Stackless Python in EVE Kristján Valur Jónsson CCP Games inc.

EVE MMORPG Space game Client / server Single shard massive server active players, > concurrent users World concurrency record on a shard Relies on Stackless Python

The Tranquility cluster 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, DS *EXP700 Windows 2000, MS SQL Server Currently being upgraded AMD x64

EVE Architecture COM-like basic architecture Python tighly integrated at an early stage Home-grown wrapping of BLUE objects

Stackless Python Tasklets Threads of execution. Not OS threads Lightweight No pre-emption Channels Tasklet rendezvous point Data passing Scheduling Synchronization

Stackless? No C stack Python stack in linked frame objects Tasklet switching by swapping frame chain Compromise stackless where possible. C stack whisked away if necessary

Channels

Channel semantics Send on a channel with no receiver blocks tasklet. Send on a channel with a (blocked) receiver, suspends tasklet and runs receiver immediately. Sender runs again in due course. Symmetric wrt. Send and Receive. “balance”, can have a queue of readers or writers. Conceptually similar to Unix pipes

Channel semantics, cont. Scheduling semantics are precise: A blocked tasklet is run immediately Usable as a building block: semaphores mutex critical section condition variables

Stackless in EVE BLUE foundation: robust, but cumbersome RAD Stackless Python: Python and so much more EVE is inconceivable without Stackless Everyone is a programmer

The main loop Establish stackless context int WinMain(...) { PyObject *myApp = new EveApp(); PyObject *r = PyStackless_CallMethod_Main(MyApp, “WinMain”, 0); return PyInt_AsLong( r );

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); } Regular Windows message loop Runs in Stackless context The “Main Tasklet” The main loop cont.

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 Tickers are BLUE modules: Trinity (the renderer) Netclient DB (on the server) Audio PyOS (special python services) …

The PyOS tick: Runs fresh tasklets (sleepers awoken elsewhere) Tick() { … mSynchro->Tick() PyObject *watchdogResult; do { watchdogResult = PyStackless_RunWatchdog( ); if (!watchdogResult) PyFlushError("PumpPython::Watchdog"); Py_XDECREF(watchdogresult); } while (!watchdogResult);

blue.pyos.synchro Synchro: Provides Thread-like tasklet utilities: Sleep(ms) Yield() BeNice()

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 cont.

blue.pyos.synchro, ctd. 4.Main tasklet in windows loop enters PyOS::Tick() 5.mSleepers are examined for all that are due we do: mSleepers.remove(sleeper); PyChannel_Send(sleepers.mChannel, Py_NONE); 6.Main tasklet is suspended (but runnable), sleeper runs.

Points to note: A tasklet goes to sleep by calling PyChannel_Receive() on a channel which has no pending sender. It will sleep there (block) until someone sends Typically the main tasklet does this, doing PyChannel_Send() on a channel with a reader Ergo: The main tasklet may not block

Socket Receive Use Windows asynchronous file API Provide a synchronous python API. A python script calls Read(). Tasklet may be blocked for a long time, (many frames) other tasklets continue running. 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

Main Tasklet 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 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 multi- threading

Co-operative multitasking Context is switched only at known points. In Stakcless, this is channel.send() and channel.receive() Also synchro.Yield(), synchro.Sleep(), BeNice(), socket and DB ops, etc. No unexpected context switches Almost no race conditions Program like you are single-threaded Very few exceptions. This extends to C state too!

Tasklets 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: Main tasklet receives window messages such as WM_CLICK Trinity invokes handler on UI elements or global handler 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

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