ixgbe and fast interrupts

[John Baldwin]

On Friday, November 18, 2011 5:04:58 pm Luigi Rizzo wrote:
> On Fri, Nov 18, 2011 at 11:16:00AM -0800, Doug Barton wrote:
> > On 11/18/2011 09:54, Luigi Rizzo wrote:
> > > One more thing (i am mentioning it here for archival purposes,
> > > as i keep forgetting to test it). Is entropy harvesting expensive ?
> > 
> > No. It was designed to be inexpensive on purpose. :)
> 
> hmmm....
> unfortunately I don't have a chance to test it until monday
> (probably one could see if the ping times change by modifying
> the value of kern.random.sys.harvest.* ).
> 
> But in the code i see the following:
> 
> - the harvest routine is this:
> 
>     void
>     random_harvest(void *entropy, u_int count, u_int bits, u_int frac,
> 	enum esource origin)
>     {
>         if (reap_func)
>                 (*reap_func)(get_cyclecount(), entropy, count, bits, frac,
>                     origin);
>     }
> 
> - the reap_func seems to be bound to
> 
>     dev/random/randomdev_soft.c::random_harvest_internal()
> 
>   which internally uses a spinlock and then moves entries between
>   two lists.
> 
> I am concerned that the get_cyclecount() might end up querying an
> expensive device (is it using kern.timecounter.hardware ?)

On modern x86 it just does rdtsc().

> So between the indirect function call, spinlock, list manipulation
> and the cyclecounter i wouldn't be surprised it the whole thing
> takes a microsecond or so.

I suspect it is not quite that expensive.

> Anyways, on monday i'll know better. in the meantime, if someone
> wants to give it a try... in our tests between two machines and
> ixgbe (10G) interfaces, an unmodified 9.0 kernel has a median ping
> time of 30us with "slow" pings (say -i 0.01 or larger) and 17us with
> a ping -f .

Did you time it with harvest.interrupt disabled?

-- 
John Baldwin