[refactor-put] send several blocks in parallel to storage nodes

src/api/s3/put.rs

@@ -3,6 +3,7 @@ use std::sync::Arc;
 
 use base64::prelude::*;
 use futures::prelude::*;
+use futures::stream::FuturesOrdered;
 use futures::try_join;
 use md5::{digest::generic_array::*, Digest as Md5Digest, Md5};
 use sha2::Sha256;
@@ -37,6 +38,8 @@ use crate::helpers::*;
 use crate::s3::api_server::{ReqBody, ResBody};
 use crate::s3::error::*;
 
+const PUT_BLOCKS_MAX_PARALLEL: usize = 3;
+
 pub async fn handle_put(
 	garage: Arc<Garage>,
 	req: Request<ReqBody>,
@@ -376,12 +379,52 @@ pub(crate) async fn read_and_put_blocks<S: Stream<Item = Result<Bytes, Error>> +
 	};
 
 	let put_blocks = async {
+		// Structure for handling several concurrent writes to storage nodes
+		let mut write_futs = FuturesOrdered::new();
 		let mut written_bytes = 0u64;
-		while let Some(next) = block_rx3.recv().await {
-			let (block, hash) = next?;
+		loop {
+			// Simultaneously write blocks to storage nodes & await for next block to be written
+			let currently_running = write_futs.len();
+			let write_futs_next = async {
+				if write_futs.is_empty() {
+					futures::future::pending().await
+				} else {
+					write_futs.next().await.unwrap()
+				}
+			};
+			let recv_next = async {
+				// If more than a maximum number of writes are in progress, don't add more for now
+				if currently_running >= PUT_BLOCKS_MAX_PARALLEL {
+					futures::future::pending().await
+				} else {
+					block_rx3.recv().await
+				}
+			};
+			let (block, hash) = tokio::select! {
+				result = write_futs_next => {
+					result?;
+					continue;
+				},
+				recv = recv_next => match recv {
+					Some(next) => next?,
+					None => break,
+				},
+			};
+
+			// For next block to be written: count its size and spawn future to write it
 			let offset = written_bytes;
 			written_bytes += block.len() as u64;
-			put_block_and_meta(garage, version, part_number, offset, hash, block).await?;
+			write_futs.push_back(put_block_and_meta(
+				garage,
+				version,
+				part_number,
+				offset,
+				hash,
+				block,
+			));
+		}
+		while let Some(res) = write_futs.next().await {
+			res?;
 		}
 		Ok::<_, Error>(written_bytes)
 	};