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CPU Question about L1 and L2Cache (AMD specific)

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July 20, 2006 2:49:34 AM

Come Monday, when the price cuts arrive,I am going to purchase a Athlon x2.

A question though: What is the purpose of L1 and L2 cache?

I've looked are various CPU's and they all vary:

some are 128kb + 128kb
others are: 512 x 2 (see this on L2)
others are: 1mb x 2

Can someone help me out and explain what they do exactly?

Since im buying a AM2 cpu, i've noticed only one of the CPU's as a 1mb x2 for the L2 cache.

Appreciate it.
July 20, 2006 4:23:30 AM

For L1 (Primary cache/Inside processor/Instruction Cache),
L2 (Secondary cache/between processor and main memory/Data Cache)
July 20, 2006 5:08:17 AM

The access time for the CPU core to get data from the L1 and L2 caches is very quick - typically just a few clock cycles, versus 20-30 clock cycles from main memory and <<an eternity>> from harddrive.

For this reason very very commonly used data (for example the "working set" data that is being computed upon) and/or decoded instructions are kelp first in L1 and then in L2 to save time for CPU when it needs to access that data.

Generally speaking, all things being equal, more cache gives you a faster CPU, but the effects are quite small.

AMD says they are worth about 200 PR points, IE a 4200+ cpu has 2 x 512 kb, while the identically clocked 4400+ has 2 x 1 megs - ie AMD suggests the extra cache only adds +/- 5% in performance

This 5% meshes pretty well with real world testing.

There are exceptions, but fairly few. For example doing a Fast Forrier Transform (or other itterative calculation) that fit in the 2 x 1 megs caches, but did NOT fit in 2 x 512 cache migh yield a rather dramatic difference in speeds, but again this is the exception, not the rule.

AMD chips, by virtue of their integrated memory controllers, are less sensitive to cache size since they access main memory very quickly.

Intel uses much larger caches (2 x 2 megs or 1 x 4 megs) to compensate for their somewhat slower northbridge based memory controllers.
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July 20, 2006 5:49:05 AM

Quote:
Come Monday, when the price cuts arrive,I am going to purchase a Athlon x2.

A question though: What is the purpose of L1 and L2 cache?

I've looked are various CPU's and they all vary:

some are 128kb + 128kb
others are: 512 x 2 (see this on L2)
others are: 1mb x 2

Can someone help me out and explain what they do exactly?

Since im buying a AM2 cpu, i've noticed only one of the CPU's as a 1mb x2 for the L2 cache.

Appreciate it.


L2 cache is the amount of information your system can hold waiting to be processed. It's all relavent to the other parts which compose your computer and your computer's ability to process. It's all about bigger, faster, dual, quad, etc., etc.... AND STYLE AND TECHNIQUE OF PROCESSING. Intel and AMD use different methods, styles and techniques of processing if you will (Ha, his should get some responses). For the past few years AMD has the superior system and reached up and punched Intel in the nose. Now Intel has Conroe. But that'll be history in about 9 months. That's a fact. Pretty simple. I'm sure someone will add there 2 cents.
July 20, 2006 8:00:42 AM

Quote:
Come Monday, when the price cuts arrive,I am going to purchase a Athlon x2.

A question though: What is the purpose of L1 and L2 cache?

I've looked are various CPU's and they all vary:

some are 128kb + 128kb
others are: 512 x 2 (see this on L2)
others are: 1mb x 2

Can someone help me out and explain what they do exactly?

Since im buying a AM2 cpu, i've noticed only one of the CPU's as a 1mb x2 for the L2 cache.

Appreciate it.
The L1/L2 cache are for (all intents and purposes) the same as RAM. The L1 cache gets the data/instructions first. They are then handed off to the L2 cache if they are too big for the L1. L1 cache is the fastest cache...therefore more expensive...therefore less capacity. L2 is slower than L1, but of course, cheaper...therefore more capacity. AMD has 64KB + 64KB of L1 vs Conroe's 32KB + 32KB L1. Conroe's L1 is 8-way associative vs K8's 2-way associativity(higher associativity is faster) so both end up closer in performance than the #'s would suggest.
July 20, 2006 5:01:21 PM

Quote:
would it be wise to look for a AM2 cpu that has 1mb x 2 on the L2 cache then?

Here is my mobo and RAM:

http://www.newegg.com/Product/Product.asp?Item=N82E1681...

http://www.newegg.com/Product/Product.asp?Item=N82E1682...


Now, its just a matter of grabbing a sweet AM2 cpu, for cheapo!


AMD discontinued 1MBx2 cache desktop CPU's to focus more on budget chips...

That would be I only see one CPU that has 1MBx2: the 4000+Just get a 2x512KB processor. The extra 512 doesn't warrant the extra cost.
July 20, 2006 5:49:24 PM

L1 cache never changes for most processors of the same line. L2 cache makes a difference in performance because its storage space on the processor hence "cache." As a general rule more cache usually = better performance. This is why more people buy the x2 4400+ and overclock it instead of buy the x2 4600+
a b à CPUs
July 20, 2006 6:04:38 PM

Quote:
would it be wise to look for a AM2 cpu that has 1mb x 2 on the L2 cache then?

Here is my mobo and RAM:

http://www.newegg.com/Product/Product.asp?Item=N82E1681...

http://www.newegg.com/Product/Product.asp?Item=N82E1682...


Now, its just a matter of grabbing a sweet AM2 cpu, for cheapo!


AMD discontinued 1MBx2 cache desktop CPU's to focus more on budget chips...

That would be I only see one CPU that has 1MBx2: the 4000+

I have three AMD chips curently running with 1024 L2 Cache or greater. I have 2 AMD's I use currently using 512 x 2 L2 cache. None are of the AM2 variety

AMD 4800+ X2 1024 x 2 L2 Cache
AMD 64 3700 2.4 1024 L2 Cache socket 754
AMD 64 3400 2.2 1024 L2 Cache socket 754 Clawhammer

AMD 64 3400 2.4 512 L2 Cache socket 754 I have 2 of these running

I use these five computer everyday depending one which office I am in. The 4800x2 is in my home. Sitting at the computer and using it for things like word processing, excel, surfing the net, etc. you can not tell the difference in the L2 cache, 512 compared to 1024 meg. I do think when playing games or using other large software applications, I use Pinacle Studios 9 for video editing, the 1024 L2 cache processors perform somewhat better. I wouldn't know how to measure these things. Just my opinion. Like one of the prior posters mentyioned, Intel has larger L2 Caches on their latest processors. AMD 64 processors have the RAM controller on the processor, eliminating the northbridge chip for the most part. Intel uses the northbridge to run between the RAM and the processor. There are guys that will tell you this method is junk. I have four Intel processors running everyday. The latest, a Pentium D 805 Smithfield, which has 2 x 1024 L2 cache runs very nice. I think it depends on the processor itslf how well it runs with or without more L2 cache. AMD's 64 processors work fantastic to this point. I would think getting an AM2 with 512 x 2 (x2) should do you well. Then you look at Intels and they have stuff with 2048 x 2 out there. Perhaps RAM speeds of 800 mhz and above make a difference within the two systems, Intel and AMD. I was thinking we would see processors in the 5 mhz. range sometime soon. Conroe doesn't do that. The next round maybe. Just my 2 cents. Enjoy your new AM2.
July 20, 2006 10:13:02 PM

Quote:
would it be wise to look for a AM2 cpu that has 1mb x 2 on the L2 cache then?

Here is my mobo and RAM:

http://www.newegg.com/Product/Product.asp?Item=N82E1681...

http://www.newegg.com/Product/Product.asp?Item=N82E1682...


Now, its just a matter of grabbing a sweet AM2 cpu, for cheapo!


AMD discontinued 1MBx2 cache desktop CPU's to focus more on budget chips...

That would be I only see one CPU that has 1MBx2: the 4000+

I have three AMD chips curently running with 1024 L2 Cache or greater. I have 2 AMD's I use currently using 512 x 2 L2 cache. None are of the AM2 variety

AMD 4800+ X2 1024 x 2 L2 Cache
AMD 64 3700 2.4 1024 L2 Cache socket 754
AMD 64 3400 2.2 1024 L2 Cache socket 754 Clawhammer

AMD 64 3400 2.4 512 L2 Cache socket 754 I have 2 of these running

I use these five computer everyday depending one which office I am in. The 4800x2 is in my home. Sitting at the computer and using it for things like word processing, excel, surfing the net, etc. you can not tell the difference in the L2 cache, 512 compared to 1024 meg. I do think when playing games or using other large software applications, I use Pinacle Studios 9 for video editing, the 1024 L2 cache processors perform somewhat better. I wouldn't know how to measure these things. Just my opinion. Like one of the prior posters mentyioned, Intel has larger L2 Caches on their latest processors. AMD 64 processors have the RAM controller on the processor, eliminating the northbridge chip for the most part. Intel uses the northbridge to run between the RAM and the processor. There are guys that will tell you this method is junk. I have four Intel processors running everyday. The latest, a Pentium D 805 Smithfield, which has 2 x 1024 L2 cache runs very nice. I think it depends on the processor itslf how well it runs with or without more L2 cache. AMD's 64 processors work fantastic to this point. I would think getting an AM2 with 512 x 2 (x2) should do you well. Then you look at Intels and they have stuff with 2048 x 2 out there. Perhaps RAM speeds of 800 mhz and above make a difference within the two systems, Intel and AMD. I was thinking we would see processors in the 5 mhz. range sometime soon. Conroe doesn't do that. The next round maybe. Just my 2 cents. Enjoy your new AM2.

Thanks for the wealth of information. I greatly appreciate it.

I am probably going to decide between a 4000+ or 4200+, maybe a step above if the price is right.

I've alread bought a Asus M2N-E motherboard. The only gripes i've heard from this board is that it does not work well with OCZ memory. Lucky for me, I bought 2 gigs of Corsair XMS memory (even read a review where a guy has same memory, board and 4200+ CPU and works like a charm.)

The only thing I keep wonder is, what CPU should I buy if I want to OC it a bit? Does one of those OC better than the other?

The board appear to be able to OC the CPU rather easily.

Anyone like to comment on that?

thx
July 20, 2006 10:39:17 PM

Large cache allows faster data access if the CPU core has a very good Branch Prediction Unit (and Intel hasn't, excluding Conroe that none has already tested for this).
If the core hasn't a good BPU it will most likely load a lot of data in the cache that it must later discard to make room for the new incoming and not predicted data.

This is one the issues related to big cache sizes and yes, big caches serves to the purpouse of compensating the ten times slower external DRAM.
July 21, 2006 10:06:13 PM

So, for gaming purposes, how would you compare a 4000+ (2x1MB L2 cache and 2.0 GHz speed) and a 4200+ (2x512MB L2 cache and 2.2 GHz speed). They are within $1 of each other at Newegg.
July 21, 2006 10:28:33 PM

I would opt for 200MHz slower speed but larger 2x1MB cache...
You can always overclock back the 200MHz, but never can you add on another 1MB of cache... you know what I mean...
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