[erlang-questions] Millions of processes?

Bengt Kleberg <>
Wed Sep 24 12:23:55 CEST 2008


Greetings,

It is not only _ideas_ in computer science that are held in contempt (as
quoted below). Ages ago (before 2000) I read an article about Linux in
embedded environments. The article quoted Linus Torvalds on why not to
use micro kernels. The reasons where that they are:
1 Experimental
2 Complex
3 Slow

After looking around for a while I found plenty of articles about
commercial micro kernels, and benchmarks showing micro kernels running
workloads faster than monolithic kernels. So 1 and 3 seemed to be
incorrect.
I submitted these findings to the magazine, which prompted an answer
from Mr Torvalds. He assert that all three where true, but did not
discuss what I had found. So (IMHO) it is also facts that are not held
in very high regard.


bengt
 
On Tue, 2008-09-23 at 19:09 +0200, Ulf Wiger (TN/EAB) wrote:
> NPTL is fast, but AFAIK uses a minimum stack size of 8KB
> per thread (the minimum heap size for erlang processes
> seems to be 932 bytes on a 32-bit system). There also seem
> to be other limits, making it very difficult in practice
> to reach anywhere near 100,000 threads, and it's not
> encouraged either.
> 
> http://nptl.bullopensource.org/Tests/NPTL-limits.html
> 
> In the Linux kernel FAQ, the philosophy on threads is
> explained thus:
> 
> "Avoid the temptation to create large numbers of threads in your
> application. Threads should only be used to take advantage of multiple
> processors or for specialised applications (i.e. low-latency real-time),
> not as a way of avoiding programmer effort (writing a state machine or
> an event callback system is quite easy). A good rule of thumb is to have
> up to 1.5 threads per processor and/or one thread per RT input stream.
> On a single processor system, a normal application would have at most
> two threads, over 10 threads is seriously flawed and hundreds or
> thousands of threads is progressively more insane.
> A common request is to modify the Linux scheduler to better handle large
> numbers of running processes/threads. This is always rejected by the
> kernel developer community because it is, frankly, stupid to have large
> numbers of threads. Many noted and respected people will extol the
> virtues of large numbers of threads. They are wrong. Some languages and
> toolkits create a thread for each object, because it fits into a
> particular ideology. A thread per object may be appealing in the
> abstract, but is in fact inefficient in the real world. Linux is not a
> good computer science project. It is, however, good engineering.
> Understand the distinction, and you will understand why many widely
> acclaimed ideas in computer science are held with contempt in the Linux
> kernel developer community. "
> 
> http://www.kernel.org/pub/linux/docs/lkml/#s7-21
> 
> BR,
> Ulf W
> 
> 
> Zvi skrev:
> > I'm no Linux expert, but
> > 
> > http://en.wikipedia.org/wiki/Native_POSIX_Thread_Library
> > 
> > "The Native POSIX Thread Library (NPTL) is a software feature that
> > enables the Linux kernel to run programs written to use POSIX Threads
> > fairly efficiently. In tests, NPTL succeeded in starting 100,000
> > threads on a IA-32 in two seconds. In comparison, this test under a
> > kernel without NPTL would have taken around 15 minutes."
> > 
> > I guess future Erlang VM will offer some more generic MxN threading
> > model, i.e. M Erlang user-level processes implemented on N
> > "schedulers" - native threads. Today in SMP Erlang is only limited
> > support (i.e. command line options) to specify number o scheduler and
> > no programmatic support for affinity of schedulers per core and
> > Erlang processes per schedulers.
> > 
> > Zvi
> > 
> > 
> > Bob Ippolito wrote:
> >> We've got a couple applications that use thousands of processes per
> >>  node. If those were pthreads, we'd be out of RAM before actually
> >> doing anything.
> >> 
> >> 2008/9/23 Bard Bloom <>:
> >>> I've seen in Erlang promotional materials some rather impressive
> >>> claims about how cheap Erlang processes are, and how many of them
> >>> one can spawn. Which is pretty cool. But, what Erlang programs
> >>> take advantage of that kind of power? Are there any examples of
> >>> programs which use huge numbers of processes in interesting ways?
> >>> (I am the local Erlang fancier. I got challenged on that point,
> >>> and didn't have a very good answer.)
> >>> 
> >>> Thanks very much, Bard Bloom
> >>> 
> >>> _______________________________________________ erlang-questions
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> >>> 
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> >> 
> > 
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