Bartosz Milewski Missing post

[Sean Kelly]

Michel Fortin wrote:
> On 2009-05-28 12:52:06 -0400, Andrei Alexandrescu 
> <SeeWebsiteForEmail at erdani.org> said:
> 
>> What happens is that memory is less shared as cache hierarchies go 
>> deeper. It was a great model when there were a couple of processors 
>> hitting on the same memory because it was close to reality. Cache 
>> hierarchies reveal the hard reality that memory is shared to a 
>> decreasing extent and that each processor would rather deal with its 
>> own memory. Incidentally, message-passing-style protocols are 
>> prevalent in such architectures even at low level. It follows that 
>> message passing is not only an attractive model for programming at 
>> large, but also a model that's closer to machine than memory sharing.
> 
> While message-passing might be useful for some applications, I have a 
> hard time seeing how it could work for others. Try split processing of a 
> 4 Gb array over 4 processors, or implement multi-threaded access to an 
> in-memory database. Message passing by copying all the data might happen 
> at the very low-level, but shared memory is more the right abstraction 
> for these cases.

Perhaps at an implementation level in some instances, yes.  But look at 
Folding at home, etc.  The approach is based on message passing.  Just if 
you were Folding at OnOnePCOnly then you might pass references to array 
regions around instead of copying the data.  I suppose what I'm getting 
at is that an interface doesn't typically necessitate a particular 
implementation.

> There's a reason why various operating systems support shared memory 
> between different processes: sometime it's easier to deal with shared 
> memory than messaging, even with all the data races you have to deal with.
> 
> Shared memory becoming more and more implemented as message passing at 
> the very low level might indicate that some uses of shared memory will 
> migrate to message passing at the application level and get some 
> performance gains, but I don't think message passing will ever 
> completely replace shared memory for dealing with large data sets. It's 
> more likely that shared memory will become a scarse resource for some 
> systems while it'll continue to grow for others.

Well sure.  At some level, sharing is going to be happening even in 
message-passing oriented applications.  The issue is more what the 
"encouraged" approach to solving problems that a language supports than 
what the language allows.  D will always allow all sorts of wickedness 
because it's a systems language.  But that doesn't mean this stuff has 
to be the central feature of the language.