Level 3 Cache Purpose

Discussion in 'General Mac Discussion' started by willmore, May 5, 2002.

  1. willmore macrumors newbie

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    #1
    I've been a Mac nut for as long as I can remember, but I for the most part stayed away from the rather technical stuff, such as caches. My question is this: what is the purpose of level 3 cache? Also, how does L3 cache differ from L1 and L2 cache in placement, purpose, design, etc.? Thanks in advance.
     
  2. Ensign Paris macrumors 68000

    Ensign Paris

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    Europe
    #2
    According to apple.com:

    The L3 cache uses 2MB of high-speed, Double Data Rate (DDR) SDRAM running at a data rate of up to 500MHz. The L3 cache boosts processor function by providing fast access to data and application code at speeds of up to 4 gigabytes per second (GBps). And the L3 cache is even more effective at speeding up processing because it has a dedicated bus to the G4 chip, making the full 4GBps data throughput always available — unimpeded by bottlenecks caused by other data (unlike Pentium 4-based systems, which don’t have L3 cache — a disadvantage that leads to congestion between various data streams, and slowdowns in the overall rate of data transfer).
    The high speed L3 cache with its dedicated bus enables the PowerPC G4 processor to receive data up to five times faster than it could from main memory. This low latency keeps the processors constantly fed with data, so it isn’t idling while waiting for the next task to arrive. The L3 cache is large enough to store active application code and data. When you run an application, most of the active code for the program — along with most of the data being used — is in L3 cache. Thus the information most required by the processor is close at hand. It’s analogous to the caching of web pages on your hard disk drive: Click the back button on your web browser, and your computer will use the data you loaded two pages ago — skipping the step of reloading the same data again — making the page appear quicker
     
  3. Hemingray macrumors 68030

    Hemingray

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    #3
    So does this mean that the caches would be pretty much unneeded if we had a truly fast system bus? That's the impression I'm getting.
     
  4. britboy macrumors 68030

    britboy

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    #4
    If the bus could deliver the same amount of data that the processor churns out, then you wouldn't need the caches. However, a 400Mhz G4 processes up to 12.8Gb of data per second, and even RIMM 4200 rambus RAM maxes out at 4.2GB/s.
     
  5. Catfish_Man macrumors 68030

    Catfish_Man

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    #5
    Faster buses...

    ...and on chip memory controllers both reduce the need for cache. With Apple's current SDR 133MHz bus a large (expensive) cache is almost a necessity. A faster bus (or even better, an on chip memory controller) will reduce, but not eliminate, the need for cache.
     
  6. Choppaface macrumors 65816

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    #6
    when you're talking about moving data, isnt it gigaBITS and not gigaBYTES ??
     
  7. mozez macrumors member

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    Sep 18, 2001
    #7
    actually you are wrong

    what was explained above is actually l2 cahce, not l3, the g4s have 256k l2 chaceh which is half the amount of a p4 or athlon, and the fact that the cache is on the motherboard and not on chip makes it twice as slow because it has to travel, so no, the l3 cache does not go at full speed and does not feed at 12.8, did you just read that on apple's site and trya nd repeat it or soemthing. what is kept in the cache, instructions sets, the most commonly used ones to be exact, a larger cache makes it easier on the proicessor to store used functions in the last milli second so that it can repeat it, this is especially handy when using graphics or for system stability and it helps tons in art world, now if you buy a zeon processor, you will get 2mb of L2 cahce and an l3 cache so let's not get our information mixed up, do some pc research before thinging you know what they have, and don't read off aplle's site either, nor intels nor amds, what they put on their is for consumers, not techs, if you are a techa nd have to use and study this stuff you'd know it's all bull.
     
  8. mozez macrumors member

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    Sep 18, 2001
    #8
    and i know a made a million spelling errors, i probably should have looked it over before submitting it.
     
  9. macstudent macrumors 6502

    macstudent

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    #9
    You can always go back and edit your posts if you want to correct any spelling errors that you may have.
     
  10. Beej macrumors 68020

    Beej

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    #10
    Good to see a bit of the ol' "leaving out bits of information to make pentiums look bad" going on here :)

    There is less need for a L3 cache in Pentium systems because of the CISC architecture. Macs use a RISC architecture, so cache performance is more important.

    Eg. using DDR in a Mac will make a bigger performance difference than using DDR in a PC.
     
  11. Nipsy macrumors 65816

    Nipsy

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    #11
    Re: actually you are wrong

    Hey, while we're on the subject of doing research before posting, I would like to point out that you are wrong in calling us wrong.

    The G4 has 256k of on die L2 cache (not on the mobo as you erroneously assert). The above posts are explaing the function of the L3 cache, not the L2 cache. L2 is best in branch recovery and similar operations. L3 is best when you don't want to pipe out to RAM, but know you'll reuse the data in short order.

    Furthermore, the 2MB cache must sit on the daughtercard, next to the processor, as there certainly isn't a slot for it on the mobo. Therefore that extra journey you mentioned takes place on a dedicated circuit, on the processor card. What's 186,000 miles per second divided by 1 inch? It's not a long trip, and while not as fast as on die cache, is not like mobo cache from the beige PPC days.
     
  12. britboy macrumors 68030

    britboy

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    Kent, UK
    #12
    Re: actually you are wrong


    actually you may notice i didn't say "L3 cache" once. What i was referring to was the processor itself. My point was that the bus cannot deliver enough data for the processor to be working at 100% capacity, which is why you need the L2 and L3 cache, to store instructions sets somewhere fast.
     
  13. barkmonster macrumors 68020

    barkmonster

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    Dec 3, 2001
    Location:
    Lancashire
    #13
    Pentium 4 cache is a little different now...

    The G4 has this cache scheme:

    L1 Cache : 32K x 2 @ CPU speed
    L2 Cache : 256K @ CPU speed
    L3 Cache : 2Mb DDR @ 25% of CPU speed x 2
    FSB : 133Mhz
    Motherboard speed / RAM type : FSB speed / SDRAM

    The Original Pentium 4 had this one :

    L1 Cache: 12K + 8K @ CPU speed
    L2 Cache: 256K @ CPU speed
    L3 Cache : N/A
    FSB : 100Mhz
    Motherboard speed / RAM type : FSB x 4 / RDRAM

    The New Pentium 4 has this scheme :

    L1 Cache: 12K + 8K @ CPU speed
    L2 Cache: 256K @ CPU speed
    L3 Cache : N/A
    FSB : 133Mhz
    Motherboard speed / RAM type : FSB x 4 / RDRAM

    So while a 1Ghz G4 has a 2Mb Cache running at 500Mhz, the main memory and motherboard on a pentium 4 is running at either 400 or 533Mhz.

    L3 cache is very important, without it the 667Mhz TiBook couldn't match the old 500Mhz Model. The 933Mhz G4 is signicantly more than a mere 16.6% faster than the 800Mhz Model because of that extra 2Mb of DDR ram between the CPU and motherboard.

    Hope I havn't ruffled any feathers pointing out the fact we've got more than CPU speed to worry about with those old Mac vs PC arguments. Back in the days of the 604, it was a general opinion with some PC people I know that it's only strength was the large cache it had, of course they also claimed with a bigger cache they're PCs could beat it and we had to wait till the Pentium 3 and Athlon before the mac wasn't at least twice as fast as a similarly clocked PC.

    Things are more even now, almost slipping away in favour of the PC if we don't see at least a 40 - 50% CPU speed hike and a faster motherboard by MWNY, I won't go too much into that here though, it's been discussed a million times already.
     

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