From a1a820d3c98a7f2bdb5547ebede3b2f281e5e6e2 Mon Sep 17 00:00:00 2001
From: Jonathan McDowell <noodles-github@earth.li>
Date: Mon, 11 Apr 2022 05:22:05 +0100
Subject: [PATCH] Avoid heap allocation in GetRequestablePieces (#734)

The calculation of whether we should ignore a piece in
GetRequestablePieces ends up doing an allocation for every piece, when
all we really need to do is query the index in the torrent. Provide an
IgnorePiece function instead, which avoids the need for a temporary
allocation.

Observed to cut out 40% of object allocations in some workloads (large
download, lots of seeds).
---
 request-strategy-impls.go   | 11 +++++++++--
 request-strategy/order.go   |  3 +--
 request-strategy/torrent.go |  2 +-
 3 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/request-strategy-impls.go b/request-strategy-impls.go
index f4c12646..bf3e4083 100644
--- a/request-strategy-impls.go
+++ b/request-strategy-impls.go
@@ -44,8 +44,15 @@ type requestStrategyTorrent struct {
 	t *Torrent
 }
 
-func (r requestStrategyTorrent) Piece(i int) request_strategy.Piece {
-	return requestStrategyPiece{r.t, i}
+func (r requestStrategyTorrent) IgnorePiece(i int) bool {
+	if r.t.ignorePieceForRequests(i) {
+		return true
+	}
+	if r.t.pieceNumPendingChunks(i) == 0 {
+		return true
+	}
+
+	return false
 }
 
 func (r requestStrategyTorrent) ChunksPerPiece() uint32 {
diff --git a/request-strategy/order.go b/request-strategy/order.go
index 130c698b..09f1ff0c 100644
--- a/request-strategy/order.go
+++ b/request-strategy/order.go
@@ -54,7 +54,6 @@ func GetRequestablePieces(input Input, pro *PieceRequestOrder, f func(ih metainf
 	pro.tree.Scan(func(_i pieceRequestOrderItem) bool {
 		ih := _i.key.InfoHash
 		var t Torrent = input.Torrent(ih)
-		var piece Piece = t.Piece(_i.key.Index)
 		pieceLength := t.PieceLength()
 		if storageLeft != nil {
 			if *storageLeft < pieceLength {
@@ -62,7 +61,7 @@ func GetRequestablePieces(input Input, pro *PieceRequestOrder, f func(ih metainf
 			}
 			*storageLeft -= pieceLength
 		}
-		if !piece.Request() || piece.NumPendingChunks() == 0 {
+		if t.IgnorePiece(_i.key.Index) {
 			// TODO: Clarify exactly what is verified. Stuff that's being hashed should be
 			// considered unverified and hold up further requests.
 			return true
diff --git a/request-strategy/torrent.go b/request-strategy/torrent.go
index 51fc1a6c..2460f2e4 100644
--- a/request-strategy/torrent.go
+++ b/request-strategy/torrent.go
@@ -1,7 +1,7 @@
 package request_strategy
 
 type Torrent interface {
-	Piece(int) Piece
+	IgnorePiece(int) bool
 	ChunksPerPiece() uint32
 	PieceLength() int64
 }
-- 
2.48.1