Revert to fa_cache for node retriever (#203)

src/hnsw/external_index.c

@@ -6,6 +6,7 @@
 #include <access/generic_xlog.h>  // GenericXLog
 #include <assert.h>
 #include <common/relpath.h>
+#include <hnsw/fa_cache.h>
 #include <pg_config.h>       // BLCKSZ
 #include <storage/bufmgr.h>  // Buffer
 #include <utils/hsearch.h>
@@ -583,8 +584,7 @@ void *ldb_wal_index_node_retriever(void *ctxp, int id)
     OffsetNumber    offset, max_offset;
     Buffer          buf = InvalidBuffer;
     bool            idx_page_prelocked = false;
-    void           *cached_node = cache_get_item(&ctx->node_cache, &id);
-
+    void           *cached_node = fa_cache_get(&ctx->fa_cache, id);
     if(cached_node != NULL) {
         return cached_node;
     }
@@ -629,7 +629,7 @@ void *ldb_wal_index_node_retriever(void *ctxp, int id)
                 LockBuffer(buf, BUFFER_LOCK_UNLOCK);
             }
 
-            cache_set_item(&ctx->node_cache, &id, nodepage->node);
+            fa_cache_insert(&ctx->fa_cache, id, nodepage->node);
 
             return nodepage->node;
 #endif
@@ -671,7 +671,7 @@ void *ldb_wal_index_node_retriever_mut(void *ctxp, int id)
     for(offset = FirstOffsetNumber; offset <= max_offset; offset = OffsetNumberNext(offset)) {
         nodepage = (HnswIndexTuple *)PageGetItem(page, PageGetItemId(page, offset));
         if(nodepage->id == (uint32)id) {
-            cache_set_item(&ctx->node_cache, &id, nodepage->node);
+            fa_cache_insert(&ctx->fa_cache, id, nodepage->node);
 
             return nodepage->node;
         }

src/hnsw/external_index.h

@@ -10,6 +10,7 @@
 #include <utils/relcache.h>  // Relation
 
 #include "extra_dirtied.h"
+#include "fa_cache.h"
 #include "hnsw.h"
 #include "htab_cache.h"
 #include "options.h"
@@ -89,7 +90,7 @@ typedef struct
 
     ExtraDirtiedBufs *extra_dirted;
 
-    HTABCache node_cache;
+    FullyAssociativeCache fa_cache;
 
     dlist_head takenbuffers;
 } RetrieverCtx;

src/hnsw/fa_cache.h

@@ -0,0 +1,51 @@
+#ifndef LDB_HNSW_FA_CACHE_H
+#define LDB_HNSW_FA_CACHE_H
+#include <stddef.h>
+
+/* A fixed-size fully associative FIFO cache meant to be embedded
+ * in other data structures for inline caching.
+ * Currently a naive loop is used for lookup, but we could use
+ * intrinsics to speed this up
+ * We could also experiment with better cache replacement policies here
+ * (CLOCK, 2 lists, etc)
+ */
+
+#define FA_CACHE_SIZE 64
+
+typedef struct
+{
+    int   keys[ FA_CACHE_SIZE ];
+    void* values[ FA_CACHE_SIZE ];
+    int   next;
+} FullyAssociativeCache;
+
+// Initalize the cache so all lookups return NULL
+static inline void fa_cache_init(FullyAssociativeCache* cache)
+{
+    // All values are set to NULL with the below
+    // so if key 0 is looked up before key 0 is inserted
+    // the data strucutre will returned the default value for key
+    // which will be NULL
+    MemSet(cache, 0, sizeof(FullyAssociativeCache));
+}
+
+// Insert the key value pair into an already initialized cache
+static inline void fa_cache_insert(FullyAssociativeCache* cache, int key, void* value)
+{
+    cache->keys[ cache->next ] = key;
+    cache->values[ cache->next ] = value;
+    cache->next = (cache->next + 1) % FA_CACHE_SIZE;
+}
+
+// Get the value associated with the key
+static inline void* fa_cache_get(FullyAssociativeCache* cache, int key)
+{
+    for(int i = 0; i < FA_CACHE_SIZE; i++) {
+        if(cache->keys[ i ] == key) {
+            return cache->values[ i ];
+        }
+    }
+    return NULL;
+}
+
+#endif  // LDB_HNSW_FA_CACHE_H

src/hnsw/retriever.c

@@ -20,7 +20,7 @@ RetrieverCtx *ldb_wal_retriever_area_init(Relation index_rel, HnswIndexHeaderPag
     ctx->header_page_under_wal = header_page_under_wal;
     ctx->extra_dirted = extra_dirtied_new();
 
-    ctx->node_cache = cache_create("NodeCache");
+    fa_cache_init(&ctx->fa_cache);
 
     dlist_init(&ctx->takenbuffers);
 
@@ -55,7 +55,6 @@ void ldb_wal_retriever_area_reset(RetrieverCtx *ctx, HnswIndexHeaderPage *header
 void ldb_wal_retriever_area_fini(RetrieverCtx *ctx)
 {
     cache_destroy(&ctx->block_numbers_cache);
-    cache_destroy(&ctx->node_cache);
     dlist_mutable_iter miter;
     dlist_foreach_modify(miter, &ctx->takenbuffers)
     {