* Pavel Machek (pavel@xxxxxx) wrote: > On Mon 2008-01-07 13:59:54, Mathieu Desnoyers wrote: > > * Ingo Molnar (mingo@xxxxxxx) wrote: > > > > > > * Frank Ch. Eigler <fche@xxxxxxxxxx> wrote: > > > > > > > > [...] this is a general policy matter. It is _so much easier_ to add > > > > > markers if they _can_ have near-zero overhead (as in 1-2 > > > > > instructions). Otherwise we'll keep arguing about it, especially if > > > > > any is added to performance-critical codepath. (where we are > > > > > counting instructions) > > > > > > > > The effect of the immediate-values patch, combined with gcc > > > > CFLAGS+=-freorder-blocks, *is* to keep the overhead at 1-2 > > > > dcache-impact-free instructions. The register saves, parameter > > > > evaluation, the function call, can all be moved out of line. > > > > > > well, -freorder-blocks seems to be default-enabled at -O2 on gcc 4.2, so > > > we should already be getting that, right? > > > > > > There's one thing that would make out-of-line tracepoints have a lot > > > less objectionable to me: right now the 'out of line' area is put to the > > > end of functions. That splinters the kernel image with inactive, rarely > > > taken areas of code - blowing up its icache footprint considerably. For > > > example sched.o has ~100 functions, with the average function size being > > > 200 bytes. At 64 bytes L1 cacheline size that's a 10-20% icache waste > > > already. > > > > Hrm, I agree this can be a problem on architectures with more standard > > associative icaches, but aren't most x86_64 machines (and modern x86_32) > > using an instruction trace cache instead ? This makes the problem > > irrelevant. > > > > But I agree that, as Frank proposed, -freorder-blocks-and-partition > > could help us in that matter for the architectures using an associative > > L1 icache. > > I thought trace cache died with P4? > -- And you are absolutely right. We would have to figure out if enabling -freorder-blocks-and-partition makes sense kernel-wide. Mathieu -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - 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
