cvs commit: src/sys/dev/em if_em.c if_em.h

[Andrew Gallatin]

Scott Long [scottl at FreeBSD.org] wrote:
> scottl      2006-01-11 00:30:25 UTC
> 
>   FreeBSD src repository
> 
>   Modified files:
>     sys/dev/em           if_em.c if_em.h 
>   Log:
>   Significant performance improvements for the if_em driver:

Very cool.

>   - If possible, use a fast interupt handler instead of an ithread handler.  Use
>     the interrupt handler to check and squelch the interrupt, then schedule a
>     taskqueue to do the actual work.  This has three benefits:
>     - Eliminates the 'interrupt aliasing' problem found in many chipsets by
>       allowing the driver to mask the interrupt in the NIC instead of the
>       OS masking the interrupt in the APIC.

Neat.  Just like Windows..

<....>


>     - Don't hold the driver lock in the RX handler.  The handler and all data
>       associated is effectively serialized already.  This eliminates the cost of
>       dropping and reaquiring the lock for every receieved packet.  The result
>       is much lower contention for the driver lock, resulting in lower CPU usage
>       and lower latency for interactive workloads.

This seems orthogonal to using a fastintr/taskqueue, or am I missing 
something?

Assuming a system where interrupt aliasing is not a problem, how much
does using a fastintr/taskqueue change interrupt latency as compared
to using an ithread?  I would (naively) assume that using an ithread
would be faster & cheaper.  Or is disabling/enabling interrupts in the
apic really expensive?

Do you have a feel for how much of the increase was do to the other
changes (rx lock, avoiding register reads)?

Drew