From 67774d660d09748d8b2deb9d82bc7769216535b2 Mon Sep 17 00:00:00 2001
From: arvidn <arvid@cs.umu.se>
Date: Sun, 6 Mar 2016 16:31:18 -0500
Subject: [PATCH] always keep at least 2 async hash jobs outstanding when
 rechecking torrents (to keep the bandwidth delay product full)

---
 src/disk_io_thread.cpp | 12 ++++++------
 src/torrent.cpp        |  5 ++++-
 2 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/src/disk_io_thread.cpp b/src/disk_io_thread.cpp
index 57ac499c0..9845a67e0 100644
--- a/src/disk_io_thread.cpp
+++ b/src/disk_io_thread.cpp
@@ -573,7 +573,7 @@ namespace libtorrent
 
 		int piece_size = pe->storage->files()->piece_size(pe->piece);
 		TORRENT_PIECE_ASSERT(piece_size > 0, pe);
-		
+
 		int iov_len = 0;
 		// the blocks we're flushing
 		int num_flushing = 0;
@@ -2839,17 +2839,17 @@ namespace libtorrent
 		if (no_pieces == false)
 		{
 			int block_size = m_disk_cache.block_size();
-   
+
 			if (storage)
 			{
 				ret->pieces.reserve(storage->num_pieces());
-   
+
 				for (boost::unordered_set<cached_piece_entry*>::iterator i
 					= storage->cached_pieces().begin(), end(storage->cached_pieces().end());
 					i != end; ++i)
 				{
 					TORRENT_ASSERT((*i)->storage.get() == storage);
-   
+
 					if ((*i)->cache_state == cached_piece_entry::read_lru2_ghost
 						|| (*i)->cache_state == cached_piece_entry::read_lru1_ghost)
 						continue;
@@ -2860,10 +2860,10 @@ namespace libtorrent
 			else
 			{
 				ret->pieces.reserve(m_disk_cache.num_pieces());
-   
+
 				std::pair<block_cache::iterator, block_cache::iterator> range
 					= m_disk_cache.all_pieces();
-   
+
 				for (block_cache::iterator i = range.first; i != range.second; ++i)
 				{
 					if (i->cache_state == cached_piece_entry::read_lru2_ghost
diff --git a/src/torrent.cpp b/src/torrent.cpp
index e51edbc66..b9c99794c 100644
--- a/src/torrent.cpp
+++ b/src/torrent.cpp
@@ -2682,7 +2682,10 @@ namespace libtorrent
 
 		int num_outstanding = settings().get_int(settings_pack::checking_mem_usage) * block_size()
 			/ m_torrent_file->piece_length();
-		if (num_outstanding <= 0) num_outstanding = 1;
+		// if we only keep a single read operation in-flight at a time, we suffer
+		// significant performance degradation. Always keep at least two jobs
+		// outstanding
+		if (num_outstanding < 2) num_outstanding = 2;
 
 		// we might already have some outstanding jobs, if we were paused and
 		// resumed quickly, before the outstanding jobs completed