Date: Fri, 18 Nov 2011 23:04:58 +0100 From: Luigi Rizzo <rizzo@iet.unipi.it> To: Doug Barton <dougb@FreeBSD.org> Cc: Matteo Landi <matteo@matteolandi.net>, freebsd-current@freebsd.org Subject: Re: ixgbe and fast interrupts Message-ID: <20111118220458.GA21152@onelab2.iet.unipi.it> In-Reply-To: <4EC6AEF0.1010402@FreeBSD.org> References: <CALJ8J_HPZewO12uanb=kctQYwepMssr63E0DQh9CqV6PGaC=JA@mail.gmail.com> <201111180800.06593.jhb@freebsd.org> <20111118170615.GA9762@onelab2.iet.unipi.it> <201111181220.04846.jhb@freebsd.org> <20111118175433.GA13459@onelab2.iet.unipi.it> <4EC6AEF0.1010402@FreeBSD.org>
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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 ?)
> sysctl -a | grep timecounter
kern.timecounter.tick: 1
kern.timecounter.choice: TSC(-100) HPET(900) ACPI-fast(1000) i8254(0) dummy(-1000000)
kern.timecounter.hardware: ACPI-fast
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.
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 .
BTW the reason for the difference is totally unclear to me (ping
-f uses a non-blocking select() but i don't think it can explain
such a large delta).
cheers
luigi
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