Mailing-List: contact dev-help@httpd.apache.org; run by ezmlm
Precedence: bulk
Reply-To: dev@httpd.apache.org
From: Stefan Eissing <stefan.eissing@greenbytes.de>
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: quoted-printable
Subject: async mpms + http2
Message-Id: <703ED97E-4B43-4AC1-94B1-2BB55893DBAC@greenbytes.de>
Date: Tue, 15 Dec 2015 16:44:27 +0100
To: dev@httpd.apache.org
Mime-Version: 1.0 (Mac OS X Mail 9.2 \(3112\))

I just committed a change in mod_http2 that will return idle connections =
back to the mpm, expecting to be invoked again when new data arrives. I =
learned quite something about event (have not looked much at motorz yet) =
and am thinking about improvements. If knowledgeable people could give =
me a hand here, I'd highly appreciate it.

Observation 1: mapping conn_states
  As I understand it, the connection state model, as defined in =
conn_state_e, enables asynchronous processing in two states:
  a. CONN_STATE_CHECK_REQUEST_LINE_READABLE, check read, invoke =
ap_run_process_connection() on data
  b. CONN_STATE_WRITE_COMPLETION, check write, if data is still in out =
filters

  An invocation of ap_run_process_connection() is expected to return, =
usually in WRITE_COMPLETION state. Then any data is attempted to be =
written. When this succeeds, the next state depends on secondary flags =
such as c->aborted, c->keepalive and c->data_in_input_filters.

  HTTP/2 processing has one state where it fits into this model: when no =
more streams are open. This is what is currently implemented in trunk.=20=


  However, there is another state in HTTP/2 processing, where a =
BLOCK_READ is performed: flow control on streaming out responses. =
Basically, the client has a window of n bytes, the server sends n =
response bytes and then needs to wait for the client to increase the =
window again. This would be a good time to resume processing back to the =
mpm, and initially I did so. But that breaks under load.

  Under load, the event mpm feels free to close connections in certain =
states - I think CONN_STATE_CHECK_REQUEST_LINE_READABLE. That is fine =
for a HTTP/1 connection as it always is between requests here. But =
HTTP/2 would also like to use read events, but unloading a child by =
closing such connections is not the right thing to do.

  Is there a way mod_http2 can achieve this or do we need another flag =
in conn_rec/cs?

Observation 2: controlled close
  HTTP/2 would like to send out a last frame when closing a connection =
in a controlled/timeout way. Do we need another hook for that? Something =
like "pre_close_connection"?
=20
Observation 3: timeouts
  a. If admins would like to have another Timeout for HTTP/2 =
connections, they are currently stuck with configuring one Timeout which =
goes to server_rec->timeout, right? For idle connections, clients =
already try to keep it open longer. It would be nice to give admins a =
choice here for different values based on the selected protocol. How to =
best do this?

  b. Handling own timeouts: if I want, during processing the connection, =
a different socket timeout, I can just set it using =
apr_socket_timeout_set()? Shall I restore it back to server->timeout =
before returning?

  c. What would be the best way to wait for a file description *and* =
conditional event simultaneously? mod_http2 has now a sort of pool loop =
with double backup timer for situations where it needs to watch both =
client and backend processes.


Cheers,

  Stefan