dealing with lots of sockets (was Re: [Beowulf] automount on high ports)

Perry E. Metzger perry at piermont.com
Wed Jul 2 14:37:41 EDT 2008


"Robert G. Brown" <rgb at phy.duke.edu> writes:
>> Well, it actually kind of is. Typically, a box in an HPC cluster is
>> running stuff that's compute bound and who's primary job isn't serving
>> vast numbers of teeny high latency requests. That's much more what a
>> web server does. However...
>
> I'd have to disagree.  On some clusters, that is quite true.  On others,
> it is very much not true, and whole markets of specialized network
> hardware that can manage vast numbers of teeny communications requests
> with acceptably low latency have come into being.  And in between, there
> is, well, between, and TCP/IP at gigabit speeds is at least a contender
> for ways to fill it.

I have to admit my experience here is limited. I'll take your word for
it that there are systems where huge numbers of small, high latency
requests are processed. (I thought that teeny stuff in HPC land was
almost always where you brought in the low latency fabric and used
specialized protocols, but...)

>> Myself, I'm a believer in event driven code. One thread, one core. All
>> other concurrency management should be handled by events, not by
>> multiple threads.[....]

> Interesting.  Makes sense, but a lot of boilerplate code for daemons has
> always used the fork approach.  Of course, things were "smaller" back
> when the approach was dominant.  The forking approach is easy to program
> and reminiscent of pipe code and so on.

Sure, but it is way inefficient. Every single process you fork means
another data segment, another stack segment, which means lots of
memory. Every process you fork also means that concurrency is achieved
only by context switching, which means loads of expense on changing
MMU state and more. Even thread switching is orders of magnitude worse
than a procedure call. Invoking an event is essentially just a
procedure call, so that wins big time.

Event driven systems can also avoid locking if you keep global data
structures to a minimum, in a way you really can't manage well with
threaded systems. That makes it easier to write correct code.

The price you pay is that you have to think in terms of events, and
few programmers have been trained that way.

Perry
-- 
Perry E. Metzger		perry at piermont.com
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