Re: [Chicken-users] Re: An alternative thread system?

[Alaric Snell-Pym]

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On 12 Aug 2008, at 4:13 am, Ivan Raikov wrote:

>   Guys, I am glad to see that old flame wars never die ;-) Pipes used
> to be slow back in the day, around the time when forking and context
> switching was slow. Nowadays, modern kernels rely extensively on
> zero-copy techniques, and I am pretty sure that Linux has had
> zero-copy pipes since at least the 2.4 series. Passing data between
> threads may be as simple as an assignment, but it almost never is. In
> practice, you always end up having some complicated synchronization
> protocol. Why not let the OS handle that for you?

For comparison, a project I worked on a while ago (closed-source,
alas) was a fast inter-process communications system that involved
all participating processes sharing a memory segment (folks, forget
shmget(); everyone mmap() the same file is where it's at these days,
not least because you can then hexdump -C the file to see what's
going on, preserve it across reboots, more portable OS support, etc)
in which a specialist memory manager controls a heap, and the system
provides a namespace of caches, FIFOs, and LIFOs.

So you can either allocate a shared memory object and serialise stuff
into it, or allocate your stuff in a shared memory object in the
first place, then use a FIFO or a LIFO to pass it to a server process
as a message - or store things in the caches.

It's *almost* all implemented in a lock-free manner (using cmpxchg8b
and friends on x86, and ll/sc instructions on sparc); there are a
couple of locks that the heap manager uses occasionally (but not in
the 'main line' of allocations and deallocations), so processes can
communicate without *any* syscalls using it.

The speed advantages of shared memory *can* coexist with the safe-by-
default nature of independent processes.

>   -Ivan


ABS

- --
Alaric Snell-Pym
Work: http://www.snell-systems.co.uk/
Play: http://www.snell-pym.org.uk/alaric/
Blog: http://www.snell-pym.org.uk/?author=4


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