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    SubjectRe: [PATCH 2/2] Markers Implementation for Preempt RCU Boost Tracing
    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?
    --
    (english) http://www.livejournal.com/~pavelmachek
    (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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