On Mon, Jun 19, 2006 at 05:14:15PM -0400, Chris Browne wrote: > [EMAIL PROTECTED] (Robert Lor) writes: > > For DTrace, probes can be enabled using a D script. When the probes > > are not enabled, there is absolutely no performance hit whatsoever. > > That seems inconceivable. > > In order to have a way of deciding whether or not the probes are > enabled, there has *got* to be at least one instruction executed, and > that can't be costless.
I think the trick is that the probe are enabled by overwriting bits of code. So by default you might put a No-Op instruction and if you want to trace you replace that with an illegal instruction or the special one-byte INT3 instruction x86 system have for this purpose. With a 17-stage pipelined processor I imagine the cost of a no-op would indeed be almost unmeasurable (increase code size I suppose). Have a nice day, -- Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to > litigate.
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