perf: use timer to flush outbuf to avoid taking lock frequently (#8423)

* perf: use timer to flush outbuf to avoid taking lock frequently

* code review: extract to `flush_outbuf_soon()`

* code review: make `flush_outbuf_trigger_` const

libtransmission/peer-io.cc

@@ -32,6 +32,7 @@
 #include "libtransmission/peer-io.h"
 #include "libtransmission/peer-socket.h" // tr_peer_socket, tr_netOpen...
 #include "libtransmission/session.h"
+#include "libtransmission/timer.h"
 #include "libtransmission/tr-assert.h"
 #include "libtransmission/utils.h" // for _()
 
@@ -84,6 +85,7 @@ tr_peerIo::tr_peerIo(
     : bandwidth_{ parent_bandwidth }
     , info_hash_{ info_hash != nullptr ? *info_hash : tr_sha1_digest_t{} }
     , session_{ session }
+    , flush_outbuf_trigger_{ session->timerMaker().create() }
     , client_is_seed_{ client_is_seed }
     , is_incoming_{ is_incoming }
 {
@@ -101,8 +103,18 @@ std::shared_ptr<tr_peerIo> tr_peerIo::create(
     TR_ASSERT(session != nullptr);
     auto lock = session->unique_lock();
 
-    auto io = std::make_shared<tr_peerIo>(session, std::move(socket), parent, info_hash, is_incoming, is_seed);
+    auto const io = std::shared_ptr<tr_peerIo>{
+        new tr_peerIo{ session, std::move(socket), parent, info_hash, is_incoming, is_seed }
+    };
     io->bandwidth().set_peer(io);
+    io->flush_outbuf_trigger_->set_callback(
+        [weak = io->weak_from_this()]
+        {
+            if (auto const ptr = weak.lock())
+            {
+                ptr->try_write(SIZE_MAX);
+            }
+        });
     tr_logAddTraceIo(io, fmt::format("bandwidth is {}; its parent is {}", fmt::ptr(&io->bandwidth()), fmt::ptr(parent)));
     return io;
 }
@@ -573,6 +585,11 @@ size_t tr_peerIo::flush_outgoing_protocol_msgs()
     return flush(tr_direction::Up, byte_count);
 }
 
+void tr_peerIo::flush_outbuf_soon()
+{
+    flush_outbuf_trigger_->start_single_shot(std::chrono::milliseconds::zero());
+}
+
 void tr_peerIo::write_bytes(void const* bytes, size_t n_bytes, bool is_piece_data)
 {
     if (n_bytes == 0U)
@@ -586,14 +603,7 @@ void tr_peerIo::write_bytes(void const* bytes, size_t n_bytes, bool is_piece_dat
     filter_.encrypt(reinterpret_cast<std::byte const*>(bytes), n_bytes, resbuf);
     outbuf_.commit_space(n_bytes);
 
-    session_->queue_session_thread(
-        [ptr = weak_from_this()]()
-        {
-            if (auto io = ptr.lock(); io)
-            {
-                io->try_write(SIZE_MAX);
-            }
-        });
+    flush_outbuf_soon();
 }
 
 // ---

libtransmission/peer-io.h

@@ -34,10 +34,15 @@ struct tr_error;
 struct tr_session;
 struct tr_socket_address;
 
-namespace tr::test
+namespace tr
+{
+class Timer;
+
+namespace test
 {
 class HandshakeTest;
-} // namespace tr::test
+}
+} // namespace tr
 
 enum class ReadState : uint8_t
 {
@@ -63,14 +68,6 @@ class tr_peerIo final : public std::enable_shared_from_this<tr_peerIo>
     using GotError = void (*)(tr_peerIo* io, tr_error const& error, void* user_data);
 
 public:
-    tr_peerIo(
-        tr_session* session,
-        tr_peer_socket&& socket,
-        tr_bandwidth* parent_bandwidth,
-        tr_sha1_digest_t const* info_hash,
-        bool is_incoming,
-        bool client_is_seed);
-
     ~tr_peerIo();
 
     tr_peerIo(tr_peerIo const&) = delete;
@@ -325,6 +322,14 @@ private:
 
     friend class tr::test::HandshakeTest;
 
+    tr_peerIo(
+        tr_session* session,
+        tr_peer_socket&& socket,
+        tr_bandwidth* parent_bandwidth,
+        tr_sha1_digest_t const* info_hash,
+        bool is_incoming,
+        bool client_is_seed);
+
     [[nodiscard]] constexpr auto client_is_seed() const noexcept
     {
         return client_is_seed_;
@@ -345,6 +350,8 @@ private:
 
     void close();
 
+    void flush_outbuf_soon();
+
     static void event_read_cb(evutil_socket_t fd, short /*event*/, void* vio);
     static void event_write_cb(evutil_socket_t fd, short /*event*/, void* vio);
 
@@ -388,6 +395,8 @@ private:
     GotError got_error_ = nullptr;
     void* user_data_ = nullptr;
 
+    std::unique_ptr<tr::Timer> const flush_outbuf_trigger_;
+
     tr::evhelpers::event_unique_ptr event_read_;
     tr::evhelpers::event_unique_ptr event_write_;