Messages in this thread | | | Date | Mon, 14 Jan 2008 10:35:05 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 2/2] Markers Implementation for Preempt RCU Boost Tracing |
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* Pavel Machek (pavel@ucw.cz) wrote: > On Mon 2008-01-07 13:59:54, Mathieu Desnoyers wrote: > > * Ingo Molnar (mingo@elte.hu) wrote: > > > > > > * Frank Ch. Eigler <fche@redhat.com> 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
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