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? -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html - 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
