Stackless Python in EVE
Kristján Valur Jónsson
[email protected]
CCP Games inc.
EVE
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MMORPG Space game
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Client / server
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Single shard massive server
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120.000 active players, >24.000 concurrent users
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World concurrency record on a shard
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Relies on Stackless Python
The Tranquility cluster
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400 GHz CPU / 200 Gb
RAM
2 Routers (CISCO Alteon)
14 Proxy servers (IBM
Blade)
55 Sol servers (IBM x335)
2 DB servers (clustered,
IBM Brick x445)
FastT600 Fiber, 56 x FC
15k disks, DS4300 +
3*EXP700
Windows 2000, MS SQL
Server
Currently being upgraded
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AMD x64
EVE Architecture
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COM-like basic architecture
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Python tighly integrated at an early stage
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Home-grown wrapping of BLUE objects
blue modules
ExeFile.exe
Netclient.dll
blue.dll
Pythonlib.lib
python modules
autoexec.py
foo.py
Trinity.dll
bar.py
...
...
Stackless Python
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Tasklets
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Threads of execution. Not OS threads
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Lightweight
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No pre-emption
Channels
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Tasklet rendezvous point
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Data passing
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Scheduling
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Synchronization
Stackless?
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No C stack
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Python stack in linked
frame objects
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Tasklet switching by
swapping frame chain
Compromise
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stackless where
possible.
C stack whisked away
if necessary
main()
PyStackless_CallMethod_Main(“MyMain”)
def MyMain(self):
cmodule.Frobnicate(self.Callback)
PyFrobnicate() {
PyObject_CallObject(cb, …); }
def Callback(self):
self.channel.send(42)
Stack switch
Channels
Readers
Writers
Tasklet 2
Tasklet 1
Channel
Channel semantics
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Send on a channel with no receiver blocks tasklet.
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Send on a channel with a (blocked) receiver,
suspends tasklet and runs receiver immediately.
Sender runs again in due course.
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Symmetric wrt. Send and Receive.
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“balance”, can have a queue of readers or writers.
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Conceptually similar to Unix pipes
Channel semantics, cont.
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Scheduling semantics are precise:
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A blocked tasklet is run immediately
Usable as a building block:
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semaphores
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mutex
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critical section
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condition variables
Stackless in EVE
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BLUE foundation: robust, but cumbersome
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RAD
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Stackless Python: Python and so much more
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EVE is inconceivable without Stackless
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Everyone is a programmer
The main loop
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Establish stackless context
int WinMain(...) {
PyObject *myApp = new EveApp();
PyObject *r =
PyStackless_CallMethod_Main(MyApp,
“WinMain”, 0);
return PyInt_AsLong( r );
The main loop cont.
PyObject* EveApp::WinMain(PyObject *self, PyObject
*args) {
PyOS->ExecFile("script:/sys/autoexec.py");
MSG msg;
while(PeekMessage(&msg, 0, 0, 0, PM_REMOVE)){
TranslateMessage(&msg);
DispatchMessage(&msg);
}
for (TickIt i = mTickers.begin(; i !=
mTickers.end(); i++)
i->mCb->OnTick(mTime, (void*)taskname);
}
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Regular Windows message loop
Runs in Stackless context
The “Main Tasklet”
Autoexec.py
import blue
def Startup():
import service
srvMng = service.ServiceManager()
run = ["dataconfig", "godma", “ui", …]
srvMng.Run(run)
#Start up the client in a tasklet!
if CheckDXVersion():
import blue
blue.pyos.CreateTasklet(Startup, (), {})
Tickers
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Tickers are BLUE modules:
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Trinity (the renderer)
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Netclient
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DB (on the server)
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Audio
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PyOS (special python services)
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…
The PyOS tick:
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Runs fresh tasklets
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(sleepers awoken elsewhere)
Tick() {
…
mSynchro->Tick()
PyObject *watchdogResult;
do {
watchdogResult =
PyStackless_RunWatchdog(20000000);
if (!watchdogResult)
PyFlushError("PumpPython::Watchdog");
Py_XDECREF(watchdogresult);
} while (!watchdogResult);
blue.pyos.synchro
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Synchro:
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Provides Thread-like tasklet utilities:
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Sleep(ms)
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Yield()
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BeNice()
blue.pyos.synchro cont.
1. Sleep: A python script makes the call
blue.pyos.Sleep(200)
2. C++ code runs:
1. Main tasklet check
2. sleeper = New Sleeper();
mSleepers.insert(sleeper);
PyObject *r =
PyChannel_Receive(sleeper>mChannel);
3. Another tasklet runs
blue.pyos.synchro, ctd.
4.
5.
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Main tasklet in windows loop enters PyOS::Tick()
mSleepers are examined for all that are due we
do:
mSleepers.remove(sleeper);
PyChannel_Send(sleepers.mChannel,
Py_NONE);
Main tasklet is suspended (but runnable), sleeper
runs.
Points to note:
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A tasklet goes to sleep by calling
PyChannel_Receive() on a channel which has
no pending sender.
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It will sleep there (block) until someone sends
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Typically the main tasklet does this, doing
PyChannel_Send() on a channel with a reader
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Ergo: The main tasklet may not block
Socket Receive
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Use Windows asynchronous file API
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Provide a synchronous python API. A python script
calls Read().
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Tasklet may be blocked for a long time, (many
frames) other tasklets continue running.
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Do this using channels.
Receive, cont.
1.
Python script runs:
foo, bar = socket.Read()
2.
C code executes the request:
Request *r = new Request(this);
WSAReceive(mSocket, …);
mServe->insert( r );
PyChannel_Receive(r->mChannel);
3.
Tasklet is suspended
Receive, cont.
4. Socket server is ticked from main loop
5. For all requests that are marked completed, it
transfers the data to the sleeping tasklets:
PyObject *r =
PyString_FromStringAndSize(req->mData,
req->mDataLen);
PyChannel_Send(req->mChannel, r);
Py_DECREF(data);
delete req;
4. The sleeping tasklet wakes up, main tasklet is
suspended (but runnable)
Receive completed
def TaskletMain():
def ProcessStuff:
Cmodule::Work(PyObject *cb)
EveApp::WinMain()
def Callback():
Netclient::OnTick()
Socket::Receive(PyObject *cb)
PyChannel_Send(mReceiver, data);
d = PyChannel_Receive(mReceiver);
Stack switch
Stack switch
Main Tasklet
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The one running the windows loop
Can be suspended, allowing other tasklets to run
Can be blocked, as long as there is another tasklet to
unblock it (dangerous)
Is responsible for waking up Sleepers, Yielders, IO
tasklets, etc. therefore cannot be one of them
Is flagged as non-blockable
(stackless.get_current().block_trap = True)
Channel magic
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Channels perform the stackless context switch.
If there is a C stack in the call chain, it will magically
swap the stacks.
Your entire C stack (with C and python invocations)
is whisked away and stored, to be replaced with a
new one.
This allows stackless to simulate cooperative multithreading
Co-operative multitasking
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Context is switched only at known points.
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In Stakcless, this is channel.send() and
channel.receive()
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Also synchro.Yield(), synchro.Sleep(), BeNice(),
socket and DB ops, etc.
No unexpected context switches
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Almost no race conditions
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Program like you are single-threaded
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Very few exceptions.
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This extends to C state too!
Tasklets
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Tasklets are cheap
Used liberally to reduce perceived lag
• UI events forked out to tasklets
• A click can have heavy consequences.
• Heavy logic
• DB Access
• Networks access
• special rendering tasks forked out to tasklets.
• controlling an audio track
“tasklet it out”
Use blue.pyos.synchro.BeNice() in large loops
Example: UI Event:
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Main tasklet receives window messages such as
WM_CLICK
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Trinity invokes handler on UI elements or global
handler
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Handler “tasklets out” any action to allow main
thread to continue immediately.
def OnGlobalUp(self, *args):
if not self or self.destroyed:
return
mo = eve.triapp.uilib.mouseOver
if mo in self.children:
uthread.new(mo._OnClick)
class Action(xtriui.QuickDeco):
def _OnClick(self, *args):
pass
That’s all
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For more info:
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http://www.ccpgames.com
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http://www.eve-online.com
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http://www.stackless.com
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[email protected]