Date: Mon, 21 Nov 2011 11:29:29 -0500 From: John Baldwin <jhb@freebsd.org> To: Luigi Rizzo <rizzo@iet.unipi.it> Cc: Matteo Landi <matteo@matteolandi.net>, Doug Barton <dougb@freebsd.org>, freebsd-current@freebsd.org Subject: Re: ixgbe and fast interrupts Message-ID: <201111211129.29362.jhb@freebsd.org> In-Reply-To: <20111118220458.GA21152@onelab2.iet.unipi.it> References: <CALJ8J_HPZewO12uanb=kctQYwepMssr63E0DQh9CqV6PGaC=JA@mail.gmail.com> <4EC6AEF0.1010402@FreeBSD.org> <20111118220458.GA21152@onelab2.iet.unipi.it>
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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
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