i < MAX_DELTA_CACHE ; i++) {
-               struct delta_base_cache_entry *f = delta_base_cache + i;
+
+       for (lru = delta_base_cache_lru.next;
+            delta_base_cached > delta_base_cache_limit
+            && lru != &delta_base_cache_lru;
+            lru = lru->next) {
+               struct delta_base_cache_entry *f = (void *)lru;
                if (f->type == OBJ_BLOB)
                        release_delta_base_cache(f);
        }
-       for (i = 0; delta_base_cached > delta_base_cache_limit
-                   && i < MAX_DELTA_CACHE ; i++)
-               release_delta_base_cache(delta_base_cache + i);
+       for (lru = delta_base_cache_lru.next;
+            delta_base_cached > delta_base_cache_limit
+            && lru != &delta_base_cache_lru;
+            lru = lru->next) {
+               struct delta_base_cache_entry *f = (void *)lru;
+               release_delta_base_cache(f);
+       }
 
        ent->p = p;
        ent->base_offset = base_offset;
        ent->type = type;
        ent->data = base;
        ent->size = base_size;
+       ent->lru.next = &delta_base_cache_lru;
+       ent->lru.prev = delta_base_cache_lru.prev;
+       delta_base_cache_lru.prev->next = &ent->lru;
+       delta_base_cache_lru.prev = &ent->lru;
 }
 
 static void *unpack_delta_entry(struct packed_git *p,