added session setting to control disk buffer pool allocation sizes

docs/manual.rst

@@ -3369,6 +3369,7 @@ session_settings
 		bool auto_upload_slots_rate_based;
 		bool use_parole_mode;
 		int cache_size;
+		int cache_buffer_chunk_size;
 		int cache_expiry;
 		bool use_read_cache;
 		bool disk_io_no_buffer;
@@ -3613,6 +3614,11 @@ send or receive buffer also count against this limit. Send and receive buffers
 will never be denied to be allocated, but they will cause the actual cached blocks
 to be flushed or evicted.
 
+Disk buffers are allocated using a pool allocator, the number of blocks that
+are allocated at a time when the pool needs to grow can be specified in
+``cache_buffer_chunk_size``. This defaults to 16 blocks. Lower numbers
+saves memory at the expense of more heap allocations. It must be at least 1.
+
 ``cache_expiry`` is the number of seconds from the last cached write to a piece
 in the write cache, to when it's forcefully flushed to disk. Default is 60 second.
 

include/libtorrent/session_settings.hpp

@@ -124,6 +124,7 @@ namespace libtorrent
 			, auto_upload_slots_rate_based(true)
 			, use_parole_mode(true)
 			, cache_size(1024)
+			, cache_buffer_chunk_size(16)
 			, cache_expiry(60)
 			, use_read_cache(true)
 			, disk_io_write_mode(0)
@@ -383,6 +384,12 @@ namespace libtorrent
 		// default is 512 (= 8 MB)
 		int cache_size;
 
+		// this is the number of disk buffer blocks (16 kiB)
+		// that should be allocated at a time. It must be
+		// at least 1. Lower number saves memory at the expense
+		// of more heap allocations
+		int cache_buffer_chunk_size;
+
 		// the number of seconds a write cache entry sits
 		// idle in the cache before it's forcefully flushed
 		// to disk. Default is 60 seconds.

src/disk_io_thread.cpp

@@ -52,7 +52,7 @@ namespace libtorrent
 		: m_block_size(block_size)
 		, m_in_use(0)
 #ifndef TORRENT_DISABLE_POOL_ALLOCATOR
-		, m_pool(block_size, 16)
+		, m_pool(block_size, m_settings.cache_buffer_chunk_size)
 #endif
 	{
 #if defined TORRENT_DISK_STATS || defined TORRENT_STATS
@@ -95,7 +95,7 @@ namespace libtorrent
 		char* ret = page_aligned_allocator::malloc(m_block_size);
 #else
 		char* ret = (char*)m_pool.ordered_malloc();
-		m_pool.set_next_size(16);
+		m_pool.set_next_size(m_settings.cache_buffer_chunk_size);
 #endif
 		++m_in_use;
 #if TORRENT_USE_MLOCK
@@ -162,7 +162,7 @@ namespace libtorrent
 		char* ret = page_aligned_allocator::malloc(m_block_size * num_blocks);
 #else
 		char* ret = (char*)m_pool.ordered_malloc(num_blocks);
-		m_pool.set_next_size(16);
+		m_pool.set_next_size(m_settings.cache_buffer_chunk_size);
 #endif
 		m_in_use += num_blocks;
 #if TORRENT_USE_MLOCK

src/session.cpp

@@ -148,6 +148,7 @@ namespace libtorrent
 
 		// don't use any disk cache
 		set.cache_size = 0;
+		set.cache_buffer_chunk_size = 1;
 		set.use_read_cache = false;
 
 		set.close_redundant_connections = true;
@@ -190,12 +191,12 @@ namespace libtorrent
 		// use 128 MB of cache
 		set.cache_size = 8192;
 		set.use_read_cache = true;
+		set.cache_buffer_chunk_size = 64;
 
 		set.close_redundant_connections = true;
 
 		set.max_rejects = 10;
 
-		// use less memory when checking pieces
 		set.optimize_hashing_for_speed = true;
 
 		return set;