L2 Cache, System Bus and Video Memory

redchannel · Feb 11, 2009

I'm about to purchase my first mac, and im still debating between the
15-inch: 2.4GHz and 15-inch: 2.53GHz models. I'm not exactly sure on how to justify the 2499 model. What is L2 cache and how does having twice the amount of it make a difference on the system ? Is there a big difference between the 2.4 and 2.53 dual cores ?

"NVIDIA GeForce 9600M GT graphics processor; and NVIDIA GeForce 9400M graphics processor with 256MB of DDR3 SDRAM shared with main memory"

- Does this mean that is comes with two video cards ?

Techguy172 · Feb 11, 2009

It is good to have more cache but it doesn't make that big of a difference. It sounds like the 2.4 is best for you. Yes they both come with 2 graphics cards one is integrated(9400M) the Other being dedicated(9600M GT). The clock speed difference between 2.53 and 2.4 is not too much of a difference and generally not justifiable by the price. Unless your a big gamer get the 2.4.

redchannel · Feb 11, 2009

I am a big gamer but i dont game on the PC

. I just wanna make sure I'm getting a good system that will last a long time and perform at a great deal. What is Cache BTW ?

J the Ninja · Feb 11, 2009

redchannel said:
I am a big gamer but i dont game on the PC . I just wanna make sure I'm getting a good system that will last a long time and perform at a great deal. What is Cache BTW ?

L2 Cache is a small of amount high-speed RAM that is directly attached to the CPU cores themselves ("on-die" is the term). It's much faster to access than the normal RAM, due to both the higher speed, more direct link, and shorter distance from the processor. (When things are spinning two and half billion times a second, the time it takes an electrical signal to go 4 inches counts). So, the CPU uses this to keep data it needs close by. The idea is, the more you have, the less you have to change things in and out of the main RAM, and work gets done faster.

In all honesty though, the high-end 15" MBP isn't worth it in my mind. The extra L2 doesn't count for all that much, neither does the extra 256MB of video memory. The 4GB of RAM and the bigger HDD are nice, but those can be had elsewhere for a lot less than $500. (as in, around $150-$200). Other than that, there is really no difference. My advice, get the 2.4 model. It's just a plain better deal, especially in the long run. (those RAM and HDD upgrades will get even cheaper with time, and the extra L2 and vRAM will mean less and less in the grand scheme of things over the next few years).

redchannel · Feb 11, 2009

J the Ninja said:
L2 Cache is a small of amount high-speed RAM that is directly attached to the CPU cores themselves ("on-die" is the term). It's much faster to access than the normal RAM, due to both the higher speed, more direct link, and shorter distance from the processor. (When things are spinning two and half billion times a second, the time it takes an electrical signal to go 4 inches counts). So, the CPU uses this to keep data it needs close by. The idea is, the more you have, the less you have to change things in and out of the main RAM, and work gets done faster.

In all honesty though, the high-end 15" MBP isn't worth it in my mind. The extra L2 doesn't count for all that much, neither does the extra 256MB of video memory. The 4GB of RAM and the bigger HDD are nice, but those can be had elsewhere for a lot less than $500. (as in, around $150-$200). Other than that, there is really no difference. My advice, get the 2.4 model. It's just a plain better deal, especially in the long run. (those RAM and HDD upgrades will get even cheaper with time, and the extra L2 and vRAM will mean less and less in the grand scheme of things over the next few years).

So are you suggesting that cache is a RAM assistant or RAM for the RAM

?

Techguy172 · Feb 11, 2009

redchannel said:
So are you suggesting that cache is a RAM assistant or RAM for the RAM ?

What it does is sits right beside the CPU, and the CPU store instructions there until it is needed however it is much faster than ram because it runs 1:1 with the processor meaning they are the same speed, So instead of going to the ram which will take long/be slower.

redchannel · Feb 11, 2009

Techguy172 said:
What it does is sits right beside the CPU, and the CPU store instructions there until it is needed however it is much faster than ram because it runs 1:1 with the processor meaning they are the same speed, So instead of going to the ram which will take long/be slower.

OKay, i guess i kind of understand that

. So is there really a noticable diffrence between 3MB and 6MB shared L2 Cache then ?

Eidorian · Feb 11, 2009

redchannel said:
OKay, i guess i kind of understand that . So is there really a noticable diffrence between 3MB and 6MB shared L2 Cache then ?

You get diminishing returns in performance above ~3 MB of L2 cache.

For hte most part games are GPU bound today.

Amdahl · Feb 11, 2009

Eidorian said:
You get diminishing returns in performance above ~3 MB of L2 cache.

For hte most part games are GPU bound today.

It depends on the size of the problem. Databases love cache. I also would guess that encoding 1080P video would benefit, unless those codecs work exclusively on smaller tiles. 1920*1080*4-bytes = 8,294,400 bytes. Multiply that by two (you've got to be comparing two frames to each other), and you've got 16MB of data that is accessed repeatedly to generate one frame delta.

Encoding likely works on smaller tile sizes, so the scenario is probably not so dire as needing 16MB cache to get good performance, but you can see why certain levels of cache are needed for certain types of work.

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