Implement dynamic socket buffer sizes

Cowboy will set the socket's buffer size dynamically to
better fit the current workload. When the incoming data
is small, a low buffer size reduces the memory footprint
and improves responsiveness and therefore performance.
When the incoming data is large, such as large HTTP
request bodies, a larger buffer size helps us avoid
doing too many binary appends and related allocations.

Setting a large buffer size for all use cases is
sub-optimal because allocating more than needed
necessarily results in a performance hit (not just
increased memory usage).

By default Cowboy starts with a buffer size of 8192 bytes.
It then doubles or halves the buffer size depending on
the size of the data it receives from the socket. It
stops decreasing at 8192 and increasing at 131072 by
default.

To keep track of the size of the incoming data Cowboy
maintains a moving average. It allows Cowboy to avoid
changing the buffer too often but still react quickly
when necessary. Cowboy will increase the buffer size
when the moving average is above 90% of the current
buffer size, and decrease when the moving average is
below 40% of the current buffer size.

The current buffer size and moving average are
propagated when switching protocols. The dynamic buffer
is implemented in HTTP/1, HTTP/2 and HTTP/1 Websocket.
HTTP/2 Websocket has it disabled because it doesn't
interact directly with the socket; in that case it
is HTTP/2 that has a dynamic buffer.

The dynamic buffer provides a very large performance improvement
in many scenarios, at minimal cost for others. Because it largely
depend on the underlying protocol the improvements are no all equal.
TLS and compression also impact the results.

The improvement when reading a large request body, with the
requests repeated in a fast loop are:

* HTTP: 6x to 20x faster
* HTTPS: 2x to 6x faster
* H2: 4x to 5x faster
* H2C: 20x to 40x faster

I am not sure why H2C's performance was so bad, especially compared
to H2, when using default buffer sizes. Dynamic buffers make H2C a
lot more viable with default settings.

The performance impact on "hello world" type requests is minimal,
it goes from -5% to +5% roughly.

Websocket improvements vary again depending on the protocol, but
also depending on whether compression is enabled:

* HTTP echo: roughly 2x faster
* HTTP send: roughly 4x faster
* H2C echo: roughly 2x faster
* H2C send: 3x to 4x faster

In the echo test we reply back, and Gun doesn't have the dynamic
buffer optimisation, so that probably explains the x2 difference.

With compression however there isn't much improvement. The results
are roughly within -10% to +10% of each other. Zlib compression
seems to be a bottleneck, or at least to modify the performance
profile to such an extent that the size of the buffer does not
matter. This happens to randomly generated binary data as well
so it is probably not caused by the test data.

5 files changed, 194 insertions, 28 deletions

diff --git a/src/cowboy.erl b/src/cowboy.erl
index e5ed831..c685649 100644
--- a/src/cowboy.erl
+++ b/src/cowboy.erl
@@ -51,8 +51,12 @@
 
 start_clear(Ref, TransOpts0, ProtoOpts0) ->
 	TransOpts1 = ranch:normalize_opts(TransOpts0),
-	{TransOpts, ConnectionType} = ensure_connection_type(TransOpts1),
-	ProtoOpts = ProtoOpts0#{connection_type => ConnectionType},
+	{TransOpts2, DynamicBuffer} = ensure_dynamic_buffer(TransOpts1, ProtoOpts0),
+	{TransOpts, ConnectionType} = ensure_connection_type(TransOpts2),
+	ProtoOpts = ProtoOpts0#{
+		connection_type => ConnectionType,
+		dynamic_buffer => DynamicBuffer
+	},
 	ranch:start_listener(Ref, ranch_tcp, TransOpts, cowboy_clear, ProtoOpts).
 
 -spec start_tls(ranch:ref(), ranch:opts(), opts())
@@ -60,12 +64,13 @@ start_clear(Ref, TransOpts0, ProtoOpts0) ->
 
 start_tls(Ref, TransOpts0, ProtoOpts0) ->
 	TransOpts1 = ranch:normalize_opts(TransOpts0),
-	SocketOpts = maps:get(socket_opts, TransOpts1, []),
-	TransOpts2 = TransOpts1#{socket_opts => [
-		{alpn_preferred_protocols, [<<"h2">>, <<"http/1.1">>]}
-	|SocketOpts]},
-	{TransOpts, ConnectionType} = ensure_connection_type(TransOpts2),
-	ProtoOpts = ProtoOpts0#{connection_type => ConnectionType},
+	{TransOpts2, DynamicBuffer} = ensure_dynamic_buffer(TransOpts1, ProtoOpts0),
+	TransOpts3 = ensure_alpn(TransOpts2),
+	{TransOpts, ConnectionType} = ensure_connection_type(TransOpts3),
+	ProtoOpts = ProtoOpts0#{
+		connection_type => ConnectionType,
+		dynamic_buffer => DynamicBuffer
+	},
 	ranch:start_listener(Ref, ranch_ssl, TransOpts, cowboy_tls, ProtoOpts).
 
 %% @todo Experimental function to start a barebone QUIC listener.
@@ -77,6 +82,7 @@ start_tls(Ref, TransOpts0, ProtoOpts0) ->
 -spec start_quic(ranch:ref(), #{socket_opts => [{atom(), _}]}, cowboy_http3:opts())
 	-> {ok, pid()}.
 
+%% @todo Implement dynamic_buffer for HTTP/3 if/when it applies.
 start_quic(Ref, TransOpts, ProtoOpts) ->
 	{ok, _} = application:ensure_all_started(quicer),
 	Parent = self(),
@@ -139,11 +145,32 @@ port_0() ->
 	end,
 	Port.
 
+ensure_alpn(TransOpts) ->
+	SocketOpts = maps:get(socket_opts, TransOpts, []),
+	TransOpts#{socket_opts => [
+		{alpn_preferred_protocols, [<<"h2">>, <<"http/1.1">>]}
+	|SocketOpts]}.
+
 ensure_connection_type(TransOpts=#{connection_type := ConnectionType}) ->
 	{TransOpts, ConnectionType};
 ensure_connection_type(TransOpts) ->
 	{TransOpts#{connection_type => supervisor}, supervisor}.
 
+%% Dynamic buffer was set; accept transport options as-is.
+%% Note that initial 'buffer' size may be lower than dynamic buffer allows.
+ensure_dynamic_buffer(TransOpts, #{dynamic_buffer := DynamicBuffer}) ->
+	{TransOpts, DynamicBuffer};
+%% Dynamic buffer was not set; define default dynamic buffer
+%% only if 'buffer' size was not configured. In that case we
+%% set the 'buffer' size to the lowest value.
+ensure_dynamic_buffer(TransOpts=#{socket_opts := SocketOpts}, _) ->
+	case proplists:get_value(buffer, SocketOpts, undefined) of
+		undefined ->
+			{TransOpts#{socket_opts => [{buffer, 8192}|SocketOpts]}, {8192, 131072}};
+		_ ->
+			{TransOpts, false}
+	end.
+
 -spec stop_listener(ranch:ref()) -> ok | {error, not_found}.
 
 stop_listener(Ref) ->
diff --git a/src/cowboy_dynamic_buffer.hrl b/src/cowboy_dynamic_buffer.hrl
new file mode 100644
index 0000000..cb07aab
--- /dev/null
+++ b/src/cowboy_dynamic_buffer.hrl
@@ -0,0 +1,80 @@
+%% Copyright (c) 2025, Loïc Hoguin <[email protected]>
+%%
+%% Permission to use, copy, modify, and/or distribute this software for any
+%% purpose with or without fee is hereby granted, provided that the above
+%% copyright notice and this permission notice appear in all copies.
+%%
+%% THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+%% WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+%% MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+%% ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+%% WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+%% ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+%% OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+%% These functions are common to cowboy_http, cowboy_http2 and
+%% cowboy_websocket. It requires the options and the state
+%% to use the same field names.
+
+%% Experiments have shown that the size of the 'buffer' can greatly
+%% impact performance: a buffer too small leads to more messages
+%% being handled and typically more binary appends; and a buffer
+%% too large results in inefficient use of memory which in turn
+%% reduces the throughput, presumably because large binary appends
+%% are not as efficient as smaller ones, and because while the
+%% buffer gets allocated only when there is data, the allocated
+%% size remains until the binary is GC and so under-use hurts.
+%%
+%% The performance of a given 'buffer' size will also depend on
+%% how the client is sending data, and on the protocol. For example,
+%% HTTP/1.1 doesn't need a very large 'buffer' size for reading
+%% request headers, but it does need one for reading large request
+%% bodies. At the same time, HTTP/2 performs best reading large
+%% request bodies when the 'buffer' size is about half that of
+%% HTTP/1.1.
+%%
+%% It therefore becomes important to resize the buffer dynamically
+%% depending on what is currently going on. We do this based on
+%% the size of data packets we received from the transport. We
+%% maintain a moving average and when that moving average is
+%% 90% of the current 'buffer' size, we double the 'buffer' size.
+%% When things slow down and the moving average falls below
+%% 40% of the current 'buffer' size, we halve the 'buffer' size.
+%%
+%% To calculate the moving average we do (MovAvg + DataLen) div 2.
+%% This means that the moving average will change very quickly when
+%% DataLen increases or decreases rapidly. That's OK, we want to
+%% be reactive, but also setting the buffer size is a pretty fast
+%% operation. The formula could be changed to the following if it
+%% became a problem: (MovAvg * N + DataLen) div (N + 1).
+%%
+%% Note that this works best when active,N uses low values of N.
+%% We don't want to accumulate too much data because we resize
+%% the buffer.
+
+init_dynamic_buffer_size(#{dynamic_buffer_initial_size := DynamicBuffer}) ->
+	DynamicBuffer;
+init_dynamic_buffer_size(#{dynamic_buffer := {LowDynamicBuffer, _}}) ->
+	LowDynamicBuffer;
+init_dynamic_buffer_size(_) ->
+	false.
+
+maybe_resize_buffer(State=#state{dynamic_buffer_size=false}, _) ->
+	State;
+maybe_resize_buffer(State=#state{transport=Transport, socket=Socket,
+		opts=#{dynamic_buffer := {LowDynamicBuffer, HighDynamicBuffer}},
+		dynamic_buffer_size=BufferSize0, dynamic_buffer_moving_average=MovingAvg0}, Data) ->
+	DataLen = byte_size(Data),
+	MovingAvg = (MovingAvg0 + DataLen) div 2,
+	if
+		BufferSize0 < HighDynamicBuffer andalso MovingAvg > BufferSize0 * 0.9 ->
+			BufferSize = min(BufferSize0 * 2, HighDynamicBuffer),
+			ok = maybe_socket_error(State, Transport:setopts(Socket, [{buffer, BufferSize}])),
+			State#state{dynamic_buffer_moving_average=MovingAvg, dynamic_buffer_size=BufferSize};
+		BufferSize0 > LowDynamicBuffer andalso MovingAvg < BufferSize0 * 0.4 ->
+			BufferSize = max(BufferSize0 div 2, LowDynamicBuffer),
+			ok = maybe_socket_error(State, Transport:setopts(Socket, [{buffer, BufferSize}])),
+			State#state{dynamic_buffer_moving_average=MovingAvg, dynamic_buffer_size=BufferSize};
+		true ->
+			State#state{dynamic_buffer_moving_average=MovingAvg}
+	end.
diff --git a/src/cowboy_http.erl b/src/cowboy_http.erl
index 7c62b13..de268a6 100644
--- a/src/cowboy_http.erl
+++ b/src/cowboy_http.erl
@@ -28,6 +28,9 @@
 	compress_buffering => boolean(),
 	compress_threshold => non_neg_integer(),
 	connection_type => worker | supervisor,
+	dynamic_buffer => false | {pos_integer(), pos_integer()},
+	dynamic_buffer_initial_average => non_neg_integer(),
+	dynamic_buffer_initial_size => pos_integer(),
 	env => cowboy_middleware:env(),
 	http10_keepalive => boolean(),
 	idle_timeout => timeout(),
@@ -137,6 +140,10 @@
 	%% Flow requested for the current stream.
 	flow = infinity :: non_neg_integer() | infinity,
 
+	%% Dynamic buffer moving average and current buffer size.
+	dynamic_buffer_size :: pos_integer() | false,
+	dynamic_buffer_moving_average :: non_neg_integer(),
+
 	%% Identifier for the stream currently being written.
 	%% Note that out_streamid =< in_streamid.
 	out_streamid = 1 :: pos_integer(),
@@ -181,12 +188,16 @@ init(Parent, Ref, Socket, Transport, ProxyHeader, Opts) ->
 		parent=Parent, ref=Ref, socket=Socket,
 		transport=Transport, proxy_header=ProxyHeader, opts=Opts,
 		peer=Peer, sock=Sock, cert=Cert,
+		dynamic_buffer_size=init_dynamic_buffer_size(Opts),
+		dynamic_buffer_moving_average=maps:get(dynamic_buffer_initial_average, Opts, 0),
 		last_streamid=maps:get(max_keepalive, Opts, 1000)},
 	safe_setopts_active(State),
 	loop(set_timeout(State, request_timeout)).
 
+-include("cowboy_dynamic_buffer.hrl").
+
 setopts_active(#state{socket=Socket, transport=Transport, opts=Opts}) ->
-	N = maps:get(active_n, Opts, 100),
+	N = maps:get(active_n, Opts, 1),
 	Transport:setopts(Socket, [{active, N}]).
 
 safe_setopts_active(State) ->
@@ -220,11 +231,13 @@ loop(State=#state{parent=Parent, socket=Socket, transport=Transport, opts=Opts,
 	receive
 		%% Discard data coming in after the last request
 		%% we want to process was received fully.
-		{OK, Socket, _} when OK =:= element(1, Messages), InStreamID > LastStreamID ->
-			loop(State);
+		{OK, Socket, Data} when OK =:= element(1, Messages), InStreamID > LastStreamID ->
+			State1 = maybe_resize_buffer(State, Data),
+			loop(State1);
 		%% Socket messages.
 		{OK, Socket, Data} when OK =:= element(1, Messages) ->
-			parse(<< Buffer/binary, Data/binary >>, State);
+			State1 = maybe_resize_buffer(State, Data),
+			parse(<< Buffer/binary, Data/binary >>, State1);
 		{Closed, Socket} when Closed =:= element(2, Messages) ->
 			terminate(State, {socket_error, closed, 'The socket has been closed.'});
 		{Error, Socket, Reason} when Error =:= element(3, Messages) ->
@@ -885,12 +898,12 @@ is_http2_upgrade(_, _) ->
 
 %% Prior knowledge upgrade, without an HTTP/1.1 request.
 http2_upgrade(State=#state{parent=Parent, ref=Ref, socket=Socket, transport=Transport,
-		proxy_header=ProxyHeader, opts=Opts, peer=Peer, sock=Sock, cert=Cert}, Buffer) ->
+		proxy_header=ProxyHeader, peer=Peer, sock=Sock, cert=Cert}, Buffer) ->
 	case Transport:secure() of
 		false ->
 			_ = cancel_timeout(State),
-			cowboy_http2:init(Parent, Ref, Socket, Transport,
-				ProxyHeader, Opts, Peer, Sock, Cert, Buffer);
+			cowboy_http2:init(Parent, Ref, Socket, Transport, ProxyHeader,
+				opts_for_upgrade(State), Peer, Sock, Cert, Buffer);
 		true ->
 			error_terminate(400, State, {connection_error, protocol_error,
 				'Clients that support HTTP/2 over TLS MUST use ALPN. (RFC7540 3.4)'})
@@ -898,7 +911,7 @@ http2_upgrade(State=#state{parent=Parent, ref=Ref, socket=Socket, transport=Tran
 
 %% Upgrade via an HTTP/1.1 request.
 http2_upgrade(State=#state{parent=Parent, ref=Ref, socket=Socket, transport=Transport,
-		proxy_header=ProxyHeader, opts=Opts, peer=Peer, sock=Sock, cert=Cert},
+		proxy_header=ProxyHeader, peer=Peer, sock=Sock, cert=Cert},
 		Buffer, HTTP2Settings, Req) ->
 	%% @todo
 	%% However if the client sent a body, we need to read the body in full
@@ -907,13 +920,22 @@ http2_upgrade(State=#state{parent=Parent, ref=Ref, socket=Socket, transport=Tran
 	try cow_http_hd:parse_http2_settings(HTTP2Settings) of
 		Settings ->
 			_ = cancel_timeout(State),
-			cowboy_http2:init(Parent, Ref, Socket, Transport,
-				ProxyHeader, Opts, Peer, Sock, Cert, Buffer, Settings, Req)
+			cowboy_http2:init(Parent, Ref, Socket, Transport, ProxyHeader,
+				opts_for_upgrade(State), Peer, Sock, Cert, Buffer, Settings, Req)
 	catch _:_ ->
 		error_terminate(400, State, {connection_error, protocol_error,
 			'The HTTP2-Settings header must contain a base64 SETTINGS payload. (RFC7540 3.2, RFC7540 3.2.1)'})
 	end.
 
+opts_for_upgrade(#state{opts=Opts, dynamic_buffer_size=false}) ->
+	Opts;
+opts_for_upgrade(#state{opts=Opts, dynamic_buffer_size=Size,
+		dynamic_buffer_moving_average=MovingAvg}) ->
+	Opts#{
+		dynamic_buffer_initial_average => MovingAvg,
+		dynamic_buffer_initial_size => Size
+	}.
+
 %% Request body parsing.
 
 parse_body(Buffer, State=#state{in_streamid=StreamID, in_state=
@@ -1210,7 +1232,7 @@ commands(State0=#state{socket=Socket, transport=Transport, streams=Streams, out_
 	commands(State, StreamID, Tail);
 %% Protocol takeover.
 commands(State0=#state{ref=Ref, parent=Parent, socket=Socket, transport=Transport,
-		out_state=OutState, opts=Opts, buffer=Buffer, children=Children}, StreamID,
+		out_state=OutState, buffer=Buffer, children=Children}, StreamID,
 		[{switch_protocol, Headers, Protocol, InitialState}|_Tail]) ->
 	%% @todo If there's streams opened after this one, fail instead of 101.
 	State1 = cancel_timeout(State0),
@@ -1234,7 +1256,8 @@ commands(State0=#state{ref=Ref, parent=Parent, socket=Socket, transport=Transpor
 	%% Turn off the trap_exit process flag
 	%% since this process will no longer be a supervisor.
 	process_flag(trap_exit, false),
-	Protocol:takeover(Parent, Ref, Socket, Transport, Opts, Buffer, InitialState);
+	Protocol:takeover(Parent, Ref, Socket, Transport,
+		opts_for_upgrade(State), Buffer, InitialState);
 %% Set options dynamically.
 commands(State0=#state{overriden_opts=Opts},
 		StreamID, [{set_options, SetOpts}|Tail]) ->
diff --git a/src/cowboy_http2.erl b/src/cowboy_http2.erl
index 0e110cd..a6ffca7 100644
--- a/src/cowboy_http2.erl
+++ b/src/cowboy_http2.erl
@@ -29,6 +29,9 @@
 	connection_type => worker | supervisor,
 	connection_window_margin_size => 0..16#7fffffff,
 	connection_window_update_threshold => 0..16#7fffffff,
+	dynamic_buffer => false | {pos_integer(), pos_integer()},
+	dynamic_buffer_initial_average => non_neg_integer(),
+	dynamic_buffer_initial_size => pos_integer(),
 	enable_connect_protocol => boolean(),
 	env => cowboy_middleware:env(),
 	goaway_initial_timeout => timeout(),
@@ -133,6 +136,10 @@
 	%% Flow requested for all streams.
 	flow = 0 :: non_neg_integer(),
 
+	%% Dynamic buffer moving average and current buffer size.
+	dynamic_buffer_size :: pos_integer() | false,
+	dynamic_buffer_moving_average :: non_neg_integer(),
+
 	%% Currently active HTTP/2 streams. Streams may be initiated either
 	%% by the client or by the server through PUSH_PROMISE frames.
 	streams = #{} :: #{cow_http2:streamid() => #stream{}},
@@ -169,12 +176,15 @@ init(Parent, Ref, Socket, Transport, ProxyHeader, Opts) ->
 	{inet:ip_address(), inet:port_number()}, {inet:ip_address(), inet:port_number()},
 	binary() | undefined, binary()) -> ok.
 init(Parent, Ref, Socket, Transport, ProxyHeader, Opts, Peer, Sock, Cert, Buffer) ->
+	DynamicBuffer = init_dynamic_buffer_size(Opts),
 	{ok, Preface, HTTP2Machine} = cow_http2_machine:init(server, Opts),
 	%% Send the preface before doing all the init in case we get a socket error.
 	ok = maybe_socket_error(undefined, Transport:send(Socket, Preface)),
 	State = set_idle_timeout(init_rate_limiting(#state{parent=Parent, ref=Ref, socket=Socket,
 		transport=Transport, proxy_header=ProxyHeader,
 		opts=Opts, peer=Peer, sock=Sock, cert=Cert,
+		dynamic_buffer_size=DynamicBuffer,
+		dynamic_buffer_moving_average=maps:get(dynamic_buffer_initial_average, Opts, 0),
 		http2_status=sequence, http2_machine=HTTP2Machine}), 0),
 	safe_setopts_active(State),
 	case Buffer of
@@ -216,12 +226,15 @@ add_period(Time, Period) -> Time + Period.
 	binary() | undefined, binary(), map() | undefined, cowboy_req:req()) -> ok.
 init(Parent, Ref, Socket, Transport, ProxyHeader, Opts, Peer, Sock, Cert, Buffer,
 		_Settings, Req=#{method := Method}) ->
+	DynamicBuffer = init_dynamic_buffer_size(Opts),
 	{ok, Preface, HTTP2Machine0} = cow_http2_machine:init(server, Opts),
 	{ok, StreamID, HTTP2Machine}
 		= cow_http2_machine:init_upgrade_stream(Method, HTTP2Machine0),
 	State0 = #state{parent=Parent, ref=Ref, socket=Socket,
 		transport=Transport, proxy_header=ProxyHeader,
 		opts=Opts, peer=Peer, sock=Sock, cert=Cert,
+		dynamic_buffer_size=DynamicBuffer,
+		dynamic_buffer_moving_average=maps:get(dynamic_buffer_initial_average, Opts, 0),
 		http2_status=upgrade, http2_machine=HTTP2Machine},
 	State1 = headers_frame(State0#state{
 		http2_machine=HTTP2Machine}, StreamID, Req),
@@ -241,11 +254,14 @@ init(Parent, Ref, Socket, Transport, ProxyHeader, Opts, Peer, Sock, Cert, Buffer
 		_ -> parse(State, Buffer)
 	end.
 
+-include("cowboy_dynamic_buffer.hrl").
+
 %% Because HTTP/2 has flow control and Cowboy has other rate limiting
 %% mechanisms implemented, a very large active_n value should be fine,
 %% as long as the stream handlers do their work in a timely manner.
+%% However large active_n values reduce the impact of dynamic_buffer.
 setopts_active(#state{socket=Socket, transport=Transport, opts=Opts}) ->
-	N = maps:get(active_n, Opts, 100),
+	N = maps:get(active_n, Opts, 1),
 	Transport:setopts(Socket, [{active, N}]).
 
 safe_setopts_active(State) ->
@@ -258,7 +274,8 @@ loop(State=#state{parent=Parent, socket=Socket, transport=Transport,
 	receive
 		%% Socket messages.
 		{OK, Socket, Data} when OK =:= element(1, Messages) ->
-			parse(State#state{idle_timeout_num=0}, << Buffer/binary, Data/binary >>);
+			State1 = maybe_resize_buffer(State, Data),
+			parse(State1#state{idle_timeout_num=0}, << Buffer/binary, Data/binary >>);
 		{Closed, Socket} when Closed =:= element(2, Messages) ->
 			Reason = case State#state.http2_status of
 				closing -> {stop, closed, 'The client is going away.'};
diff --git a/src/cowboy_websocket.erl b/src/cowboy_websocket.erl
index 8d3ed08..d2eb99f 100644
--- a/src/cowboy_websocket.erl
+++ b/src/cowboy_websocket.erl
@@ -69,6 +69,9 @@
 	active_n => pos_integer(),
 	compress => boolean(),
 	deflate_opts => cow_ws:deflate_opts(),
+	dynamic_buffer => false | {pos_integer(), pos_integer()},
+	dynamic_buffer_initial_average => non_neg_integer(),
+	dynamic_buffer_initial_size => pos_integer(),
 	idle_timeout => timeout(),
 	max_frame_size => non_neg_integer() | infinity,
 	req_filter => fun((cowboy_req:req()) -> map()),
@@ -97,6 +100,11 @@
 	timeout_num = 0 :: 0..?IDLE_TIMEOUT_TICKS,
 	messages = undefined :: undefined | {atom(), atom(), atom()}
 		| {atom(), atom(), atom(), atom()},
+
+	%% Dynamic buffer moving average and current buffer size.
+	dynamic_buffer_size = false :: pos_integer() | false,
+	dynamic_buffer_moving_average = 0 :: non_neg_integer(),
+
 	hibernate = false :: boolean(),
 	frag_state = undefined :: cow_ws:frag_state(),
 	frag_buffer = <<>> :: binary(),
@@ -270,7 +278,7 @@ websocket_handshake(State, Req=#{ref := Ref, pid := Pid, streamid := StreamID},
 	%% @todo We don't want date and server headers.
 	Headers = cowboy_req:response_headers(#{}, Req),
 	Pid ! {{Pid, StreamID}, {switch_protocol, Headers, ?MODULE, {State, HandlerState}}},
-	takeover(Pid, Ref, {Pid, StreamID}, undefined, undefined, <<>>,
+	takeover(Pid, Ref, {Pid, StreamID}, undefined, #{}, <<>>,
 		{State, HandlerState}).
 
 %% Connection process.
@@ -295,8 +303,8 @@ websocket_handshake(State, Req=#{ref := Ref, pid := Pid, streamid := StreamID},
 -spec takeover(pid(), ranch:ref(), inet:socket() | {pid(), cowboy_stream:streamid()},
 	module() | undefined, any(), binary(),
 	{#state{}, any()}) -> no_return().
-takeover(Parent, Ref, Socket, Transport, _Opts, Buffer,
-		{State0=#state{handler=Handler, req=Req}, HandlerState}) ->
+takeover(Parent, Ref, Socket, Transport, Opts, Buffer,
+		{State0=#state{opts=WsOpts, handler=Handler, req=Req}, HandlerState}) ->
 	case Req of
 		#{version := 'HTTP/3'} -> ok;
 		%% @todo We should have an option to disable this behavior.
@@ -308,7 +316,11 @@ takeover(Parent, Ref, Socket, Transport, _Opts, Buffer,
 	end,
 	State = set_idle_timeout(State0#state{parent=Parent,
 		ref=Ref, socket=Socket, transport=Transport,
-		key=undefined, messages=Messages}, 0),
+		opts=WsOpts#{dynamic_buffer => maps:get(dynamic_buffer, Opts, false)},
+		key=undefined, messages=Messages,
+		%% Dynamic buffer only applies to HTTP/1.1 Websocket.
+		dynamic_buffer_size=init_dynamic_buffer_size(Opts),
+		dynamic_buffer_moving_average=maps:get(dynamic_buffer_initial_average, Opts, 0)}, 0),
 	%% We call parse_header/3 immediately because there might be
 	%% some data in the buffer that was sent along with the handshake.
 	%% While it is not allowed by the protocol to send frames immediately,
@@ -319,6 +331,12 @@ takeover(Parent, Ref, Socket, Transport, _Opts, Buffer,
 		false -> after_init(State, HandlerState, #ps_header{buffer=Buffer})
 	end.
 
+-include("cowboy_dynamic_buffer.hrl").
+
+%% @todo Implement early socket error detection.
+maybe_socket_error(_, _) ->
+	ok.
+
 after_init(State=#state{active=true}, HandlerState, ParseState) ->
 	%% Enable active,N for HTTP/1.1, and auto read_body for HTTP/2.
 	%% We must do this only after calling websocket_init/1 (if any)
@@ -340,7 +358,7 @@ after_init(State, HandlerState, ParseState) ->
 setopts_active(#state{transport=undefined}) ->
 	ok;
 setopts_active(#state{socket=Socket, transport=Transport, opts=Opts}) ->
-	N = maps:get(active_n, Opts, 100),
+	N = maps:get(active_n, Opts, 1),
 	Transport:setopts(Socket, [{active, N}]).
 
 maybe_read_body(#state{socket=Stream={Pid, _}, transport=undefined, active=true}) ->
@@ -414,7 +432,8 @@ loop(State=#state{parent=Parent, socket=Socket, messages=Messages,
 	receive
 		%% Socket messages. (HTTP/1.1)
 		{OK, Socket, Data} when OK =:= element(1, Messages) ->
-			parse(?reset_idle_timeout(State), HandlerState, ParseState, Data);
+			State1 = maybe_resize_buffer(State, Data),
+			parse(?reset_idle_timeout(State1), HandlerState, ParseState, Data);
 		{Closed, Socket} when Closed =:= element(2, Messages) ->
 			terminate(State, HandlerState, {error, closed});
 		{Error, Socket, Reason} when Error =:= element(3, Messages) ->