ixgbe and fast interrupts

John Baldwin jhb at freebsd.org
Fri Nov 18 13:14:45 UTC 2011

On Friday, November 18, 2011 3:46:02 am Matteo Landi wrote:
> > you probably want to be using MSI-X for a 10G NIC instead of INTx anyway.
> Why do you say that? Is MSI-X faster than INTx in terms of interrupt
> latency? When should I use MSI-X, instead of fast filters interrupts
> (fast interrupt?), instead of ithread interrupts? Thanks in advace.

With MSI-X you can have more than one interrupt and those interrupts can be 
distributed across CPUs.  This means you can (somewhat) tie each queue on your 
NIC to a different CPU.

MSI-X vs INTx is orthogonal to fast vs filter, but in general MSI and MSI-X 
interrupts are not shared, and require no interrupt masking in hardware (they 
are effectively edge-triggered), so using a filter for MSI is rather pointless 
and only adds needless complexity.  For MSI I would just use a theraded 
interrupt handler.  For INTx, I would only use a fast interrupt handler if 
there is a really good reason to do so (e.g. em(4) does so to work around 
broken Intel Host-PCI bridges).

John Baldwin

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