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SubjectRe: [PATCH 0/5] ftrace: to kill a daemon

On Fri, 8 Aug 2008, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@goodmis.org) wrote:
> >
> > I originally used jumps instead of nops, but unfortunately, they actually
> > hurt performance more than adding nops. Ingo told me it was probably due
> > to using up the jump predictions of the CPU.
> >
>
> Hrm, are you sure you use a single 5-bytes nop instruction then, or do
> you use a mix of various nop sizes (add_nops) on some architectures ?

I use (for x86) what is in include/asm-x86/nops.h depending on what the
cpuid gives us.

>
> You can consume the branch prediction buffers for conditional branches,
> but I doubt static jumps have this impact ? I don't see what "jump
> predictions" you are referring to here exactly.

I don't know the details, but we definitely saw a drop in preformance
between using nops and static jumps.

-- Steve



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