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Why don't computers have more than 2 processors?

Shake 'n' Bake

Shake 'n' Bake

macrumors 68020
Original poster
Mar 2, 2009
2,186
2
Albany
Not sure if this is the right place, but here goes:

Why don't computers have more than 2 processors? Instead of 2 quad core processors, why not 8 1 core? I know that would be expensive.

The Power Mac G4 had dual single core, so why aren't there quad or more single core processors?

If any computers have ever had more than 2 processors, let me know about them, please.

Any insight is appreciated!
 
Because that would be about four times as expensive and nothing uses multiple cores right now.

It doesn't affect the average consumer, they wouldn't notice the extra power, and it would be more expensive. You can't explain price to people who don't need power.
 
Shake 'n' Bake said:
Not sure if this is the right place, but here goes:

Why don't computers have more than 2 processors? Instead of 2 quad core processors, why not 8 1 core? I know that would be expensive.

The Power Mac G4 had dual single core, so why aren't there quad or more single core processors?

If any computers have ever had more than 2 processors, let me know about them, please.

Any insight is appreciated!
Click to expand...

Many high-end workstations and rack-mount systems have 4 sockets. I've used a bunch in the past. AMD chips I designed, for example, were designed to work with up to 8 sockets.

It's always better to have multiple cores than an equivalent number of cores spread across sockets. First, information travels at approximately 6ps/mm in semiconductors (this is a rough estimate - it varies depending on substrate, dielectric, etc.). So multiple sockets increases the communication delay from one core to the other, because the cores are spread further apart.

Multiple sockets are also more expensive - bigger motherboards, more wires (more RF problems, etc.), more bypass capacitors, etc.

Also, you burn more power because you replicate circuitry instead of sharing it among multiple cores.
 
I think it's cuz of efficiency and production costs. But some computers have more than two. I had a DELL PowerEdge with 4 xeons in it. :)
 
Plenty of computers have more than 2 processors per board. Look at the Intel 7000 series Xeons and AMD 8000 series Opterons. Also look at Sun and IBM's enterprise product pages online.

Multiple cores are the most efficent way to improve processing power, then multiple core processors in multiple sockets then multiple boards (afaik).
 
There are many really expensive servers with 64 or more processors in a single OS image. Just look at Sun, SGI, and Cray, for example.
 
Those are some good reasons.

Its cool that some computers have more than 2. I've just always wanted to know that.

I just thought of another reason, by the way: Heat. It would get very hot without a good cooling system.
 
Shake 'n' Bake said:
Those are some good reasons.

Its cool that some computers have more than 2. I've just always wanted to know that.

I just thought of another reason, by the way: Heat. It would get very hot without a good cooling system.
Click to expand...

Sorry - as an engineer I forgot that people don't always understand that "burning power" means "heat." :)

There's actually an ADVANTAGE to multiple cpus, re: heat, however.

The ability to get rid of heat is based not on the watts, but on the watts/area. By spreading cores across multiple chips, it's actually EASIER to get rid of the excess heat.

Of course, the amount of heat to get rid of also goes up due to replication of circuitry.

However, overall, if you are worried about thermal dissipation, you probably prefer to spread out the chips. That's why nehalem has "turbo mode." It can't always just run at the higher speed because of heat. If you took the nehalem cores and spread them onto 4 chips, you wouldn't have THAT problem (you'd have others :)
 
IMO then should only put 3+ cores in if they are all maxed out 8 Cores, and they just need it for expandability.

For even the Hardcore user, an Octo2.93 Mac Pro would more than suffice.
 
In addition to the distance beteen them being slower than being on the same die, I suspect it makes it more expensive to share cache with more discreet processors. Or, with separate cache, maintaining cache coherrancy (sp).

Even for workstation class systems like the Mac Pro, it probably isn't worth the expense to go up to a motherboard that can hold more than 2 processors, plus I think that the current Xeons the Mac uses are limited to 2 physical processors.
 
kevink2 said:
In addition to the distance beteen them being slower than being on the same die, I suspect it makes it more expensive to share cache with more discreet processors. Or, with separate cache, maintaining cache coherrancy (sp).

Even for workstation class systems like the Mac Pro, it probably isn't worth the expense to go up to a motherboard that can hold more than 2 processors, plus I think that the current Xeons the Mac uses are limited to 2 physical processors.
Click to expand...

Cache coherency is a whole other issue. In some cases it's better to share caches, but often not, since processes generally don't share memory (instruction memory is a different situation, but since instruction memory isn't modified, the coherency problem goes away).
 
cmaier said:
Cache coherency is a whole other issue. In some cases it's better to share caches, but often not, since processes generally don't share memory (instruction memory is a different situation, but since instruction memory isn't modified, the coherency problem goes away).
Click to expand...

I can think of cases where it matters. Cases where it doesn't matter. I think it all ends up relating to plusses and minuses of whether to buy the 4 core or 8 core Mac Pro, hyperthreading, what benefits Snow Leopard gives to multicores, etc.

Different physical chips probably has the edge for running different processes, since on a given CPU you aren't contending with a different process's requirements. But a multi-threaded application with a shared memory space probably works better on a multi-core system, especially if they share some data.
 
BlizzardBomb said:
Umm... What?
Click to expand...

Generalization; I meant to change it, sorry.

On the consumer side of things, software that uses multiple cores/processors is a rarity. The rest of my argument was based on the consumer perception of a computer's hardware, and thus it made sense in the context of what I said.

Final Cut Studio 3, Logic Studio 2, Phenomenon, and CS5 will most definitely be multithreaded, so you'll want what cores you can get, but other than HandBrake, you're not going to find consumer software that uses eight cores. :D
 
About 13-14 years ago, there was an option for a Mac (I think it was a clone) with 4 CPUs (if I'm right, these were 150 Mhz CPUs) ...
 
kevink2 said:
I can think of cases where it matters. Cases where it doesn't matter. I think it all ends up relating to plusses and minuses of whether to buy the 4 core or 8 core Mac Pro, hyperthreading, what benefits Snow Leopard gives to multicores, etc.

Different physical chips probably has the edge for running different processes, since on a given CPU you aren't contending with a different process's requirements. But a multi-threaded application with a shared memory space probably works better on a multi-core system, especially if they share some data.
Click to expand...

What you refer to is shared vs. independent caches, not multi-core vs. multi-socket.

Whether a cache should be shared or not is a function of: the size of the cache, the penalty for accessing the cache, the penalty paid when needed data is not in the cache, whether the cache is for data, instructions, or both, whether the cache has higher affinity with some memory than other memory, the type of software being used, etc.


For example, the last chip I worked on was a multi-core chip, which was designed so that you could have multiple sockets without glue logic chips. In other words, pretty much what you see in the MP's.

Each core had its own, unshared, L1 instruction cache, and a separate unshared L1 data cache.

All of the cores on each chip shared a single L2 cache for data and instructions.

All of the chips shared a single, external, L3 cache.

We traded off memory access time, vs. chip size/power/clock rate, vs. number of pins, vs. time-to-market, vs. etc.
 
Tallest Skil said:
Generalization; I meant to change it, sorry.

On the consumer side of things, software that uses multiple cores/processors is a rarity. The rest of my argument was based on the consumer perception of a computer's hardware, and thus it made sense in the context of what I said.

Final Cut Studio 3, Logic Studio 2, Phenomenon, and CS5 will most definitely be multithreaded, so you'll want what cores you can get, but other than HandBrake, you're not going to find consumer software that uses eight cores. :D
Click to expand...

Fair enough. But I'm pretty sure most new games use 2 cores as well as other bits and bobs out there for consumers. That and if you're running several things at the same time, multiple cores will obviously help there.
 
Shake 'n' Bake said:
Not sure if this is the right place, but here goes:

Why don't computers have more than 2 processors? Instead of 2 quad core processors, why not 8 1 core? I know that would be expensive.

The Power Mac G4 had dual single core, so why aren't there quad or more single core processors?

If any computers have ever had more than 2 processors, let me know about them, please.

Any insight is appreciated!
Click to expand...

These do exist. Most people have no need for even 8 cores.

Quad socket motherboard are easy enough to find though.
 
Tallest Skil said:
Because that would be about four times as expensive and nothing uses multiple cores right now.

It doesn't affect the average consumer, they wouldn't notice the extra power, and it would be more expensive. You can't explain price to people who don't need power.
...
On the consumer side of things, software that uses multiple cores/processors is a rarity.
Click to expand...

That is pure ignorance, every application for OSX will use at least 2 cores. For some 19 year old claiming to have bought a $5,000 Mac Pro, you should know that.
 
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