prefetch: limit the amount of prefetches we have in flight to 2

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>

lib/libnfs.c

@@ -2103,8 +2103,6 @@ static void nfs_open_cb(struct rpc_context *rpc, int status, void *command_data,
 	nfsfh->fh = data->fh;
 	data->fh.data.data_val = NULL;
 
-	nfs_set_streaming_mode(nfsfh, 200 * NFS_STREAM_BUF_SIZE);
-
 	data->cb(0, nfs, nfsfh, data->private_data);
 	free_nfs_cb_data(data);
 }
@@ -2374,12 +2372,24 @@ static void nfs_stream_cb(struct rpc_context *rpc, int status, void *command_dat
 
 static void prefetch_streaming_blocks(struct nfs_context *nfs, struct nfsfh *nfsfh, uint64_t next_offset, int num_blocks)
 {
-	int i;
+	int i, num_pending = 0;
 
 	for (i = 0; i < nfsfh->sr->num_blocks && num_blocks; i++) {
 		struct stream_cb_data *stream_data;
 		READ3args args;
 
+		/*
+		 * BSS_PENDING means we have a request for prefetch in flight.
+		 * We don't want an unlimited amount of requests in flight
+		 * since it can cause wild latency spikes while initially
+		 * filling the prefetch buffer.
+		 */
+		if (nfsfh->sr->blocks[i].state == BSS_PENDING) {
+			num_pending++;
+		}
+		if (num_pending >= num_blocks) {
+			continue;
+		}
 		if (nfsfh->sr->blocks[i].state != BSS_UNUSED) {
 			continue;
 		}

nfs-stream.c

@@ -18,6 +18,11 @@ int main(int argc, char *argv[])
 	struct timeval t1, t2;
 	uint64_t delta, tpc;
 
+	if (argc != 3) {
+		fprintf(stderr, "Usage: nfs-stream <nfs-url> <bytes-per-second>\n");
+		exit(1);
+	}
+
 	nfs = nfs_init_context();
 	url = nfs_parse_url_full(nfs, argv[1]);
 	if (!url) {
@@ -36,6 +41,8 @@ int main(int argc, char *argv[])
 			nfs_get_error(nfs));
 		exit(1);
 	}
+	nfs_set_streaming_mode(nfsfh, 5 * 1024 * 1024);
+
 	if (nfs_fstat64(nfs, nfsfh, &st)) {
 		fprintf(stderr, "Failed to stat file %s: %s\n",
 			url->file,