what is the DMA bandwidth of Corsair Vengeance ?

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prudhvirazz

prudhvirazz

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I am upgrading my RAM to 64gb Corsair Vengeance Series so that I can do Machine Learning / Deep Learning. I came across a post where it says even though you have a fast RAM, it has somehow bottleneck such as DMA Bandwidth. (quote and link below)

what is the DMA bandwidth of this ram ?

PCPartPicker part list / Price breakdown by merchant

Memory: Corsair Vengeance LED 64GB (4 x 16GB) DDR4-3000 Memory ($499.99 @ Newegg)
Total: $499.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-03-03 02:35 EST-0500

Blog Link

The memory bandwidth of your RAM determines how fast a mini-batch can be overwritten and allocated for initiating a GPU transfer, but the next step, CPU-RAM-to-GPU-RAM is the true bottleneck – this step makes use of direct memory access (DMA). As quoted above, the memory bandwidth for my RAM modules are 51.2GB/s, but the DMA bandwidth is only 12GB/s!
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Leaps-from-Shadows
Doesn't work that way. The bandwidth is the same when on the same platform at the same speed.

It would increase if you increased the number of memory channels (use different chipset -- Z170 versus X99) or the speed of the RAM (overclock it).

Example: 2x8GB 3200 kit and 4x8GB 3200 kit on Z170 chipset would have the same DMA bandwidth, as long as both channels are used.
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prudhvirazz

prudhvirazz

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are you referring to the blog that I have shared ? I didn't find anything else there.
 
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Leaps-from-Shadows

Leaps-from-Shadows

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The blog entry tells you where to find the answer to your question:

The DMA bandwidth relates to the regular bandwidth, but the details are unnecessary and I will just refer you to this Wikipedia entry, in which you can look up the DMA bandwidth for RAM modules (peak transfer limit).
Click to expand...
The table in the Wikipedia entry is old, but it tells us that the DMA bandwidth is basically the PCxxxx number (compare peak transfer rate to the "module name" in the table). And for the RAM you are asking about, the PCxxxx number is PC 24000 or ~24GB/s.
 
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prudhvirazz

prudhvirazz

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what is difference between 1 X 16 gb ram vs 4 x 16 gb ram in terms od DMA. 24 GB/s for 1 stick and 24 * 4 = 96gb/s for 4 sticks ?

what does the DMA numbers look like if its 8 gb ram instead of 16 ? 4 X 8 gb ?
 
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Leaps-from-Shadows

Leaps-from-Shadows

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Jan 10, 2011
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Doesn't work that way. The bandwidth is the same when on the same platform at the same speed.

It would increase if you increased the number of memory channels (use different chipset -- Z170 versus X99) or the speed of the RAM (overclock it).

Example: 2x8GB 3200 kit and 4x8GB 3200 kit on Z170 chipset would have the same DMA bandwidth, as long as both channels are used.
 
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