Gevent what's the point

GeventWhat’s the point? Sean McQuillan sean@bittorrent.com

Bottom LineGevent is fast (libev)Pythonic styleSingle threaded event loop

Gevent is Fast application code geventlibev greenlet kernel

Pythonic Styledef get_db(): return db.query(...) vsdef _db_cb(result): consumer.send(result)def get_db(): x = db.query(...) x.addCallback(_db_cb)

Single Threaded Event LoopSingle threaded

Single Threaded Event LoopSingle threaded wait

Single Threaded Event LoopSingle threaded wait what?

Execution ModelOne OS threadOnly ONE thing happens at a time... Switching constantly

More Execution Modeldef example_function(sock): r = sock.recv(4096) count = len(r) for c in xrange(count): tokenizer.send(c) r += sock.recv(4096)

More Execution Model def get_db(): return db.query(...)

Basic Example

from flask import Flaskapp = Flask(__name__)@app.route(/)def hello_world(): import time; time.sleep(1) return Hello World!if __name__ == __main__: app.run()

from flask import Flaskapp = Flask(__name__)import gevent.monkeygevent.monkey.patch_all()@app.route(/)def hello_world(): import time; time.sleep(1) return Hello World!if __name__ == __main__: app.run()

from flask import Flaskapp = Flask(__name__)@app.route(/)def hello_world(): import time; time.sleep(1) return Hello World!if __name__ == __main__: app.run()

from flask import Flaskapp = Flask(__name__)

from flask import Flaskapp = Flask(__name__)import gevent.monkeygevent.monkey.patch_all()@app.route(/)def hello_world(): import time; time.sleep(1) return Hello World!if __name__ == __main__: app.run()

from flask import Flaskapp = Flask(__name__)import gevent.monkeygevent.monkey.patch_all()

Interlude:Monkey Patching

Monkey PatchingReplace existing librariesNot generally done in python

Monkey Patchingsys.modules[socket] = gevent.socketsys.modules[time] = gevent.sleep...

Part II: Event LoopsLow overheadSingle threadedHandle LOTS of connections

poll, select, epoll, kqueue “Given a list of file descriptors which are ready for read or write.”

Lets dig in application code geventlibev greenlet kernel

Lets dig insocket 1 write callback event socket 1 read callbacksocket 2 write callback loop socket 2 read callback socket 1, socket 2, ... kernel write buffer 1 read buffer 1 write buffer 2 read buffer 2

ExampleContrived event loop exampleSee t wisted event loop for real examplet wisted/interent/selectreactor.py

Examplerlist = []wlist = []callbacks = {}pending = defaultdict(str)def want_to_read(sock, cb): callbacks[sock.fileno] = cb global rlist rlist = list(set(rlist + [sock.fileno]))def want_to_write(sock, data): pending[sock.fileno] += data global wlist wlist = list(set(wlist + [sock.fileno])))

Exampledef event_loop(): while True: reads, writes, _ = select(rlist, wlist, []) for readfd in reads: data = _do_actual_read(readfd) callbacks[readfd](data) for writefd in writes: _do_actual_write(pending[writefd])

Part III: SocketsSocketsWhat magic lies here

patched socket.recvdef recv(self, *args): sock = self._sock while True: try: return sock.recv(*args) except error: ex = sys.exc_info()[1] if ex.args[0] == EBADF: return if (ex.args[0] != EWOULDBLOCK or self.timeout == 0.0 ): raise self._wait(self._read_event)

patched socket.recvdef recv(self, *args): sock = self._sock while True: try: return sock.recv(*args) except error: ex = sys.exc_info()[1] if ex.args[0] == EBADF: return if (ex.args[0] != EWOULDBLOCK or self.timeout == 0.0 ): raise self.want_to_read(sock, self.mygreenthread.switch)

Part IV: Greenlets“green” threads

Part IV: Greenlets“green” threads(that means fake threads)

Green Threads GoodNever switch accidentallyNever spend time blocked on IOHigh performance switching

Green Threads BadNever preemptively interruptNo SMP

Green Thread Execution Python Initial Stack

Green Thread Execution Gevent Initialized Python Initial Stack

Green Thread ExecutionGreenThread Stack Gevent Initialized Python Initial Stack

Green Thread Execution Green ThreadGreen StackThread Stack Gevent Initialized Python Initial Stack

Green Thread Execution Green Green Thread ThreadGreen Stack StackThread Stack Gevent Initialized Python Initial Stack

How does it switch? (x86 assembly) void *ebp, *ebx; unsigned short cw; register int *stackref, stsizediff; __asm__ volatile ("" : : : "esi", "edi"); __asm__ volatile ("fstcw %0" : "=m" (cw)); __asm__ volatile ("movl %%ebp, %0" : "=m" (ebp)); __asm__ volatile ("movl %%ebx, %0" : "=m" (ebx)); __asm__ ("movl %%esp, %0" : "=g" (stackref)); { SLP_SAVE_STATE(stackref, stsizediff); __asm__ volatile ( "addl %0, %%espn" "addl %0, %%ebpn" : : "r" (stsizediff) ); SLP_RESTORE_STATE(); } __asm__ volatile ("movl %0, %%ebx" : : "m" (ebx)); __asm__ volatile ("movl %0, %%ebp" : : "m" (ebp)); __asm__ volatile ("fldcw %0" : : "m" (cw)); __asm__ volatile ("" : : : "esi", "edi"); return 0;

When to use GeventReplace thread-based ser versReplace thread-based clientsLighter than multiprocessing

When not to use gevent When SMP is required High CPU load Latency guarntees Memory star ved systems

TwistedMature, async-baked, implementationsof most protocols DNS Ser ver SSH Server AWS AMQP Web

TornadoHTTPHighly performantLess protocol / library support I’m not an expert

ErlangScales more vertically (dozens of cores)Similar execution modelLess library support

Python ThreadingI’ve never tried to write a “web-scale”server with it, would love to hear yourexperience

Python MultiproccessingGreat SMP usage

GoNever used, benchmarks look good

The End

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Gevent what's the point

by seanmcq on Jun 29, 2012

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