[prototype] kv: support committing txns in parallel with writes

docs/RFCS/20180324_parallel_commit.md

@@ -1,5 +1,5 @@
 - Feature Name: parallel commit
-- Status: draft
+- Status: in-progress
 - Start Date: 2018-03-24
 - Authors: Tobias Schottdorf, Nathan VanBenschoten
 - RFC PR: #24194

docs/generated/settings/settings.html

@@ -51,6 +51,7 @@
 <tr><td><code>kv.snapshot_recovery.max_rate</code></td><td>byte size</td><td><code>8.0 MiB</code></td><td>the rate limit (bytes/sec) to use for recovery snapshots</td></tr>
 <tr><td><code>kv.transaction.max_intents_bytes</code></td><td>integer</td><td><code>262144</code></td><td>maximum number of bytes used to track write intents in transactions</td></tr>
 <tr><td><code>kv.transaction.max_refresh_spans_bytes</code></td><td>integer</td><td><code>256000</code></td><td>maximum number of bytes used to track refresh spans in serializable transactions</td></tr>
+<tr><td><code>kv.transaction.parallel_commits</code></td><td>boolean</td><td><code>true</code></td><td>if enabled, transactional commits will be parallelized with transactional writes</td></tr>
 <tr><td><code>kv.transaction.write_pipelining_enabled</code></td><td>boolean</td><td><code>true</code></td><td>if enabled, transactional writes are pipelined through Raft consensus</td></tr>
 <tr><td><code>kv.transaction.write_pipelining_max_batch_size</code></td><td>integer</td><td><code>128</code></td><td>if non-zero, defines that maximum size batch that will be pipelined through Raft consensus</td></tr>
 <tr><td><code>kv.transaction.write_pipelining_max_outstanding_size</code></td><td>byte size</td><td><code>256 KiB</code></td><td>maximum number of bytes used to track in-flight pipelined writes before disabling pipelining</td></tr>

pkg/kv/batch.go

@@ -22,6 +22,12 @@ import (
 
 var emptySpan = roachpb.Span{}
 
+const (
+	filterNone int = iota
+	filterEndTxn
+	filterNotEndTxn
+)
+
 // truncate restricts all contained requests to the given key range and returns
 // a new, truncated, BatchRequest. All requests contained in that batch are
 // "truncated" to the given span, and requests which are found to not overlap
@@ -32,9 +38,23 @@ var emptySpan = roachpb.Span{}
 // rs = [a,bb],
 //
 // then truncate(ba,rs) returns a batch (Put[a], Put[b]) and positions [0,2].
-func truncate(ba roachpb.BatchRequest, rs roachpb.RSpan) (roachpb.BatchRequest, []int, error) {
+func truncate(
+	ba roachpb.BatchRequest, rs roachpb.RSpan, filter int,
+) (roachpb.BatchRequest, []int, error) {
 	truncateOne := func(args roachpb.Request) (bool, roachpb.Span, error) {
 		header := args.Header().Span()
+		switch filter {
+		case filterNone:
+		case filterEndTxn:
+			if args.Method() != roachpb.EndTransaction {
+				return false, emptySpan, nil
+			}
+		case filterNotEndTxn:
+			if args.Method() == roachpb.EndTransaction {
+				return false, emptySpan, nil
+			}
+		}
+
 		if !roachpb.IsRange(args) {
 			// This is a point request.
 			if len(header.EndKey) > 0 {