Topic Title: Memory Bit Rate
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Created On: 03/28/2013 03:13 PM
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 03/28/2013 03:13 PM
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Buford T. Justice
Peon

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I use to own an NVIDIA 9600 GT.  I think it used 256-bit or maybe 192-bit DDR3 memory.  I just installed a new HD 7770 which I know is DDR5 and 128-bit.

Is 128-bit in DDR5 more powerful than 192 or 256-bit in DDR3?

 03/28/2013 03:59 PM
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black_zion
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It's GDDR, separate and not equal to DDR. GDDR5 is Quadruple Data Rate, GDDR3 is Double Data Rate. Over the same interface, in this case 128 bit, GDDR5 will have a little more than twice the bandwidth of GDDR3.

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 03/28/2013 06:27 PM
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Thanny
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GDDR3 and GDDR5 actually have the same raw speed for a given bus width and clock.  The difference is that GDDR3 can only transfer in one direction per clock cycle, while GDDR5 can transfer in both directions.  So if you read a bunch of data from VRAM or write a bunch of data to VRAM, GDDR3 and GDDR5 will look the same, all else being equal.  That's not what actually happens when rendering, though.  There's always a simultaneous need to read and write memory.  GDDR3 has to take turns, while GDDR5 can perform them simultaneousl.  So the overall memory throughput for GDDR5 is twice that of GDDR3, given the same bus width and clock speed.

It's rather like half-duplex and full-duplex with a network switch.  If you're connected at 1Gbps at half-duplex, you can send data at 1Gbps or receive data at 1Gbps, but not at the same time.  So if you're trying to send and receive at the same time, the aggregate speed of a 1Gbps full-duplex link is 2Gbps - still 1Gbps in either direction, though.  In contrast, if sending and receiving simultaneously with a half-duplex connection, the one-way transfer speed would be 0.5Gbps.

In your specific case, you have a 9600 GT with a 256-bit memory bus at 900MHz, which means 57.6Gbps total theoretical throughput (i.e. read and write combined).  Then you moved to a 7770 which has a 128-bit memory bus at 1125MHz, which means 72.0Gbps total throughput.  In any kind of test which involved only reading or only writing, the GDDR3 would be faster - 57.6Gbps max versus 36Gbps max with GDDR5.  Under real world conditions, the GDDR5 will be faster.

In rendering power, the 7770 should be considerably faster as well.  It's hard to find any common benchmarks between the two given the age difference, but it looks like a factor of two to three difference.

 

 03/29/2013 06:41 PM
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Buford T. Justice
Peon

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So other than bi-directional memory, I only got a modest increase in performance?  Damn

I did notice the AUTO setting in Battlefield 3 went from low for the 9600 to a combination of medium and high with the 7770.  Gameplay seems a few FPS faster.  It does ULTRA but I am only around 20-30 FPS.

 03/29/2013 08:03 PM
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black_zion
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The bulk of a card's power comes from the processor count (USP or CUDA depending on manufacturer) and clock rates. You could have XDR2 chips on the card but if the processors aren't capable of shifting in enough information then it isn't giving any additional benefit.

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 03/30/2013 12:53 PM
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Thanny
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I would not characterize 200% to 300% performance increase as modest, though I should stress again that I'm not sure about those numbers.

The fact that you can increase the quality settings dramatically and still be slightly faster shows that you've made a significant upgrade.  If you want to do a direct comparison, though, you'd have to run games at the same quality settings with each card.

 

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