Bruce mostly has it right -- ITR is the minimum latency between interrupts. But, it does actually guarantee a minimum period between interrupts. Though, Fortville actually is unique a little bit in that there is another ITR setting that can ensure a certain average number of interrupts per second (called Interrupt Rate Limiting), though, but I don't think this is used in the current version of the driver.
I see that the sysctl does clobber the global value, but have you tried lowering the interval / raising the rate? You could try something like 10usecs, and see if that helps. We'll do some more investigation here -- 3Gb/s on a 40Gb/s using default settings is terrible, and we shouldn't let that be happening. - Eric On Thu, Oct 22, 2015 at 10:36 PM Bruce Evans <b...@optusnet.com.au> wrote: > On Wed, 21 Oct 2015, Bruce Evans wrote: > > > Fix for em: > > > > X diff -u2 if_em.c~ if_em.c > > X --- if_em.c~ 2015-09-28 06:29:35.000000000 +0000 > > X +++ if_em.c 2015-10-18 18:49:36.876699000 +0000 > > X @@ -609,8 +609,8 @@ > > X em_tx_abs_int_delay_dflt); > > X em_add_int_delay_sysctl(adapter, "itr", > > X - "interrupt delay limit in usecs/4", > > X + "interrupt delay limit in usecs", > > X &adapter->tx_itr, > > X E1000_REGISTER(hw, E1000_ITR), > > X - DEFAULT_ITR); > > X + 1000000 / MAX_INTS_PER_SEC); > > X X /* Sysctl for limiting the amount of work done in the taskqueue */ > > > > "delay limit" is fairly good wording. Other parameters tend to give long > > delays, but itr limits the longest delay due to interrupt moderation to > > whatever the itr respresents. > > Everything in the last paragraph is backwards (inverted). Other > parameters tend to give short delays. They should be set to small > values to minimise latency. Then under load, itr limits the interrupt > _rate_ from above. The interrupt delay is the inverse of the interrupt > rate, so it is limited from below. So "delay limit" is fairly bad > wording. Normally, limits are from above, but the inversion makes > the itr limit from below. > > This is most easily understood by converting itr to a rate: itr = 125 > means a rate limit of 8000 Hz. It doesn't quite mean that the latency > is at least 125 usec. No one wants to ensure large latencies, and the > itr setting only ensures a minimal average latency them under load. > > Bruce > _______________________________________________ freebsd-net@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-net To unsubscribe, send any mail to "freebsd-net-unsubscr...@freebsd.org"
