On Fri, Apr 2, 2010 at 4:16 PM, Ingo Molnar <mingo@xxxxxxx> wrote: > > * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > >> On Sun, 2010-03-28 at 23:37 -0400, Matt Turner wrote: >> > include/asm-generic/bitops/sched.h says >> > /* >> > * Every architecture must define this function. It's the fastest >> > * way of searching a 100-bit bitmap. It's guaranteed that at least >> > * one of the 100 bits is cleared. >> > */ >> > >> > arch/alpha/include/asm/bitops.h says >> > /* >> > * Every architecture must define this function. It's the fastest >> > * way of searching a 140-bit bitmap where the first 100 bits are >> > * unlikely to be set. It's guaranteed that at least one of the 140 >> > * bits is set. >> > */ >> > >> > Is the guarantee that one of the first 100-bits set (and that the last >> > 40 are useless?), or 140-bits? If it's just the first 100 bits we care >> > about, then the alpha version needs to be fixed. >> > >> > I'm just checking this out, because gcc produces horrendous code for >> > sched_find_first_bit on alpha. I rewrote it in assembly and it's >> > better than 4 times faster. >> > >> > Also, is it even worth optimizing that function? It looks like it's >> > only used in kernel/sched_rt.c. >> >> (might help if you CC the scheduler people on scheduler functions :-) >> >> Right, so it used to be 140 bits with the old O(1) scheduler, currently >> (as you noted) sched_rt is the only user left and will therefore only >> care about the first 100 bits. >> >> As it stands I think it might still make sense to optimize this as for >> rt loads it still on a hot path. >> >> As to the 100 vs 140 length, would it really make much of difference to >> shorten the implementation to 100? If not I'd leave it at 140. >> >> Ingo, any comments? > > I guess getting below the 128 bits boundary would shave an instruction and a > branch off or so? > > Ingo > That's right. I should be able to get rid of a cmov, which kind of counts as two instructions in EV6 scheduling. So I should send a patch to reduce this to the first 100 (128) bits? Thanks guys, Matt -- To unsubscribe from this list: send the line "unsubscribe linux-alpha" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
