On 2012-05-09, Joachim Achtzehnter <joachima@xxxxxxxxxxxxxx> wrote: > Grant Edwards wrote: > >> I've been loaned a clue by somebody on the OSADL mailing list: the RT >> patches are for improving _user_space_ reponse, and may do so at the >> expense of both CPU usage and interrupt latency. > > The RT patches improve *worst case* latency. They are primarily > intended for applications that must *always* meet their deadlines, > not merely most of the time. In return you tend to get increased > average latency as well as reduced throughput. Good point. I eventually figured out that my increased interrupt latency is due to my ISR being run in a kernel thread instead of as a real ISR. Adding the IRQF_NODELAY flag gets me the same average latency I had without the RT patch. [It looks like that flag has a different name in 2.6.39 and later?] >> As a result, I'm better off without the RT patch if what I care >> about is interrupt latency. > > Yes, if you only care about *typical* interrupt latency but don't > mind the occasional long delay. Unfortunately, the requirements are a bit fuzzy -- I've got an ISR deadline of about 20us that I'm trying to meet [I wouldn't mind a little chat with the person who designed _that_ requirement into the hardware]. What I don't know is how hard that deadline is. With the RT patch (and without IRQF_NODELAY), I miss the deadline most of the time (I'd guess about 80% of the time). Without RT or with RT and IRQF_NODELAY, it looks like I meet the deadline maybe 98% of the time (under test conditions). What I don't know is if once the deadline is missed it matters weather it's missed by 50us or by 250us [or if 98% is going to be anywhere close to acceptible, for that matter]. -- Grant -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
