[erlang-questions] Question about message passing paradigm
Wed Jul 2 06:27:56 CEST 2008
First of all: Wow, I never expected this much of a response.
I need some time to sort through the replies and will reply when I
have more time. I just wanted to write this short reply and say thank
you to everyone for being willing to help a newbie into this area.
On Jul 1, 6:58 pm, "Edwin Fine" <erlang-questions_ef...@REDACTED>
> On Tue, Jul 1, 2008 at 8:20 PM, Richard A. O'Keefe <o...@REDACTED>
> > On 1 Jul 2008, at 7:37 pm, Edwin Fine wrote:
> >> Thank you for a very interesting and informative analysis. I must admit
> >> that I tend to lump together Erlang/OTP as "Erlang" and see solutions in
> >> that context. In a way, I feel as if using Erlang "standalone" without OTP
> >> is very roughly analogous to using C++ without the STL: it can be done, but
> >> why on earth would one want to?
> > Because the STL
> > - is still not fully portable between compilers;
> > in theory it should not be, but it takes you into
> > deep template territory where compilers have
> > incompatible bugs (though they are improving)
> > - is in my experience less efficient than home-brew
> > code (the STL relies, like Haskell, on *serious*
> > optimisation which compilers do not always do)
> > - gets you some of the most incomprehensible
> > compiler error messages (un)imaginable when you
> > make mistakes, which you always do, because
> > - it is unpleasantly complex to use (this will, I
> > hope, be remedied in C0x, at least if I can
> > trust Stroustrup's summary of what's going to be
> > there).
> Maybe after using it for a long time, it's doesn't seem that complicated. At
> least, not compared to C++ itself. C++ itself is IMHO horribly complex and
> arguably a language to be used only by C++ experts. Just writing
> exception-safe code alone can be dreadfully difficult, never mind the
> powerful yet labyrinthine maze that is called templates. I've now used the
> STL extensively for many years, and it has a large advantage in that it is
> more or less supported by all compliant compilers (and "free" as in beer).
> It pretty much does the trick. Initially, the ability of various compilers
> to handle templates was pretty bad and inconsistent so I rolled my own
> portable libraries. I had to write a significant amount of commercial C++
> code that had to be ported across OS/390, HP/UX, Windows, Solaris, AIX, and
> Linux, so I have a feel for the issues involved. I found it amusing that
> Andrei Alexandrescu could write syntactically valid template code that no
> C++ compilers could parse at that time. After a while, though, the effort of
> maintaining (testing, documenting, fixing) all that code across all those
> systems became too much and I used the STL exclusively after that. I had to
> choose a lowest common denominator approach and avoid doing anything really
> fancy, but it was still worth it.
> The STL was efficient enough for most purposes. In the IT department of a
> large telco where I worked for some years, trying to improve quality and
> establish standards, the code was so inefficient that the benefits of using
> the STL far outweighed any efficiency concerns. They didn't even compile the
> code with optimization when it went into production, so the overhead of
> those templates must have been quite significant, yet it didn't even
> register on the radar compared to the big concerns (like, actually getting
> the code to work, and getting the developers to understand why it's slow to
> do a linear lookup in a vector of 100,000 items, in a loop that iterates
> 50,000 times). When you're in the trenches, you are grateful for anything
> that works as advertised most of the time. Gladly, I am out of the C++
> business right now and happily learning the Erlang world, which is a joy in
> > None of these applies to OTP, but the argument that
> > - it is another large and complex body of material
> > to master on top of something itself unfamiliar.
> > does apply to both.
> > I have a colleague who has done
> > serious commercial software development in C++ and
> > flatly refuses to have anything to do with the STL
> > (see reasons above).
> I did, too, until a few years ago (see above).
> > Let's take the current example and see if we can squeeze a
> > bit more out of it. Tim Watson pointed out that there is
> > an issue about "locking" and failing processes. My take
> > on this is "let it crash": when processes are coupled like
> > this they should be as a rule be linked and if one dies,
> > all should die. The OTP behaviours take care of that kind
> > of thing, which is why *once you have grasped the basics*
> > you should try them before rolling your own. What's really
> > interesting here is that the original system was written in
> > terms of threads and locking. Have you looked at thread
> > interfaces lately?
> Unfortunately, I have worked a significant amount with pthreads, Java
> threads, Windows threads, and OS/2 threads, and consider that skill set to
> be in the same domain as C++: for experts (as opposed to the "average"
> developer found in IT shops and who knows how many software companies) only.
> The fact that we are now seeing multicore processors and are being exhorted
> to write more thread-bearing code makes me shudder. I suspect that most of
> the readers of this newsgroup are highly skilled developers, hopefully
> working for great companies, and I wonder if they have been exposed to the
> horrors of the "average" developer (as evidenced in "The Daily WTF" and
> similar web sites). I hope I am not coming across conceited; I'm not like
> that at all. It is just that I have spent 25 years working hard to be as
> good a software guy (for want of a better description) as possible, and for
> my troubles have almost always been put in a position where I have had to
> teach and mentor the "average" developer (and fix literally millions of
> lines of code, usualy C++). My experience (and others I am sure will have
> differences of opinion) is that the level of the average developer is
> trending steeply downwards as developers become a globalized commodity
> ("resource") and compete more on price than on ability. These days, I find
> it rare to see halfway decent code written in ANY language, and the thought
> of zillions of lines of thread-bearing C++ and Java software being let loose
> on the world by people who can't even spell O(N^2) scares the heck out of
> me. Sorry if I am ranting a bit.
> > [Single Unix Specification version 3]
> > pthread_mutex_lock(&mutex)
> > pthread_mutex_unlock(&mutex)
> > There are four kinds of mutex in POSIX threads:
> > PTHREAD_MUTEX_NORMAL
> > recursive locking deadlocks
> > unlocking a mutex that is already unlocked
> > or that is locked by another => undefined
> > PTHREAD_MUTEX_ERRORCHECK
> > these conditions return an error code
> > PTHREAD_MUTEX_RECURSIVE
> > recursive locking works
> > unlocking a mutex that is already unlocked
> > or that is locked by another => error code
> > PTHREAD_MUTEX_DEFAULT
> > same as PTHREAD_MUTEX_NORMAL
> > Anything else
> > All behaviour undefined
> > [Solaris 2.10]
> > If
> > (1) _POSIX_THREAD_PRIO_INHERIT is defined
> > (2) the mutex was initialised with protocol
> > attribute PTHREAD_PRIO_INHERIT
> > (3) the mutex was initialised with robustness
> > attribute PTHREAD_MUTEX_ROBUST_NP
> > (4) the last holder of the lock crashed
> > then
> > (A) an attempt to lock the mutex will 'fail'
> > with the error code EOWNERDEAD
> > (B) but in fact the attempt will have succeeded
> > (C) it is up to the *new* owner of the lock to
> > try to clean up the state
> > (D) if it can, it calls pthread_mutex_consistent_np
> > (E) if it crashes, the next locker will get the
> > same error code and the same chance to recover
> > (F) if it can't, it should unlock the mutex, and
> > future lockers will get a *different* error code
> > (ENOTRECOVERABLE).
> > (G) it is possible to call pthread_mutex_consistent_np
> > on mutexes that aren't held or didn't need
> > recovery and the behaviour is undefined
> > If a mutex with the default PTHREAD_MUTEX_STALLED_NP
> > robustness value is held by a thread that dies,
> > future locks are "blocked in an unspecified manner".
> > What this means in practice I'm not sure.
> > If you reckon the Solaris 2.10 treatment of crashed
> > lock holders is a mess, perhaps you can point me to
> > something better in SUSv3. I can't find anything in
> > SUSv3 to say _what_ happens when a lock owner crashes.
> > (And don't get me onto the subject of cancellation points.)
> > What kind of building block is _this_ for building
> > reliable systems?
> One made of clay and straw and baked in the sun.
> > Message passing plus linking is so much easier to
> > have justified confidence in that pthreads and TBB start
> > to look like extremely sick jokes.
> I couldn't agree with you more. One thing that amazed me about Erlang is
> how, once I had figured out how to write something, it Just Worked (TM). I
> fell in love.
> > <off-topic>One last thing: I read the Ethics of Belief after poring over
> >> one of your posts recently, and was exceptionally impressed with the
> >> gentleman's writing and philosophy, with which I strongly agree. In that
> >> regard, I'd like to misquote John Stuart Mill, namely, "A foolish certainty
> >> is the hobgoblin of little minds". (Actually, I think my version is a slight
> >> improvement ;) I am unfortunately seeing the mechanism of insufficiently
> >> examined beliefs at work today, resulting in the persecution of a friend of
> >> mine by way of (the almost totally belief-based) Shaken Baby Syndrome. So
> >> the essay really resonated deeply with me. I daresay William K. Clifford
> >> would have had a lot to say about this. I wish he were still alive to do so.
> >> Bertrand Russel too.</offtopic>
> > <also-off-topic>Clifford takes a really good idea and pushes it beyond the
> > bounds
> > of reason; he manages, presumably without intending to, to make any belief
> > in
> > science ethically unjustifiable. Specifically and in Clifford's day
> > topically,
> > Clifford's rule about believing other people meant that it would have been
> > *bad*
> > for anyone to believe in Natural Selection. I encountered Clifford's paper
> > in
> > a book by DeMarco and someone else about risk management in software
> > engineering.
> > I find the idea that Shaken Baby is still credited deeply upsetting; please
> > convey
> > my sympathy and good wishes to your friend.
> read more »
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