[Beowulf] Help with inconsistent network performance

Greg Lindahl lindahl at pbm.com
Wed Dec 19 00:50:53 EST 2007


On Tue, Dec 18, 2007 at 09:05:41PM -0500, Patrick Geoffray wrote:

> No, it just means the NIC supports it.

Well, then how about ethtool -S? That looks like an actual count of
flow control events, so rx flow control events means the switch
must support it in some fashion.

> For RX hardware flow-control, you need enough buffer space to keep one 
> full frame plus the latency on the longest wire, for every port. It is a 
> bit more expensive to do with 10GigE, because you need faster memory and 
> more of it. Some recent 10GigE chips use a shared SRAM buffer that is 
> not big enough for the worst case with 9K packets:

Well, we know it can be done perfectly, it's done in InfiniBand
switches, and that other 10 gig non-ethernet switch, what's it called?
Oh yeah, Myrinet. They do it, too.

> Flow-control is not for everyone, and that's why it is often turned off 
> by default. When a sender is paused, it will stop sending anything, 
> including packets for different destinations. Dropping packets is 
> expensive to recover but it keeps things moving.

Can Myrinet even disable flow control? Odd that Ethrernet is any
different; dropping any packets is an utter disaster for TCP.

-- greg


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