perf: avoid extra heap alloc in block cache (#5522)

libtransmission/cache.cc

@@ -73,7 +73,8 @@ std::pair<Cache::CIter, Cache::CIter> Cache::find_contiguous(CIter const begin,
 int Cache::write_contiguous(CIter const begin, CIter const end) const
 {
     // The most common case without an extra data copy.
-    auto const* towrite = begin->buf.get();
+    auto const* out = std::data(*begin->buf);
+    auto outlen = std::size(*begin->buf);
 
     // Contiguous area to join more than one block, if any.
     auto buf = std::vector<uint8_t>{};
@@ -94,7 +95,8 @@ int Cache::write_contiguous(CIter const begin, CIter const end) const
             buf.insert(std::end(buf), std::begin(*iter->buf), std::end(*iter->buf));
         }
         TR_ASSERT(std::size(buf) == buflen);
-        towrite = &buf;
+        out = std::data(buf);
+        outlen = std::size(buf);
     }
 
     // save it
@@ -107,13 +109,13 @@ int Cache::write_contiguous(CIter const begin, CIter const end) const
 
     auto const loc = tor->block_loc(block);
 
-    if (auto const err = tr_ioWrite(tor, loc, std::size(*towrite), std::data(*towrite)); err != 0)
+    if (auto const err = tr_ioWrite(tor, loc, outlen, out); err != 0)
     {
         return err;
     }
 
     ++disk_writes_;
-    disk_write_bytes_ += std::size(*towrite);
+    disk_write_bytes_ += outlen;
     return {};
 }
 
@@ -141,7 +143,7 @@ Cache::Cache(tr_torrents& torrents, int64_t max_bytes)
 
 // ---
 
-int Cache::write_block(tr_torrent_id_t tor_id, tr_block_index_t block, std::unique_ptr<std::vector<uint8_t>> writeme)
+int Cache::write_block(tr_torrent_id_t tor_id, tr_block_index_t block, std::unique_ptr<BlockData> writeme)
 {
     auto const key = Key{ tor_id, block };
     auto iter = std::lower_bound(std::begin(blocks_), std::end(blocks_), key, CompareCacheBlockByKey{});

libtransmission/cache.h

@@ -26,6 +26,44 @@ struct tr_torrent;
 class Cache
 {
 public:
+    class BlockData
+    {
+    public:
+        BlockData(size_t size)
+            : size_{ size }
+        {
+        }
+
+        [[nodiscard]] constexpr auto size() const noexcept
+        {
+            return size_;
+        }
+
+        [[nodiscard]] constexpr auto* data() noexcept
+        {
+            return std::data(data_);
+        }
+
+        [[nodiscard]] constexpr const auto* data() const noexcept
+        {
+            return std::data(data_);
+        }
+
+        [[nodiscard]] constexpr auto* begin() const noexcept
+        {
+            return data();
+        }
+
+        [[nodiscard]] constexpr auto* end() const noexcept
+        {
+            return begin() + size();
+        }
+
+    private:
+        std::array<uint8_t, tr_block_info::BlockSize> data_;
+        size_t const size_;
+    };
+
     Cache(tr_torrents& torrents, int64_t max_bytes);
 
     int set_limit(int64_t new_limit);
@@ -36,7 +74,7 @@ public:
     }
 
     // @return any error code from cacheTrim()
-    int write_block(tr_torrent_id_t tor, tr_block_index_t block, std::unique_ptr<std::vector<uint8_t>> writeme);
+    int write_block(tr_torrent_id_t tor, tr_block_index_t block, std::unique_ptr<BlockData> writeme);
 
     int read_block(tr_torrent* torrent, tr_block_info::Location const& loc, uint32_t len, uint8_t* setme);
     int prefetch_block(tr_torrent* torrent, tr_block_info::Location const& loc, uint32_t len);
@@ -49,7 +87,7 @@ private:
     struct CacheBlock
     {
         Key key;
-        std::unique_ptr<std::vector<uint8_t>> buf;
+        std::unique_ptr<BlockData> buf;
         time_t time_added = {};
     };
 

libtransmission/peer-msgs.cc

@@ -224,7 +224,7 @@ struct tr_incoming
     struct incoming_piece_data
     {
         explicit incoming_piece_data(uint32_t block_size)
-            : buf{ std::make_unique<std::vector<uint8_t>>(block_size) }
+            : buf{ std::make_unique<Cache::BlockData>(block_size) }
             , block_size_{ block_size }
         {
         }
@@ -249,7 +249,7 @@ struct tr_incoming
             return have_.count() >= block_size_;
         }
 
-        std::unique_ptr<std::vector<uint8_t>> buf;
+        std::unique_ptr<Cache::BlockData> buf;
 
     private:
         std::bitset<tr_block_info::BlockSize> have_;
@@ -1347,7 +1347,7 @@ void peerMadeRequest(tr_peerMsgsImpl* msgs, struct peer_request const* req)
     }
 }
 
-int clientGotBlock(tr_peerMsgsImpl* msgs, std::unique_ptr<std::vector<uint8_t>> block_data, tr_block_index_t block);
+int clientGotBlock(tr_peerMsgsImpl* msgs, std::unique_ptr<Cache::BlockData> block_data, tr_block_index_t block);
 
 ReadResult read_piece_data(tr_peerMsgsImpl* msgs, PeerMessageReader& payload)
 {
@@ -1645,7 +1645,7 @@ ReadResult process_peer_message(tr_peerMsgsImpl* msgs, uint8_t id, PeerMessageRe
 }
 
 /* returns 0 on success, or an errno on failure */
-int clientGotBlock(tr_peerMsgsImpl* msgs, std::unique_ptr<std::vector<uint8_t>> block_data, tr_block_index_t const block)
+int clientGotBlock(tr_peerMsgsImpl* msgs, std::unique_ptr<Cache::BlockData> block_data, tr_block_index_t const block)
 {
     TR_ASSERT(msgs != nullptr);
 

libtransmission/webseed.cc

@@ -338,7 +338,7 @@ public:
         tr_session* session,
         tr_torrent_id_t tor_id,
         tr_block_index_t block,
-        std::unique_ptr<std::vector<uint8_t>>& data,
+        std::unique_ptr<Cache::BlockData> data,
         tr_webseed* webseed)
         : session_{ session }
         , tor_id_{ tor_id }
@@ -363,7 +363,7 @@ private:
     tr_session* const session_;
     tr_torrent_id_t const tor_id_;
     tr_block_index_t const block_;
-    std::unique_ptr<std::vector<uint8_t>> data_;
+    std::unique_ptr<Cache::BlockData> data_;
     tr_webseed* const webseed_;
 };
 
@@ -394,10 +394,9 @@ void useFetchedBlocks(tr_webseed_task* task)
         }
         else
         {
-            auto block_buf = std::make_unique<std::vector<uint8_t>>();
-            block_buf->resize(block_size);
+            auto block_buf = std::make_unique<Cache::BlockData>(block_size);
             evbuffer_remove(task->content(), std::data(*block_buf), std::size(*block_buf));
-            auto* const data = new write_block_data{ session, tor->id(), task->loc.block, block_buf, webseed };
+            auto* const data = new write_block_data{ session, tor->id(), task->loc.block, std::move(block_buf), webseed };
             session->runInSessionThread(&write_block_data::write_block_func, data);
         }
 

tests/libtransmission/move-test.cc

@@ -76,7 +76,7 @@ TEST_P(IncompleteDirTest, incompleteDir)
         tr_torrent* tor = {};
         tr_block_index_t block = {};
         tr_piece_index_t pieceIndex = {};
-        std::unique_ptr<std::vector<uint8_t>> buf = {};
+        std::unique_ptr<Cache::BlockData> buf = {};
         bool done = {};
     };
 
@@ -97,7 +97,8 @@ TEST_P(IncompleteDirTest, incompleteDir)
 
         for (tr_block_index_t block_index = begin; block_index < end; ++block_index)
         {
-            data.buf = std::make_unique<std::vector<uint8_t>>(tr_block_info::BlockSize, '\0');
+            data.buf = std::make_unique<Cache::BlockData>(tr_block_info::BlockSize);
+            std::fill_n(std::data(*data.buf), tr_block_info::BlockSize, '\0');
             data.block = block_index;
             data.done = false;
             session_->runInSessionThread(test_incomplete_dir_threadfunc, &data);