nova team mailing list archive

[Vishvananda Ishaya <vishvananda@xxxxxxxxx>]

We were initially using a multiproccessing process pool in the code.  The
ProcessPool in process.py is there because multiprocessing had wierd issues
and needed hackish workarounds to function properly with tornado's ioloop
and twisted's reactor.  We were having a lot if issues with exceptions
disappearing and processes hanging.

In general threads and processes in python seem a little bit annoying, that
is why I've been so grumbly about moving away from twisted, since the
interaction between async and signals is a bit nasty.  A lot of effort and
debugging went in to the current implementation of deferring to shell calls
and having the  message pump still work.

I like the idea of moving away from the reactor loop and having a simple
listener that waits for messages and fires of a another thread/process to
handle it.  The issues arise when this thread needs to return data.  Do we
just return a request id and switch to making clients poll for new data?
Also waiting on the queue also means there is no looping callback to update
state.  We'll need an external process to poll for state updates.

Crazy idea...forget os signals altogether, and just signal using rabbit.
 That would definitely circumvent a lot of python issues :)

Vish

On Wed, Aug 4, 2010 at 1:59 PM, Eric Day <eday@xxxxxxxxxxxx> wrote:

> We can also do multi-process for this services as well, and signals
> become a lot easier to deal with. Twisted signal handling may actually
> be easier than multi-threaded since it's running in a single thread.
>
> If you have the time, try hacking up a multi-threaded or multi-process
> worker and share to see how difficult it would be. :)
>
> -Eric
>
> On Wed, Aug 04, 2010 at 12:29:40PM -0700, Justin Santa Barbara wrote:
> >    If this is the primary issue, then I think that dealing with signals
> is
> >    surely easier than dealing with Twisted or Eventlet.
> >    I would propose that for the back-end services we write them 'simply'
> in
> >    Python threads.  For the front end services (which are effectively
> proxies
> >    to the data store and the back end services), there may be a
> performance
> >    case to be made for async code.  I don't believe that the back-end
> >    services have the high performance requirements, but they do have the
> >    requirement to be correct even when dealing with messy back-end APIs
> and
> >    things going badly wrong.  That logic will end up twisted enough as it
> is
> >    :-)
> >    I believe the signal problems must be the same for Twisted as for
> simple
> >    Python threads (in particular with threads.deferToThread), it's just
> that
> >    Twisted (hopefully) handles signals.  Maybe we can look at how they
> make
> >    it work.
> >    What are the requirements for 'correct signal handling'?  Is it "the
> >    process should exit in a timely way in response to SIGINT and SIGTERM,
> and
> >    immediately for SIGKILL?"
> >    Justin
> >
> >    On Wed, Aug 4, 2010 at 3:23 AM, Joshua McKenty <jmckenty@xxxxxxxxx>
> wrote:
> >
> >      The biggest issue is the interaction with signals and the python
> >      threading model. Multiprocess certainly works (see the nova use of
> >      process pool), but you're making your code simpler at the cost of
> more
> >      complex process supervision (which I don't object to in this case).
> >      Signals come up in deployment a lot, how to roll out code changes,
> etc.
> >      If we fix live migration, this gets much easier.
> >
> >      Sent from my iPhone
> >      On 2010-08-04, at 5:05 AM, Justin Santa Barbara <
> justin@xxxxxxxxxxxx>
> >      wrote:
> >
> >        Forgive a Python noob's question, but what's wrong with just using
> >        Python threads?  Why introduce multiple processes?
> >        It seems that Eric's benchmarks indicate that the overhead would
> be
> >        tolerable, and the code would definitely be much cleaner.
> >        The multiple process idea is another argument in favor of simple
> >        threading... if we figure out sharding, we could run multiple
> compute
> >        service processes to get around scaling limits that going with
> simple
> >        threading might introduce (e.g. GIL contention).
> >        Justin
> >
> >        On Tue, Aug 3, 2010 at 7:56 PM, Vishvananda Ishaya
> >        <vishvananda@xxxxxxxxx> wrote:
> >
> >          If we want to go with the simplest possible approach, we could
> make
> >          the compute workers synchronous and just run multiple copies on
> each
> >          host.  We could make one of them 'read only' so it only answers
> >          simple/fast requests, and a few (4?) others for other long/io
> >          intensive tasks.  The ultimate would be to have each message
> >          actually have its own worker a la erlang, but that might be a
> bit
> >          extreme.
> >          I've been doing a lot of the changes later that require
> switching
> >          everything to async.  It is a bit annoying to wrap your head
> around
> >          it, but it really isn't all that bad.  That said, I'm all for
> making
> >          things as simple as possible.
> >          Vish
> >
> >          On Tue, Aug 3, 2010 at 6:30 PM, Justin Santa Barbara
> >          <justin@xxxxxxxxxxxx> wrote:
> >
> >            Without meaning to make the twisted/eventlet flamewar any
> worse,
> >            can I just ask why we're not just using 'good old threads'?
>  I've
> >            asked Eric Day for his input based on his great benchmarks
> >            (http://oddments.org/?p=494).  My background is from the Java
> >            world, where threads work wonderfully - possibly even better
> than
> >            async:
> http://www.thebuzzmedia.com/java-io-faster-than-nio-old-is-new-again
> >            I feel like Nova is greatly complicated by the async code, and
> I'm
> >            starting to see some of the pain of Twisted: it seems that
> >            _everything_ needs to be async in the long run, because if
> >            something calls a function that is (or could be) async, it
> must
> >            itself be async.  So yields and @defer.inlineCallbacks start
> >            cropping up everywhere.
> >            One of the project goals seems to be simplicity of the code,
> for
> >            fewer bugs and to reduce barriers to entry, and it seems that
> if
> >            we could use 'plain old Python' that we would better achieve
> this
> >            goal than if we have to use an async framework.
> >            I know that Python has its issues here with the GIL, but I'm
> just
> >            wondering whether, in the case of nova, threads might be good
> >            enough, and produce much easier to understand code?  I'm
> guessing
> >            that maybe the project started with threads - what happened?
> >            Justin
> >
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