[Haskell-cafe] thespian thoughts

[Simon Michael]

Hi Alex. I'm testing thespian in rss2irc, FYI: http://joyful.com/darcsden/simon/rss2irc/browse/rss2irc.hs#L-114 . 

Maybe there's a better way to use it, but for now it brings me one extra thread and higher memory usage/leakage - an instance grew to 350M overnight where I'd normally see 50-100M. The leak may be in my code, but I haven't found it yet. Did you notice high memory usage when using thespian ?

Whether or not I end up keeping it, it has influenced the ordinary thread-based manager for the better, eg see forkMonitoredIO which is like a poor man's monitor using exceptions rather than actor messages.

Best,
- Simon


On Oct 12, 2011, at 8:44 AM, Simon Michael wrote:

> Hi Alex, thanks for the reply (it didn't seem to reach haskell-cafe, if that matters) and the update on status. I passed over CloudHaskell as it seemed (a) over-complex for my needs and (b) not ready, eg not on hackage. I need something that will still permit cabal install rss2irc. But perhaps that's not far off. I'll look again more closely, at both.
> 
> Best - Simon
> 
> 
> On Oct 11, 2011, at 10:48 PM, Alex Constandache wrote:
> 
>> Hi Simon,
>> 
>> Sorry for the late reply. I no longer work on thespian, as there is
>> another project, called Cloud Haskell which provides much more
>> functionality than thespian (including Erlang style distributed
>> computing). It is not available on Hackage, but it lives on Github at
>> 
>> https://github.com/jepst/CloudHaskell
>> 
>> Simon Peyton-Johnes himself seems to be associated with the project,
>> so you should have a look at it. If it does not match your needs, feel
>> free to hack thespian and send me a pull request on bitbucket. You are
>> the second person who emailed about thespian in the last month, so
>> maybe I should reconsider the decision to abandon it (which was
>> motivated by my desire to avoid contributing to the insane
>> fragmentation that plagues Hackage).
>> 
>> --Alex
>> 
>> On Sun, Oct 9, 2011 at 12:45 PM, Simon Michael <simon at joyful.com> wrote:
>>> Hi Alexander,
>>> I went looking for something to clean up rss2irc's thread management, and
>>> your recently released thespian package looks like the simplest, most
>>> practical actors/erlang/OTP-ish lib for haskell so far. Thanks!
>>> I need to restart threads (actors) in a controlled way when they die or
>>> hang. I think some of all of that is also part of Erlang/OTP, and maybe it's
>>> already on your todo list. Here are some notes I made, for your interest and
>>> in case you have any further thoughts.
>>> I thought of adding some actor-transforming combinators like these:
>>> -- | Spawn the specified action and restart it whenever it dies, unless
>>> --   the restart frequency exceeds any of the given maximum frequencies in
>>> --   which case throw an exception.
>>> restart :: [Frequency] -> IO a -> IO Address
>>> restart fs action = spawn $ do
>>>  setFlag TrapRemoteExceptions
>>>  let start times = []
>>>  forever
>>>   t <- getCurrentTime
>>>   update start times
>>>   when (any fs exceeded by start times) throw error
>>>   worker <- spawn action
>>>   monitor worker
>>>   receive worker [Case _ -> return ()]
>>> -- | Convert hangs to exceptions using a watchdog timer: spawn the
>>> --   specified action under the control of a watcher which monitors it and
>>> --   also kills it and throws an exception if the worker ever fails to
>>> --   send (something) to the watcher within the specified interval.
>>> watchdog :: Seconds -> (Address -> IO a) -> IO Address
>>> watchdog t action = spawn $ do
>>>  watcher <- self
>>>  worker <- spawn $ action watcher
>>>  monitor worker
>>>  forever
>>>   receiveTimeout t [Case _  -> return ()
>>>                     Default -> throw error
>>>                    ]
>>> which I would use something like:
>>> main = do
>>>  (feed, bot) <- ...
>>>  spawn $ do
>>>   reader    <- restart [MaxPerHour 2] $ watchdog 60 $ feedReader feed bot
>>>   announcer <- restart [MaxPerHour 2] $ watchdog 60 $ ircAnnouncer bot
>>>   responder <- restart [MaxPerHour 2] $ watchdog 60 $ ircResponder bot
>>>   monitor reader
>>>   monitor announcer
>>>   monitor responder
>>>   setFlag TrapRemoteExceptions
>>>   receive [Case e :: RemoteException -> exitFailure (show e) -- one of the
>>> above died or hung too frequently
>>>           ]
>>> but the types aren't right. I'm not sure if these combinators are possible
>>> with the current thespian api. Also they would complicate things, eg what is
>>> the address of a restarted actor ?
>>> On second thoughts it might be better if these features were built in to the
>>> library. I imagine something like:
>>> main = do ...
>>>  reader    <- spawnWith [RestartSpec [MinInterval (minutes 5), MaxPerHour
>>> 3, MaxPerWeek 6], Watchdog (seconds 60)] $ feedReader feed bot
>>> feedReader = do
>>>  forever
>>>    poll feed, do stuff
>>>    resetWatchdog
>>> What do you think ?
>>> Best - Simon
>> 
>> 
>> 
>> -- 
>> AC
>> I used to have a signature, but now I don't.
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> 

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