Re: How to develop a random number generation device

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.


I'd be happy to waste a little silicon if I could have an OS that
doesn't crash and that doesn't go to sleep for seconds at a time for
no obvious reason.

Multiple cores gives absolutely no benefits in terms of reliability or stability - indeed, it opens all sorts of possibilities for hard-to-debug race conditions.
They don't if you insist on running a copy of a bloated OS on each. A
system designed, from scratch, to run on a pool of cheap CPUs could be
incredibly reliable.

That's a conjecture plucked out of thin air. Of course a dedicated OS designed to be limited but highly reliable is going to be more reliable than a large general-purpose OS that must run on all hardware and support all sorts of software - but that has absolutely nothing to do with the number of cores!

Programmers have pretty much proven that they cannot write bug-free
large systems. Unless there's some serious breakthrough - which is
really prohibited by the culture - the answer is to have the hardware,
which people *do* routinely get right, take over most of the functions
that an OS now performs. One simple way to do that is to have a CPU
per process. It's going to happen.


Do you have any hints of a suggestion that anyone else thinks this is the case?

When I was just a sprout, my old mentor Melvin Goldstein told me "in
these integrated circuit things, one day transistors could cost a
penny each." I thought he was crazy. OK, one day CPUs will cost 5
cents each, and Windows is not the ultimate destiny of computing.

Hey, he wrote a book!

http://www.amazon.com/Physics-Foibles-physics-computer-students/dp/1553957768


John


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