Async I/O and Python

When you’re working on OpenStack, you’ll probably hear a lot of references to ‘async I/O’ and how eventlet is the library we use for this in OpenStack.

But, well … what exactly is this mysterious ‘asynchronous I/O’ thing?

The first thing to think about is what happens when a process calls a system call like write(). If there’s room in the write buffer, then the data gets copied into kernel space and the system call returns immediately.

But if there isn’t room in the write buffer, what happens then? The default behaviour is that the kernel will put the process to sleep until there is room available. In the case of sockets and pipes, space in the buffer usually becomes available when the other side reads the data you’ve sent.

The trouble with this is that we usually would prefer the process to be doing something useful while waiting for space to become available, rather than just sleeping. Maybe this is an API server and there are new connections waiting to be accepted. How can we process those new connections rather than sleeping?

One answer is to use multiple threads or processes – maybe it doesn’t matter if a single thread or process is blocked on some I/O if you have lots of other threads or processes doing work in parallel.

But, actually, the most common answer is to use non-blocking I/O operations. The idea is that rather than having the kernel put the process to sleep when no space is available in the write buffer, the kernel should just return a “try again later” error. We then using the select() system call to find out when space has become available and the file is writable again.

Below are a number of examples of how to implement a non-blocking write. For each example, you can run a simple socket server on a remote machine to test against:

$> ssh -L 1234:localhost:1234 some.remote.host 'ncat -l 1234 | dd of=/dev/null'

The way this works is that the client connects to port 1234 on the local machine, the connection is forwarded over SSH to port 1234 on some.remote.host where ncat reads the input, writes the output over a pipe to dd which, in turn, writes the output to /dev/null. I use dd to give us some information about how much data was received when the connection closes. Using a distant some.remote.host will help illustrate the blocking behaviour because data clearly can’t be transferred as quickly as the client can copy it into the kernel.

Blocking I/O

To start with, let’s look at the example of using straightforward blocking I/O:

import socket

sock = socket.socket()
sock.connect(('localhost', 1234))
sock.send('foo\n' * 10 * 1024 * 1024)

This is really nice and straightforward, but the point is that this process will spend a tonne of time sleeping while the send() method completes transferring all of the data.

Non-Blocking I/O

In order to avoid this blocking behaviour, we can set the socket to non-blocking and use select() to find out when the socket is writable:

import errno
import select
import socket

sock = socket.socket()
sock.connect(('localhost', 1234))
sock.setblocking(0)

buf = buffer('foo\n' * 10 * 1024 * 1024)
print "starting"
while len(buf):
    try:
        buf = buf[sock.send(buf):]
    except socket.error, e:
        if e.errno != errno.EAGAIN:
            raise e
        print "blocking with", len(buf), "remaining"
        select.select([], [sock], [])
        print "unblocked"
print "finished"

As you can see, when send() returns an EAGAIN error, we call select() and will sleep until the socket is writable. This is a basic example of an event loop. It’s obviously a loop, but the “event” part refers to our waiting on the “socket is writable” event.

This example doesn’t look terribly useful because we’re still spending the same amount of time sleeping but we could in fact be doing useful rather than sleeping in select(). For example, if we had a listening socket, we could also pass it to select() and select() would tell us when a new connection is available. That way we could easily alternate between handling new connections and writing data to our socket.

To prove this “do something useful while we’re waiting” idea, how about we add a little busy loop to the I/O loop:

        if e.errno != errno.EAGAIN:
            raise e

        i = 0
        while i < 5000000:
            i += 1

        print "blocking with", len(buf), "remaining"
        select.select([], [sock], [], 0)
        print "unblocked"

The difference is we’ve passed a timeout of zero to select() – this means select() never actually block – and any time send() would have blocked, we do a bunch of computation in user-space. If we run this using the ‘time’ command you’ll see something like:

$> time python ./test-nonblocking-write.py 
starting
blocking with 8028160 remaining
unblocked
blocking with 5259264 remaining
unblocked
blocking with 4456448 remaining
unblocked
blocking with 3915776 remaining
unblocked
blocking with 3768320 remaining
unblocked
blocking with 3768320 remaining
unblocked
blocking with 3670016 remaining
unblocked
blocking with 3670016 remaining
...
real    0m10.901s
user    0m10.465s
sys     0m0.016s

The fact that there’s very little difference between the ‘real’ and ‘user’ times means we spent very little time sleeping. We can also see that sometimes we get to run the busy loop multiple times while waiting for the socket to become writable.

Eventlet

Ok, so how about eventlet? Presumably eventlet makes it a lot easier to implement non-blocking I/O than the above example? Here’s what it looks like with eventlet:

from eventlet.green import socket

sock = socket.socket()
sock.connect(('localhost', 1234))
sock.send('foo\n' * 10 * 1024 * 1024)

Yes, that does look very like the first example. What has happened here is that by creating the socket using eventlet.green.socket.socket() we have put the socket into non-blocking mode and when the write to the socket blocks, eventlet will schedule any other work that might be pending. Hitting Ctrl-C while this
is running is actually pretty instructive:

$> python test-eventlet-write.py 
^CTraceback (most recent call last):
  File "test-eventlet-write.py", line 6, in 
    sock.send('foo\n' * 10 * 1024 * 1024)
  File ".../eventlet/greenio.py", line 289, in send
    timeout_exc=socket.timeout("timed out"))
  File ".../eventlet/hubs/__init__.py", line 121, in trampoline
    return hub.switch()
  File ".../eventlet/hubs/hub.py", line 187, in switch
    return self.greenlet.switch()
  File ".../eventlet/hubs/hub.py", line 236, in run
    self.wait(sleep_time)
  File ".../eventlet/hubs/poll.py", line 84, in wait
    presult = self.do_poll(seconds)
  File ".../eventlet/hubs/epolls.py", line 61, in do_poll
    return self.poll.poll(seconds)
KeyboardInterrupt

Yes, indeed, there’s a whole lot going on behind that innocuous looking send() call. You see mention of a ‘hub’ which is eventlet’s name for an event loop. You also see this trampoline() call which means “put the current code to sleep until the socket is writable”. And, there at the very end, we’re still sleeping in a call to poll() which is basically the same thing as select().

To show the example of doing some “useful” work rather than sleeping all the time we run a busy loop greenthread:

import eventlet
from eventlet.green import socket

def busy_loop():
    while True:
        i = 0
        while i < 5000000:
            i += 1
        print "yielding"
        eventlet.sleep()
eventlet.spawn(busy_loop)

sock = socket.socket()
sock.connect(('localhost', 1234))
sock.send('foo\n' * 10 * 1024 * 1024)

Now every time the socket isn’t writable, we switch to the busy_loop() greenthread and do some work. Greenthreads must cooperatively yield to one another so we call eventlet.sleep() in busy_loop() to once again poll the socket to see if its writable. Again, if we use the ‘time’ command to run this:

$> time python ./test-eventlet-write.py 
yielding
yielding
yielding
...
real    0m5.386s
user    0m5.081s
sys     0m0.088s

you can see we’re spending very little time sleeping.

(As an aside, I was going to take a look at gevent, but it doesn’t seem fundamentally different from eventlet. Am I wrong?)

Twisted

Long, long ago, in times of old, Nova switched from twisted to eventlet so it makes sense to take a quick look at twisted:

from twisted.internet import protocol
from twisted.internet import reactor

class Test(protocol.Protocol):
    def connectionMade(self):
        self.transport.write('foo\n' * 2 * 1024 * 1024)

class TestClientFactory(protocol.ClientFactory):
    def buildProtocol(self, addr):
        return Test()

reactor.connectTCP('localhost', 1234, TestClientFactory())
reactor.run()

What complicates the example most is twisted protocol abstraction which we need to use simply to write to the socket. The ‘reactor’ abstraction is simply twisted’s name for an event loop. So, we create a on-blocking socket, block in the event loop (using e.g. select()) until the connection completes and then
write to the socket. The transport.write() call will actually queue a writer in the reactor, return immediately and whenever the socket is writable, the writer will continue its work.

To show how you can run something in parallel, here’s how to run some code in a deferred callback:

def busy_loop():
    i = 0
    while i < 5000000:
        i += 1
    reactor.callLater(0, busy_loop)

reactor.connectTCP(...)
reactor.callLater(0, busy_loop)
reactor.run()

I’m using a timeout of zero here and it shows up a weakness in both twisted and eventlet – we want this busy_loop() code to only run when the socket isn’t writeable. In other words, we want the task to have a lower priority than the writer task. In both twisted and eventlet, the timed tasks are run before the
I/O tasks and there is no way to add a task which is only run if there are no runnable I/O tasks.

GLib

My introduction to async I/O was back when I was working on GNOME (beginning with GNOME’s CORBA ORB, called ORBit) so I can’t help comparing the above abstractions to GLib’s main loop. Here’s some equivalent code:

/* build with gcc -g -O0 -Wall $(pkg-config --libs --cflags glib-2.0) test-glib-write.c -o test-glib-write */

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

#include <glib.h>

GMainLoop    *main_loop = NULL;
static gchar *strv[10 * 1024 * 1024];
static gchar *data = NULL;
int           remaining = -1;

static gboolean
socket_writable(GIOChannel   *source,
                GIOCondition  condition,
                gpointer      user_data)
{
  int fd, sent;

  fd = g_io_channel_unix_get_fd(source);
  do
    {
      sent = write(fd, data, remaining);
      if (sent == -1)
        {
          if (errno != EAGAIN)
            {
              fprintf(stderr, "Write error: %s\n", strerror(errno));
              goto finished;
            }
          return TRUE;
        }

      data = &data[sent];
      remaining -= sent;
    }
  while (sent > 0 && remaining > 0);

  if (remaining <= 0)
    goto finished;

  return TRUE;

 finished:
  g_main_loop_quit(main_loop);
  return FALSE;
}

static gboolean
busy_loop(gpointer data)
{
  int i = 0;
  while (i < 5000000)
    i += 1;
  return TRUE;
}

int
main(int argc, char **argv)
{
  GIOChannel         *io_channel;
  guint               io_watch;
  int                 fd;
  struct sockaddr_in  addr;
  int                 i;
  gchar              *to_free;

  for (i = 0; i < G_N_ELEMENTS(strv)-1; i++)
    strv[i] = "foo\n";
  strv[G_N_ELEMENTS(strv)-1] = NULL;

  data = to_free = g_strjoinv(NULL, strv);
  remaining = strlen(data);

  fd = socket(AF_INET, SOCK_STREAM, 0);

  memset(&addr, 0, sizeof(struct sockaddr_in));
  addr.sin_family      = AF_INET;
  addr.sin_port        = htons(1234);
  addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);

  if (connect(fd, (struct sockaddr *)&addr, sizeof(addr)) == -1)
    {
      fprintf(stderr, "Error connecting to server: %s\n", strerror(errno));
      return 1;
    }

  fcntl(fd, F_SETFL, O_NONBLOCK);

  io_channel = g_io_channel_unix_new(fd);
  io_watch = g_io_add_watch(io_channel,
                            G_IO_OUT,
                            (GIOFunc)socket_writable,
                            GINT_TO_POINTER(fd));

  g_idle_add(busy_loop, NULL);

  main_loop = g_main_loop_new(NULL, FALSE);

  g_main_loop_run(main_loop);
  g_main_loop_unref(main_loop);

  g_source_remove(io_watch);
  g_io_channel_unref(io_channel);

  close(fd);

  g_free(to_free);

  return 0;
}

Here I create a non-blocking socket, set up an ‘I/O watch’ to tell me when the socket is writable and, when it is, I keep blasting data into the socket until I get an EAGAIN. This is the point at which write() would block if it was a blocking socket and I return TRUE from the callback to say “call me again when the socket is writable”. Only when I’ve finished writing all of the data do I return FALSE and quit the main loop causing the g_main_loop_run() call to return.

The point about task priorities is illustrated nicely here. GLib does have the concept of priorities and has a “idle callback” facility you can use to run some code when no higher priority task is waiting to run. In this case, the busy_loop() function will *only* run when the socket is not writable.

Tulip

There’s a lot of talk lately about Guido’s Asynchronous IO Support Rebooted (PEP3156) efforts so, of course, we’ve got to have a look at that.

One interesting aspect of this effort is that it aims to support both the coroutine and callbacks style programming models. We’ll try out both models below.

Tulip, of course, has an event loop, time-based callbacks, I/O callbacks and I/O helper functions. We can build a simple variant of our non-blocking I/O example above using tulip’s event loop and I/O callback:

import errno
import select
import socket

import tulip

sock = socket.socket()
sock.connect(('localhost', 1234))
sock.setblocking(0)

buf = memoryview(str.encode('foo\n' * 2 * 1024 * 1024))
def do_write():
    global buf
    while True:
        try:
            buf = buf[sock.send(buf):]
        except socket.error as e:
            if e.errno != errno.EAGAIN:
                raise e
            return

def busy_loop():
    i = 0
    while i < 5000000:
        i += 1
    event_loop.call_soon(busy_loop)

event_loop = tulip.get_event_loop()
event_loop.add_writer(sock, do_write)
event_loop.call_soon(busy_loop)
event_loop.run_forever()

We can go a step further and use tulip’s Protocol abstraction and connection helper:

import errno
import select
import socket

import tulip

class Protocol(tulip.Protocol):

    buf = b'foo\n' * 10 * 1024 * 1024

    def connection_made(self, transport):
        event_loop.call_soon(busy_loop)
        transport.write(self.buf)
        transport.close()

    def connection_lost(self, exc):
        event_loop.stop()
 
def busy_loop():
    i = 0
    while i < 5000000:
        i += 1
    event_loop.call_soon(busy_loop)

event_loop = tulip.get_event_loop()
tulip.Task(event_loop.create_connection(Protocol, 'localhost', 1234))
event_loop.run_forever()

This is pretty similar to the twisted example and shows up yet another example of the lack of task prioritization being an issue. If we added the busy loop to the event loop before the connection completed, the scheduler would run the busy loop every time the connection task yields.

Coroutines, Generators and Subgenerators

Under the hood, tulip depends heavily on generators to implement coroutines. It’s worth digging into that concept a bit to understand what’s going on.

Firstly, remind yourself how a generator works:

def gen():
    i = 0
    while i < 2:
        print(i)
        yield
        i += 1

i = gen()
print("yo!")
next(i)
print("hello!")
next(i)
print("bye!")
try:
    next(i)
except StopIteration:
    print("stopped")

This will print:

yo!
0
hello!
1
bye!
stopped

Now imagine a generator function which writes to a non-blocking socket and calls yield every time the write would block. You have the beginnings of coroutine based async I/O. To flesh out the idea, here’s our familiar example with some generator based infrastructure around it:

import collections
import errno
import select
import socket

sock = socket.socket()
sock.connect(('localhost', 1234))
sock.setblocking(0)

def busy_loop():
    while True:
        i = 0
        while i < 5000000:
            i += 1
        yield

def write():
    buf = memoryview(b'foo\n' * 2 * 1024 * 1024)
    while len(buf):
        try:
            buf = buf[sock.send(buf):]
        except socket.error as e:
            if e.errno != errno.EAGAIN:
                raise e
            yield
    quit()

Task = collections.namedtuple('Task', ['generator', 'wfd', 'idle'])

tasks = [
    Task(busy_loop(), wfd=None, idle=True),
    Task(write(), wfd=sock, idle=False)
]

running = True

def quit():
    global running
    running = False

while running:
    finished = []
    for n, t in enumerate(tasks):
        try:
            next(t.generator)
        except StopIteration:
            finished.append(n)
    map(tasks.pop, finished)

    wfds = [t.wfd for t in tasks if t.wfd]
    timeout = 0 if [t for t in tasks if t.idle] else None

    select.select([], wfds, [], timeout)

You can see how the generator-based write() and busy_loop() coroutines are cooperatively yielding to one another just like greenthreads in eventlet would do. But, there’s a pretty fundamental flaw here – if we wanted to refactor the code above to re-use that write() method to e.g. call it multiple times with
different input, we’d need to do something like:

def write_stuff():
    for i in write(b'foo' * 10 * 1024 * 1024):
        yield
    for i in write(b'bar' * 10 * 1024 * 1024):
        yield

but that’s pretty darn nasty! Well, that’s the whole idea behind Syntax for Delegating to a Subgenerator (PEP380). Since python 3.3, a generator can now yield to another generator using the ‘yield from’ syntax. This allows us to do:

...
def write(data):
    buf = memoryview(data)
    while len(buf):
        try:
            buf = buf[sock.send(buf):]
        except socket.error as e:
            if e.errno != errno.EAGAIN:
                raise e
            yield

def write_stuff():
    yield from write(b'foo\n' * 2 * 1024 * 1024)
    yield from write(b'bar\n' * 2 * 1024 * 1024)
    quit()

Task = collections.namedtuple('Task', ['generator', 'wfd', 'idle'])

tasks = [
    Task(busy_loop(), wfd=None, idle=True),
    Task(write_stuff(), wfd=sock, idle=False)
]
...

Conclusions?

Yeah, this is the point where I’ve figured out what we should do in OpenStack. Or not.

I really like the explicit nature of Tulip’s model – for each async task, you explicitly decide whether to block the current coroutine on its completion (or put another way, yield to another coroutine until the task has completed) or you register a callback to be notified of the tasks completion. I’d much prefer this to rather cavalier “don’t worry your little head” approach of hiding the async nature of what’s going on.

However, the prospect of porting something like Nova to this model is more than a little dauting. If you think about the call stack of an REST API request being handled and ultimately doing an rpc.cast() and that the entire call stack would need to be ported to ‘yield from’ in order for us to yield and handle another API request while waiting for the result of rpc.cast() …. as I said, daunting.

What I’m most interested in is how to design our new messaging API to be able to support any and all of these models in future. I haven’t quite figured that out either, but it feels pretty doable.