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- Subject: Coroutines, sockets and multiplexing
- From: Adrian Sietsma <adrian_groups@...>
- Date: Tue, 31 Jan 2006 17:12:13 +1100
There has been a fair bit of discussion lately about async socket io and
In an attempt to clarify the issues (and my thoughts) here is a rough
summary, with comments.
Note : for the purpose of this discussion, real-time (<n ms) response is not
required. This avoids issues such as interrupting the gc., etc.
1. Non-blocking versions of blocking api/os calls.
- Difficult, os-specific, and requires background thread(s), with all the
design issues involved (pooling, thread-per-request, etc). Ignored for now.
2. Non blocking file io.
- Not too bad if supported by the os, but still very os-specific.
3. Event-driven callbacks.
see mike pall's event api documentation
for a more detailed discussion
There are 2 fundamental design choices here
- are the callbacks to share a common Lua universe ?
- is the main processing loop in Lua or c ?
The simplest (and fastest ?) model here would be to have the main loop in
c(++), using os threading and events to run each lua callback in it's own
lua state (vm). We found this model perfect for a specific large commercial
nb. the ability to clone a lua state would be lovely in this case, as we
could load all libraries and perform initialisation of one "master" state,
which is then cloned for each child thread.
If we want the main loop to be in Lua, we need an efficient way for Lua to
sleep between events. We can use select() for socket io, but other events,
signals, etc become os-specific.
A single shared vm also increases granularity (response time) : events can
only be serviced between lua instructions. This will suffice for most
If we want the event handlers to be interruptible, we need to check for
events between lua instructions, and yield to the scheduler as required.
This can be simulated using the debug linehook, but that imposes a fair
amount of overhead - the event test should be in the core lua vm.
The callback handlers otherwise need to be written with multiplexing in
mind, and yield regularly to the scheduler.
4. Multiplexed socket/file io.
- See #2 re file io.
A simple way to handle socket multiplexing is to have a main scheduler based
around select(), and to override the read() and write() functions to yield
back to the scheduler if they would block. This allows re-use of existing
blocking socket code unchanged.
I have built a library which emulates/wraps the luasocket interface, using
this approach; i can thus re-use Diego's http libraries (thanks Diego). This
lib was first developed for a http proxy prototype, and is fine for
light-duty web serving / prototyping. I have tested it with 50 simultaneous
http requests on windows without a problem : it runs into winsock issues
when i try too many requests.
I found it a very nice model for simple multiplexing :
-- sample code ----------------------------
-- a, b are connected sockets
-- this fn is called from a couroutine
while true do
local s, err = a:read("*l") -- yields if insufficient data
if err then return nil, err
if not s then return true end
local ret, err = b:write(s.."\n") -- yields until done
if err then return nil, err
end -- sample ------------------------------
The main drawback with this approach is response time : the scheduler is
only invoked when a coro explicitly yields, or calls one of the yielding io
It is also difficult to fully exploit native async. io (IOCP in windows).
ps. A wonderful side-effect of using coroutines is the error behaviour - you
effectively get a free "try", and errors can be propgated / handled in a
very flexible manner.
pps For high performance web serving, Apache + FastCGI Lua ?
ppps These comments represent my limited problem set; actual mileage may vary.