From 6298322b1d430f3fa9f23a53a8b416e6d41b2830 Mon Sep 17 00:00:00 2001
From: Arvid Norberg <arvid@libtorrent.org>
Date: Wed, 16 Dec 2009 10:49:15 +0000
Subject: [PATCH] service disk-read requests that hit the cache immediately and
 circumvent the elevator ordering

---
 include/libtorrent/disk_io_thread.hpp |  2 ++
 src/disk_io_thread.cpp                | 47 +++++++++++++++++++++++++++
 test/test_storage.cpp                 | 11 +++++++
 3 files changed, 60 insertions(+)

diff --git a/include/libtorrent/disk_io_thread.hpp b/include/libtorrent/disk_io_thread.hpp
index 69b7d2ed7..7b75b9871 100644
--- a/include/libtorrent/disk_io_thread.hpp
+++ b/include/libtorrent/disk_io_thread.hpp
@@ -328,6 +328,8 @@ namespace libtorrent
 		cache_t::iterator find_cached_piece(
 			cache_t& cache, disk_io_job const& j
 			, mutex::scoped_lock& l);
+		bool disk_io_thread::is_cache_hit(cache_t::iterator p
+			, disk_io_job const& j, mutex::scoped_lock& l);
 		int copy_from_piece(cache_t::iterator p, bool& hit
 			, disk_io_job const& j, mutex::scoped_lock& l);
 
diff --git a/src/disk_io_thread.cpp b/src/disk_io_thread.cpp
index e7b9e5fd0..6b44cb53a 100644
--- a/src/disk_io_thread.cpp
+++ b/src/disk_io_thread.cpp
@@ -1136,6 +1136,30 @@ namespace libtorrent
 		return ret;
 	}
 
+	// this doesn't modify the read cache, it only
+	// checks to see if the given read request can
+	// be fully satisfied from the given cached piece
+	// this is similar to copy_from_piece() but it
+	// doesn't do anything but determining if it's a
+	// cache hit or not
+	bool disk_io_thread::is_cache_hit(cache_t::iterator p
+		, disk_io_job const& j, mutex::scoped_lock& l)
+	{
+		int block = j.offset / m_block_size;
+		int block_offset = j.offset & (m_block_size-1);
+		int size = j.buffer_size;
+		int min_blocks_to_read = block_offset > 0 ? 2 : 1;
+		TORRENT_ASSERT(size <= m_block_size);
+		int start_block = block;
+		// if we have to read more than one block, and
+		// the first block is there, make sure we test
+		// for the second block
+		if (p->blocks[start_block].buf != 0 && min_blocks_to_read > 1)
+			++start_block;
+
+		return p->blocks[start_block].buf != 0;
+	}
+
 	int disk_io_thread::copy_from_piece(cache_t::iterator p, bool& hit
 		, disk_io_job const& j, mutex::scoped_lock& l)
 	{
@@ -1408,6 +1432,29 @@ namespace libtorrent
 							break;
 						}
 
+						// at this point the operation we're looking
+						// at is a read operation. If this read operation
+						// can be fully satisfied by the read cache, handle
+						// it immediately
+						if (m_settings.use_read_cache)
+						{
+							// unfortunately we need to lock the cache
+							// if the cache querying function would be
+							// made asyncronous, this would not be
+							// necessary anymore
+							mutex::scoped_lock l(m_piece_mutex);
+							cache_t::iterator p
+								= find_cached_piece(m_read_pieces, *i, l);
+					
+							// if it's a cache hit, process the job immediately
+							if (p != m_read_pieces.end() && is_cache_hit(p, *i, l))
+							{
+								best_job = i;
+								best_score = 0;
+								break;
+							}
+						}
+
 						// we only need to query for physical offset
 						// for read operations, since those are
 						// the only ones we re-order
diff --git a/test/test_storage.cpp b/test/test_storage.cpp
index 9efe04bfe..017a27735 100644
--- a/test/test_storage.cpp
+++ b/test/test_storage.cpp
@@ -247,6 +247,17 @@ void run_elevator_test()
 		boost::intrusive_ptr<piece_manager> pm(new piece_manager(boost::shared_ptr<void>(), ti, ""
 			, fp, dio, &create_test_storage, storage_mode_sparse));
 
+		// we must disable the read cache in order to
+		// verify that the elevator algorithm works.
+		// since any read cache hit will circumvent
+		// the elevator order
+		session_settings set;
+		set.use_read_cache = false;
+		disk_io_job j;
+		j.buffer = (char*)&set;
+		j.action = disk_io_job::update_settings;
+		dio.add_job(j);
+
 		// test the elevator going up
 		add_job_up(dio, 0, pm);