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Comment by amelius | Hacker News Reader

> The short answer would be that the Phi might be the best choice in cases where you need to perform billions of energy efficient floating point operations on a working set small enough to fit in the cards RAM

I'm confused by this. You previously stated that memory bandwidth on the Phi is 10x as high as on the FX-9590. Based on this, I would say that the Phi would be better suited at jobs which require lots of data that doesn't fit in main memory.

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I might be wrong, but I think his speeds are from the internal ram of the Phi and the FX-9590 to their processors, not the transfer speed from the hard drive to the cards ram. So if all of the data fits in the Phi's ram it can transfer that to the processors really fast, but if you have to transfer over the PCI you are limited to those speeds.

You are right. The largest Xeon Phi's have 16GB of GDDR5 memory on the card, and the 31S1P on sale has 8GB. The quoted speeds are for 16-channel transfer between the card's on board memory and the cores on the same card. Transfer from main memory back and forth to the card is over PCIe v2 x16 which has a theoretical max of 8 GB/s --- about 3% of the maximum transfer rate on data within the card.

So if your working data set fits on the card (less than 8/16 GB), or can be partitioned so that it fits on multiple cards, you can potentially get great performance from Phi. But if it's larger than this, especially if it is less than 1TB and would thus fit in the host's RAM, transfer from the host to the card will likely destroy any performance advantage.

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