Re: [myrinet] Why DMA instead of PIO in GM ?
Chip Watson <[email protected]>
| Newsgroups | gmane.network.myrinet.general |
|---|---|
| Message-ID | <[email protected]> |
George, If you can overlap communication and computation, DMA is better in that the host is free to do other work. PIO is only better when synchronization is essential (and messages are small). GM achieves higher bandwidth for medium message sizes than other packages such as BIP. Chip geoge_zhang wrote: > Hello: > I've seen in GM document that GM uses DMA to transfer messages from user space to board SRAM. As is known that PIO transfer for small messages achieves much better performance than DMA, then why GM uses DMA for all messages ? > Compared with GM, many other user-level communication protocols such as BIP use PIO for small messages and DMA for bigger ones, and so it achieves less latency for small messages (about 4.3us for BIP, and 9.6us for GM ) and better bandwidth for bigger ones. Then what's the consideration of GM just using DMA ? > Any reply would be appreciated ! > > Best regards > Geoge. -- Chip Watson High Performance Computing Group Thomas Jefferson National Accelerator Facility Tel: (757) 269-7101 http://www.jlab.org/~watson