What is the advantage of a bigger cache?

[thermg]

Hi,

I am looking at buying a P4 dual core and wondered what difference in performance the 2MB cache for each core offered over the 1MB cache as in the case of the 805? Is there any benefit in having the extra cache?

Cheers

 

[BillytheImpaler]

I'm oversimplifying but when considering computer memory it is important to remember that you want as much as you can have as fast as you can possibly have it. In the mid 1990s on-die cache became commonplace. On-die cache is memory that is on the CPU. Since it is so close the CPU can use cache memory extremely quickly. If you are running some bit of code and it does not fit in the CPU cache, it has to go to the next level down, the main memory. Accessing this is very much sower than accessing the on die cache. If somehting does not fit in all the system memory it goes to the paging file on a hard disk. This is much slower than accessing system memory.

Having more L2 cache makes the computer faster when running code that can fit in it. The machine does not have to access main memory as frequently and thus does not have to wait for the signals to be sent, recognized, and recieved.

Some applications, especially DC apps and some benchmarks, are extremely fond on on-die cache. I find that more cache means much faster crunching. Many games, however, are not vastly improved by having larger L2 cache. In general having more cache is a very good thing.

I would definitely pay a small premium to get an extra 1024 KiB of L2 cache.

 

[Jimbo Mahoney]

I've typically found ~5% increase in performance from 1MB cache vs 512kB.

L2 cache is nice

 

[sablabra]

I've typically found ~5% increase in performance from 1MB cache vs 512kB.

L2 cache is nice

AMD? Intel's current prescott ext. arcitecture is a lot more dependant on L2 cache than A64

 

[daveydave]

mate get the 805 its cheap and fast jus make sure your running 2gb ram, im waiting on my extra gigabyte to turn up this week hopefully

 

[Jimbo Mahoney]

AMD?

yah

 

[juno_first]

I thought too much cache defeated the point, as in the CPU can only work wth small amounts of data quickly, so a lot of waiting calculations in the cache can be counter productive.

Am I right or is this old school thinking ?

 

[m3csl2004]

amd (sorry - no intel ones to hand) 754 cpus clocked at the same speed

linked for uberness

 

[BillytheImpaler]

I thought too much cache defeated the point, as in the CPU can only work wth small amounts of data quickly, so a lot of waiting calculations in the cache can be counter productive.

Am I right or is this old school thinking ?

Think of it like a notebook on a desktop. You're working some mathematical equations on the notebook and when you get an answer you write it down at the top of the page so that when you need it next you have it close at hand. You're doing a lot of equations and the order in which you do them is not necessarily the order in which you need to re-use the outputs you got earlier. You eventually run out of space at the top of the page and have to start writing on your desk in pencil. It takes you longer to lean over to read and write to the desk when compared to the notebook itself.

In this (weak) analogy the equation on which you're working os the L1 cache. It's usually pretty small as it's for the work that is most at hand. Beyond that is the L2 cache, the top of the paper. It is still close at hand and the more space you have here the less leaning over you need to do to the desk, system memory.

 

[juno_first]

Ahh, I get it now, thanks (maybe its best to have small amounts of L1 S RAM ?).

 

[Jimbo Mahoney]

amd (sorry - no intel ones to hand) 754 cpus clocked at the same speed

linked for uberness

Very nice

 

[joeyjojo]

linked for uberness

Good link. Surprised by the result - very little difference in synthetic benchmarks, but 4-7 fps in Doom3, nice.

 

[m3csl2004]

Ahh, I get it now, thanks (maybe its best to have small amounts of L1 S RAM ?).

all amd cpus have 128k of l1 cache - mainly why the difference in l2 cache doesnt make a big difference

intel dont have much l1 cache from memory