Direct Memory Access (DMA)

PIO (Programmed IO) has the lowest overhead and lowest latency accessing remote memory. The drawback of the PIO data transfers is that the CPU is busy reading or writing data from or to remote memory. An alternative is to use a DMA engine if this is available. The application specifies a queue of data transfers and passes it to the DMA engine. Then the CPU is free either to wait for the completion of the transfer or to do something else. In the latter case it is possible to specify a callback function that is invoked when the transfer has finished. DMA has high start up cost compared to using PIO and is normally only recommended for larger transfers.

DMA is an optional feature implemented with some PCIe chipsets only. Some systems also have a “System DMA” engine. The SISCI API DMA functionality are available on all platforms supporting PCIe DMA or System DMA that is integrated and supported with the SISCI driver stack. SISCI DMA transfer functions will fail if there is no supported DMA engine available.

• SCICreateDMAQueue()

• SCIStartDmaTransfer()

• SCIStartDmaTransferVec()

• SCIStartDmaTransferMem()

• SCIRemoveDMAQueue()

• SCIWaitForDMAQueue()

• SCIAbortDMAQueue()

• SCIDMAQueueState()