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Tornado without a GIL on PyPy STM

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This post is by Konstantin Lopuhin, who tried PyPy STM during the Warsaw sprint.

Python has a GIL, right? Not quite - PyPy STM is a python implementation without a GIL, so it can scale CPU-bound work to several cores. PyPy STM is developed by Armin Rigo and Remi Meier, and supported by community donations. You can read more about it in the docs.

Although PyPy STM is still a work in progress, in many cases it can already run CPU-bound code faster than regular PyPy, when using multiple cores. Here we will see how to slightly modify Tornado IO loop to use transaction module. This module is described in the docs and is really simple to use - please see an example there. An event loop of Tornado, or any other asynchronous web server, looks like this (with some simplifications):

while True:
    for callback in list(self._callbacks):
        self._run_callback(callback)
    event_pairs = self._impl.poll()
    self._events.update(event_pairs)
    while self._events:
        fd, events = self._events.popitem()
        handler = self._handlers[fd]
        self._handle_event(fd, handler, events)

We get IO events, and run handlers for all of them, these handlers can also register new callbacks, which we run too. When using such a framework, it is very nice to have a guaranty that all handlers are run serially, so you do not have to put any locks. This is an ideal case for the transaction module - it gives us guaranties that things appear to be run serially, so in user code we do not need any locks. We just need to change the code above to something like:

while True:
    for callback in list(self._callbacks):
        transaction.add(                # added
            self._run_callback, callback)
    transaction.run()                   # added
    event_pairs = self._impl.poll()
    self._events.update(event_pairs)
    while self._events:
        fd, events = self._events.popitem()
        handler = self._handlers[fd]
        transaction.add(                # added
            self._handle_event, fd, handler, events)
    transaction.run()                   # added

The actual commit is here, - we had to extract a little function to run the callback.

Part 1: a simple benchmark: primes

Now we need a simple benchmark, lets start with this - just calculate a list of primes up to the given number, and return it as JSON:

def is_prime(n):
    for i in xrange(2, n):
        if n % i == 0:
            return False
    return True

class MainHandler(tornado.web.RequestHandler):
    def get(self, num):
        num = int(num)
        primes = [n for n in xrange(2, num + 1) if is_prime(n)]
        self.write({'primes': primes})

We can benchmark it with siege:

siege -c 50 -t 20s https://localhost:8888/10000

But this does not scale. The CPU load is at 101-104 %, and we handle 30 % less request per second. The reason for the slowdown is STM overhead, which needs to keep track of all writes and reads in order to detect conflicts. And the reason for using only one core is, obviously, conflicts! Fortunately, we can see what this conflicts are, if we run code like this (here 4 is the number of cores to use):

PYPYSTM=stm.log ./primes.py 4

Then we can use print_stm_log.py to analyse this log. It lists the most expensive conflicts:

14.793s lost in aborts, 0.000s paused (1258x STM_CONTENTION_INEVITABLE)
File "/home/ubuntu/tornado-stm/tornado/tornado/httpserver.py", line 455, in __init__
    self._start_time = time.time()
File "/home/ubuntu/tornado-stm/tornado/tornado/httpserver.py", line 455, in __init__
    self._start_time = time.time()
...

There are only three kinds of conflicts, they are described in stm source, Here we see that two threads call into external function to get current time, and we can not rollback any of them, so one of them must wait till the other transaction finishes. For now we can hack around this by disabling this timing - this is only needed for internal profiling in tornado.

If we do it, we get the following results (but see caveats below):

Impl. req/s
PyPy 2.4 14.4
CPython 2.7 3.2
PyPy-STM 1 9.3
PyPy-STM 2 16.4
PyPy-STM 3 20.4
PyPy STM 4 24.2
   

As we can see, in this benchmark PyPy STM using just two cores can beat regular PyPy! This is not linear scaling, there are still conflicts left, and this is a very simple example but still, it works!

But its not that simple yet :)

First, these are best-case numbers after long (much longer than for regular PyPy) warmup. Second, it can sometimes crash (although removing old pyc files fixes it). Third, benchmark meta-parameters are also tuned.

Here we get relatively good results only when there are a lot of concurrent clients - as a results, a lot of requests pile up, the server is not keeping with the load, and transaction module is busy with work running this piled up requests. If we decrease the number of concurrent clients, results get slightly worse. Another thing we can tune is how heavy is each request - again, if we ask primes up to a lower number, then less time is spent doing calculations, more time is spent in tornado, and results get much worse.

Besides the time.time() conflict described above, there are a lot of others. The bulk of time is lost in these two conflicts:

14.153s lost in aborts, 0.000s paused (270x STM_CONTENTION_INEVITABLE)
File "/home/ubuntu/tornado-stm/tornado/tornado/web.py", line 1082, in compute_etag
    hasher = hashlib.sha1()
File "/home/ubuntu/tornado-stm/tornado/tornado/web.py", line 1082, in compute_etag
    hasher = hashlib.sha1()

13.484s lost in aborts, 0.000s paused (130x STM_CONTENTION_WRITE_READ)
File "/home/ubuntu/pypy/lib_pypy/transaction.py", line 164, in _run_thread
    got_exception)

The first one is presumably calling into some C function from stdlib, and we get the same conflict as for time.time() above, but is can be fixed on PyPy side, as we can be sure that computing sha1 is pure.

It is easy to hack around this one too, just removing etag support, but if we do it, performance is much worse, only slightly faster than regular PyPy, with the top conflict being:

83.066s lost in aborts, 0.000s paused (459x STM_CONTENTION_WRITE_WRITE)
File "/home/arigo/hg/pypy/stmgc-c7/lib-python/2.7/_weakrefset.py", line 70, in __contains__
File "/home/arigo/hg/pypy/stmgc-c7/lib-python/2.7/_weakrefset.py", line 70, in __contains__

Comment by Armin: It is unclear why this happens so far. We'll investigate...

The second conflict (without etag tweaks) originates in the transaction module, from this piece of code:

while True:
    self._do_it(self._grab_next_thing_to_do(tloc_pending),
                got_exception)
    counter[0] += 1

Comment by Armin: This is a conflict in the transaction module itself; ideally, it shouldn't have any, but in order to do that we might need a little bit of support from RPython or C code. So this is pending improvement.

Tornado modification used in this blog post is based on 3.2.dev2. As of now, the latest version is 4.0.2, and if we apply the same changes to this version, then we no longer get any scaling on this benchmark, and there are no conflicts that take any substantial time.

Comment by Armin: There are two possible reactions to a conflict. We can either abort one of the two threads, or (depending on the circumstances) just pause the current thread until the other one commits, after which the thread will likely be able to continue. The tool ``print_stm_log.py`` did not report conflicts that cause pauses. It has been fixed very recently. Chances are that on this test it would report long pauses and point to locations that cause them.

Part 2: a more interesting benchmark: A-star

Although we have seen that PyPy STM is not all moonlight and roses, it is interesting to see how it works on a more realistic application.

astar.py is a simple game where several players move on a map (represented as a list of lists of integers), build and destroy walls, and ask server to give them shortest paths between two points using A-star search, adopted from ActiveState recipie.

The benchmark bench_astar.py is simulating players, and tries to put the main load on A-star search, but also does some wall building and destruction. There are no locks around map modifications, as normal tornado is executing all callbacks serially, and we can keep this guaranty with atomic blocks of PyPy STM. This is also an example of a program that is not trivial to scale to multiple cores with separate processes (assuming more interesting shared state and logic).

This benchmark is very noisy due to randomness of client interactions (also it could be not linear), so just lower and upper bounds for number of requests are reported

Impl. req/s
PyPy 2.4 5 .. 7
CPython 2.7 0.5 .. 0.9
PyPy-STM 1 2 .. 4
PyPy STM 4 2 .. 6

Clearly this is a very bad benchmark, but still we can see that scaling is worse and STM overhead is sometimes higher. The bulk of conflicts come from the transaction module (we have seen it above):

91.655s lost in aborts, 0.000s paused (249x STM_CONTENTION_WRITE_READ)
File "/home/ubuntu/pypy/lib_pypy/transaction.py", line 164, in _run_thread
    got_exception)

Although it is definitely not ready for production use, you can already try to run things, report bugs, and see what is missing in user-facing tools and libraries.

Benchmarks setup:

Comments

Anonymous wrote on 2014-11-18 07:00:

"Clearly this is a very benchmark" - looks like you've missed a word here ;)

crusaderky wrote on 2014-11-19 00:01:

in bench_astar.py, you are doing the following queries:
- try to move: 85%
- build a wall: 10.5% [(1-.85)*.7]
- erase something: 0.45% [(1-.85)*(1-.7)*.1]
- show map: 4.05% [(1-.85)*(1-.7)*(1-.1)]

I doubt that's intentional.... :P

crusaderky wrote on 2014-11-19 01:01:

Correct me if I misunderstood the theory of PyPy-STM, but in the A* test there's nothing that prevents a get() to read the game map while MapChangeHandler.put() is running (that is, while the system is in an incoherent status)?

Shouldn't MapChangeHandler.put() be wrapped in a exclusive write lock, and all the get() handlers be wrapped with a shared read lock?

Konstantin Lopuhin wrote on 2014-11-19 20:45:

> Clearly this is a very benchmark" - looks like you've missed a word here ;)

Oh, yes, that word is "bad" :)

> Shouldn't MapChangeHandler.put() be wrapped in a exclusive write lock, and all the get() handlers be wrapped with a shared read lock?

Here all request handlers are already wrapped inside atomic blocks, but this is hidden from us in (modified) tornado. So we do not need any locks (as in normal tornado too, because normal tornado is single threaded). If request handlers conflict, then we just loose performance, not correctness. This is one of the main points of PyPy STM: it can support multithreaded code without needing to use locks.

Regarding the probabilities: yes, that's not quite intentional)

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