> On Sat, Mar 12, 2011 at 12:08:32PM -0500, John David Anglin wrote: > > The attached change fixes the "SLOW!" timer_interrupt warning that I > > occasionally see on my rp3440 (800 MHz). We need to avoid using the > > expensive div/mul method. I have seen instances where it takes more > > than 0x7000 cycles. > > Hi Dave, > Maybe the patch should be for this line of code instead? > if (unlikely(now2 - now > 0x3000)) /* 12K cycles */ > > ie increase the value slightly? I tried that initially. After bumping it a couple of times, it seemed like more iterations in the `if' alternative was better as most instructions only take one cycle. My sense is that we only exceed the current limit in rare circumstances. > TBH, I didn't spend alot of time trying to figure out the optimal balance > between the two cases that the proposed patch attempts to adjust. > > Also, if the div/mul takes up to 0x7000 cycles, another alternative > is to make the alternative faster. What I suggested in the else case: > /* TODO: Reduce this to one fdiv op */ > > doesn't seem possible with fdiv in one op. My reading of the fdiv > operator suggests it would need another FMUL and FSUB op in order > to get the remainder. Still might be vary fast. > > Looking through PA 2.0 arch book, looks like the PA2.0 > "Divide Step" (DS) operation (page 7-46) does what I was thinking of. > But that's going to require a sequence of DS instructions that > I don't quite understand at the moment and thus can't say how > fast the worst case for DS might be. Currently, I believe that the kernel does integer multiplication and division using millicode. If I remember correctly, division uses the DS instruction. The situation is worse for 64-bit operations because HP never released their 64-bit millicode code. So, gcc does long division in this case. I haven't seen any SLOW warnings with the patch I suggested but it may be a bit inefficient. I have the sense that the problem occurs on the rp3440 because it has two dual core cpus. I have never seen the warning on machines with a single processor chip. Dave -- J. David Anglin dave.anglin@xxxxxxxxxxxxxx National Research Council of Canada (613) 990-0752 (FAX: 952-6602) -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
