On Thu, Jan 28, 2016 at 2:44 PM, Taylor R Campbell <campbell+netbsd-tech-k...@mumble.net> wrote: > Date: Thu, 28 Jan 2016 11:58:05 +0900 > From: Ryota Ozaki <ozak...@netbsd.org> > > On Thu, Jan 28, 2016 at 12:17 AM, Taylor R Campbell > <campbell+netbsd-tech-k...@mumble.net> wrote: > > So vanilla might make less efficient use of the CPU cache, > > and vanilla might leave the rxq full for longer so that the device > > cannot fill it as quickly with incoming packets. > > That might be true. If so, the real question may be why the old > implementation isn't efficient compared to the new one. > > By `the old implementation', do you mean the one with pcq instead of > ifq, softint-rx?
Yes. > It's only a little bit slower than softint-rx-ifq. > My wild guess is that all the atomics and memory barriers in pcq slow > it down. Perhaps. Though, I have no motivation to investigate it that I already threw up :-/ > > > Another experiment that might be worthwhile is to bind the interrupt > > to a specific CPU, and then use splnet instead of WM_RX_LOCK to avoid > > acquiring and releasing a lock for each packet. > > In the measurements, all interrupts are already delivered to CPU#0. > Removing the lock doesn't change the results. I guess acquiring and > releasing a lock (w/o contentions) are low overhead. Note that > wm has a RX lock per HW queue, so RX processing can be done with no > lock contention basically. > > OK, makes sense. > > > (On Intel >=Haswell, > > we should use transactional memory to avoid bus traffic for that > > anyway (and maybe invent an MD pcq(9) that does the same). But the > > experiment with wm(4) is easier, and not everyone has transactional > > memory.) > > How does transactional memory help? > > It doesn't. I was just blathering aimlessly like I often do about > things I don't understand well enough but am idly thinking about. (It > also turns out -- I hadn't heard until today -- that transactional > memory is broken on Haswell!) NP. I'm also interested in how it (and persistent memory) affects OS design and facilities :) ozaki-r
