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Memory bandwidth anomaly

Question asked by realhet on Oct 9, 2018
Latest reply on Oct 11, 2018 by realhet

Hi,

Recently I did some tests about GDDR5 memory bandwidth.

In a few words:

It launches numberOfCUes*4 waveFronts. Each WF is reading 1024 bytes from a random aligned location in a large buffer.

The whole kernel reads 100GB total. The ideal bandwidth is 1.850GHz*256bit/8*4=236.8GBps

I'm varying the buffer size from 1KB to 4GB and got the following results:

gddr5_bandwidth_vs_size.png

You can easily spot to 2 cache levels on it. But I've found a third something above 1GB.

Anyone has an explanation what is that?

I have 8GB and I only tested this up until 4GB, but it seems like if I use the whole memory, the bandwidth will settle at 1/3 of the nominal bandwidth.

This behavior with PC memory would be unacceptable for sure. But what's the case with graphics memory?

 

Thank You for the answers!

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