Increasing L1 Cache

[bassboy_pete]

Hey guys,

I keep seeing more proccessors adding more and more L2 Cahce. But what about L1 Cache? Would it not be benificial to increase it more? The way I understand it is that L1 is faster memory than the L2. So increasing L1 should speed up the proccesser right? I was looking at Toms Hardware CPU History Chart and saw that AMD hasn't increased the L1 cache since the original athlon. I just don't understand how Intels L1 works with the KuOps. So I can't complain about them. But seriously, whats going on? Why wouldn't they increase it?

 

[BillLeeLee]

L1 cache, I believe is made of the most expensive type of static RAM available, and if you were to increase the L1 cache, you'd be paying a lot more for the processor. The high cost of the RAM used in the L1 cache is part of the reason why L2 cache was created way back when.

 

[EquaLiZr]

so how much faster would the cpu actually be if they increased the L1 cache to say 256 or 512kb? Would it double the cpu power?

 

[acra2]

You'll find the reason isn't to do with actually money expense (well it is a little) but mostly to do with space and power comsumption on the silicon die itself. It gets waaaaay complex, but if you get a picture and look at how a die is laid out and find out why certain parts of the processor have to close to each other in order to get high throughput it would help you understand.

 

[intercollector]

L1 cache is basically as close to a register as you can get, and as bassboy_pete mentioned, is very expensive to implement due to many different fabrication complexities.

Increasing the amount of cache is actually considered an architectural hack anyways. A much more elegant solution is to increase the hit/miss ratio of L1 to L2 cache since the speed difference is basically the difference between using the telephone to call someone and sending something in the mail.

 

[emorphien]

Originally posted by EquaLiZr
so how much faster would the cpu actually be if they increased the L1 cache to say 256 or 512kb? Would it double the cpu power?

Have you looked at the actual amout of L1 Cache on most processors? 256KB of cache would be 32 times what a Northwood pentium 4 currently has (edit: data cache). Either way its a lot more cache.

There's just no way to put that much L1 cache on a processor, it would boost performance but almost certainly not be worth the cost. After a point it may hurt more than it helps anyway.

 

[jpmkm]

Cache memory is typically made with associative memory, which is very fucking fast(simultaneous comparator and no refresh cycle needed), but also very complex(silicon-wise) and therefore EXPENSIVE. intercollector hit the nail on the head - cache is essentially a hack that developed out of the desire for (relatively) cheap memory while still upping the processor speed. If you want to know what increased cache does to the price of a chip, compare xeons and pentiums. Xeons are basically pentiums with bigger L2 caches. There is quite a performance boost(depending on what you are doing) but they are considerably more expensive. In most cases more cache would only lower the priceerformance ratio. Increasing L1 caches probably will not happen very soon, at least not for consumer general purpose cpus.

 

[USMC2Hard4U]

I think that Intel needs to increase some cache amounts on some upcomming processors.

Right now the prescott stands at 16K L1 Data Trace cache. In their next processor say the Pentium 5 or whatver, they should double it again to 32K...

AMD Athlon 64 is a 128K L1 Cache but there cache is slower or something compared to intels... so i think they may be able to double that in the future with less of a price bump.

 

[Mad_Pyro]

Yeah - Intel P4EE vs regular P4?

 

[Black Morty Rackham]

Increasing the amount of L1 cache would probably increase the latency. That would be rather not good.

 

[Terminus]

good thread, i always wanted to know how all the on-die cache works on CPUs

 

[scientificTHEgreat]

cost=m*n

 

[jpmkm]

Originally posted by Black Morty Rackham
Increasing the amount of L1 cache would probably increase the latency. That would be rather not good.

Maybe very very very slightly due to more transistors and more traces, but not due to cache lookup. Cache uses associative memory, which can do a simultaneous compare of all the cells(it doesn't really use addresses in the typical form of addressing. it just stores a bunch of data in a lot of different blocks and then compares the addresses to those blocks). So it shouldn't really increase latency by any noticable amount.

 

[Black Morty Rackham]

Originally posted by jpmkm
Maybe very very very slightly due to more transistors and more traces, but not due to cache lookup. Cache uses associative memory, which can do a simultaneous compare of all the cells(it doesn't really use addresses in the typical form of addressing. it just stores a bunch of data in a lot of different blocks and then compares the addresses to those blocks). So it shouldn't really increase latency by any noticable amount.

Ahh, well, nevermind that then.

It would still make the processor consume more power (and make it hotter), though... and the gains might not be all that good.

 

[darktiger]

I would think it would cost way to much money. Just think instead of 100 for a processor it would be like 200(maybe not that much), but a lot more.

 

[Black Morty Rackham]

Originally posted by darktiger
I would think it would cost way to much money. Just think instead of 100 for a processor it would be like 200(maybe not that much), but a lot more.

The P4 EE has two megs of L3 cache and costs, what, ~$900?