Re: How to develop a random number generation device

On Tue, 18 Sep 2007 18:15:07 +0200, David Brown
<david.brown@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Mon, 17 Sep 2007 23:04:03 +0200, David Brown
<david.brown@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Mon, 17 Sep 2007 18:40:35 +0200, David Brown
<david.brown@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Sun, 16 Sep 2007 22:07:42 +0200, David Brown
<david.brown@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
On Sun, 16 Sep 2007 11:33:21 -0700, MooseFET <kensmith@xxxxxxxxx>
wrote:

On Sep 15, 11:09 am, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
[....]
architecture. In a few years we'll have, say, 1024 processors on a
chip, and something new will be required to manage them. It will be a
thousand times simpler and more reliable than Windows.
I think that the number of virtual cores will grow faster than the
number fo real cores. With extra register banks and a bit of clever
design, a single ALU can look like two slightly slower ones.

I expect to see multicore machines with less actual floating point
ALUs than actual integer ALUs.

Sounds sort of like Sun's Niagra chips, which have (IIRC) 8 cores, each
with 4 threads, but only a few floating point units. For things like
web serving, it's ideal.

Yup. Low-horsepower tasks can just be a thread on a multithread core,
and many little tasks don't need a dedicated floating-point unit.

My point/fantasy is that OS design should change radically if many,
many real or virtual CPUs are available. One CPU would be the manager,
and every task, process, or driver could have its own, totally
confined and protected, CPU, and there would be no context switching
ever, and few interrupts in fact.

That's not going to work for Linux, anyway - there is a utility thread
spawned per cpu at the moment (work is underway to avoid this, because
it is a bit of a pain when you have thousands of cpus in one box).

However, there is no point in having a cpu (or even a virtual cpu)
dedicated to each task. Many sorts of tasks spend a lot of time
sleeping while waiting for other events - a cpu in this state is a waste
of resources.
Only if you think of a CPU as a valuable resource. As silicon shrinks,
a CPU becomes a minor bit of real estate. It makes sense to use it
when there's something to do, and put it to sleep when there's not.
Lots of power gets saved by not doing context switches.

CPUs *are* a valuable resource - modern cpu cores take up a lot of
space, even when you exclude things like the cache (which take more
space, but cost less per mm^2 since you can design in a bit of
redundancy and thus tolerate some faults).

The more CPUs you have, the more time and space it costs to keep caches
and memory accesses coherent. There are some sorts of architectures
which work well with multiple CPU cores, but these are not suitable for
general purpose computing.

My point is that large numbers of CPU cores *will* become common and
cheap, and we need a new type of OS to take advantage of this new
reality. Done right, it could be simple and astoundingly secure and
reliable.

I would be very surprised to see a system where the number of CPU cores
was greater than the number of processes. I expect to see the number of
cores increase, especially for server systems, but I don't expect to see
systems where it is planned and expected that most cores will sleep most
of the time.

Well, I remember 64-bit static rams, and 256-bit DRAMS. I can't see
any reason we couldn't have 256 or 1024 cpu's on a chip, especially if
a lot of them are simple integer RISC machines.

You can certainly get 1024 CPUs on a chip - there are chips available
today with hundreds of cores. But there are big questions about what
you can do with such a device - they are specialised systems. To make
use of something like that - you'd need a highly parallel problem (most
desktop applications have trouble making good use of two cores - and it
takes a really big web site or mail gateway to scale well beyond about
16 cores). You also have to consider the bandwidth to feed these cores,
and be careful that there are no memory conflicts (since cache coherency
does not scale well enough).

No, no, NO. You seem to be assuming that we'd use multiple cores the
way Windows would use multiple cores. I'm not talking about solving
big math problems; I'm talking about assigning one core to be a disk
controller, one to do an Ethernet/stack interface, one to be a printer
driver, one to be the GUI, one to run each user application, and one
to be the system manager, the true tiny kernal and nothing else.
Everything is dynamically loadable, unloadable, and restartable. If a
core is underemployed, it sleeps or runs slower; who cares if
transistors are wasted? This would not be a specialized system, it
would be a perfectly general OS with applications, but no process
would hog the machine, no process could crash anything else, and it
would be fundamentally reliable.


That would be an absurd setup. There is some justification for wanting
multiple simple cores in server systems (hence the Sun Niagara chips),
but not for a desktop system. The requirements for a disk controller, a
browser, and Doom are totally different. With a few fast cores like
today's machines, combined with dedicated hardware (on the graphics
card), you get a pretty good system that can handle any of these. With
your system, you'd get a chip with a couple of cores running flat out
(but without a hope of competing with a ten year old PC, as they could
not have comparable bandwidth, cache, or computing resources in each
core), along with a few hundred cores doing practically nothing. In
fact, most of the cores would *never* be used - they are only there in
case someone wants to do a few extra things at the same time since you
need a core per process.

This is not about performance; hardly anybody needs gigaflops. It's
all about reliability.

Until you can come up with some sort of justification, however vague, as
to why you think one cpu per process is more reliable than context
switches, this whole discussion is useless.

You define yourself by the ideas you refuse to consider. So I suppose
you'll still be running Windows 20 years from now.

John


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