Add readahead_distance GUC

pgxn/neon/access_stat.c

@@ -49,7 +49,6 @@ static int max_access_stat_count;
 static double min_seq_access_ratio;
 static int min_seq_access_count;
 
-
 void access_stat_init(void)
 {
 	MemoryContext memctx = AllocSetContextCreate(TopMemoryContext,

pgxn/neon/libpagestore.c

@@ -58,6 +58,7 @@ char	   *neon_auth_token;
 int			n_unflushed_requests = 0;
 int			flush_every_n_requests = 8;
 int			readahead_buffer_size = 128;
+int         readahead_distance = 10;
 
 bool	(*old_redo_read_buffer_filter) (XLogReaderState *record, uint8 block_id) = NULL;
 
@@ -452,6 +453,18 @@ pg_init_libpagestore(void)
 							PGC_USERSET,
 							0,	/* no flags required */
 							NULL, (GucIntAssignHook) &readahead_buffer_resize, NULL);
+	DefineCustomIntVariable("neon.readahead_distance",
+							"Number of read-ahead blocks",
+							NULL,
+							&readahead_distance,
+							10,
+							0,
+							INT_MAX,
+							PGC_USERSET,
+							0,
+							NULL,
+							NULL,
+							NULL);
 
 	relsize_hash_init();
 

pgxn/neon/pagestore_client.h

@@ -157,6 +157,7 @@ extern page_server_api * page_server;
 extern char *page_server_connstring;
 extern int flush_every_n_requests;
 extern int readahead_buffer_size;
+extern int readahead_distance = 10;
 extern bool seqscan_prefetch_enabled;
 extern int seqscan_prefetch_distance;
 extern char *neon_timeline;

pgxn/neon/pagestore_smgr.c

@@ -2006,7 +2006,7 @@ neon_read(SMgrRelation reln, ForkNumber forkNum, BlockNumber blkno,
 
 	/* If it is expected to be sequential access then initiate prefetch of next block */
 	if (is_sequential_access(reln->smgr_rnode.node, forkNum, blkno))
-		neon_prefetch(reln, forkNum, blkno+1);
+		neon_prefetch(reln, forkNum, blkno + readahead_distance);
 
 	/* Try to read from local file cache */
 	if (lfc_read(reln->smgr_rnode.node, forkNum, blkno, buffer))