Re: asymmetric DMA speed on myrinet

Hi,

[email protected] wrote:
> Hello. I've seen that the DMA speed of Myrinet NIC is different based on 
> the direction of DMA. 
> L2E(from NIC to host memory) is faster than E2L(from host memory to NIC), 
> especially for large messages.
> Is it a Myrinet specific feature or a wide-accepted one?

Write DMAs (NIC to host) are always faster than Read DMAs (Host to NIC), 
unless something is broken. A Read DMA is actually composed of a request 
sent from the NIC to the host, then the data is collected from host 
memory and pushed to the NIC. This request/fetch delay is a penalty 
compared to Write DMAs where the data is immediately pushed by the NIC.

It's not a feature, it's just how it works. Not only Myrinet, all PCI 
devices.

> Can I balance the DMA speed with simple operation to special registers
> on Myrinet NIC or any other method?

A good PCI implementation would keep the difference to a minimum, but 
you can't change it, it's already best effort. Actually, you could slow 
things down by various means, but you really don't want to do that :-)

Patrick
-- 

Patrick Geoffray
Myricom, Inc.
http://www.myri.com