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DMA Engine Bandwidth
The maximum performance of the DMA engine for D32 transfers is summarized in Table 4-4. Performance with D64 Block transfers is somewhat less than twice the rate shown in Table 4-4. Transfers for larger sizes are faster because the setup time is amortized over a greater number of bytes.
VME Bus Bandwidth, DMA Engine, D32 Transfer
| Transfer Size | Read | Write | Block Read | Block Write |
|---|
| 32 | 2.8 MB/sec | 2.6 MB/sec | 2.7 MB/sec | 2.7 MB/sec |
| 64 | 3.8 MB/sec | 3.8 MB/sec | 4.0 MB/sec | 3.9 MB/sec |
| 128 | 5.0 MB/sec | 5.3 MB/sec | 5.6 MB/sec | 5.8 MB/sec |
| 256 | 6.0 MB/sec | 6.7 MB/sec | 6.4 MB/sec | 7.3 MB/sec |
| 512 | 6.4 MB/sec | 7.7 MB/sec | 7.0 MB/sec | 8.0 MB/sec |
| 1024 | 6.8 MB/sec | 8.0 MB/sec | 7.5 MB/sec | 8.8 MB/sec |
| 2048 | 7.0 MB/sec | 8.4 MB/sec | 7.8 MB/sec | 9.2 MB/sec |
| 4096 | 7.1 MB/sec | 8.7 MB/sec | 7.9 MB/sec | 9.4 MB/sec |
Note: The throughput that can be achieved in VME DMA is very sensitive to several factors:
- The other activity on the VME bus.
- The blocksize (larger is better).
- Other overhead in the loop requesting DMA operations.
The loop used to generate the figures in Table 4-4 contained no activity except calls to dma_start().
- the response time of the target VME board to a read or write request, in particular the time from when the VME adapter raises Data Strobe (DS) and the time the slave device raises Data Acknowledge (DTACK).
For example, if the slave device takes 500 ns to raise DTACK, there will always be fewer than 2 M data transfers per second.
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