Messages in this thread | | | Date | Sun, 19 Jan 1997 20:47:26 +0100 (MET) | From | Ingo Molnar <> | Subject | Re: HZ=1000 & kernel profiling? |
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On Sun, 19 Jan 1997, David S. Miller wrote:
> 2) The profiling timer runs completely in assembler and does > not save/restore state, just records the profiling info > then returns from the trap, no c-code, smaller cache > footprint and less cycles burned, thus the changed timing > timing of the kernel caused by the profiling tick itself > is reduced as much as possible
some hardware sucks ... for example, on my pentium system, alone the cost of getting to the first instruction of the IRQ handler costs ... 8 microseconds :(( I guess it's due to the legacy PIC chip still sitting still on the ISA bus ...
really, 8 microseconds, from the point where CPU execution stops, to the point where the interrupt vector shows. It's 800 wasted cycles. PC hardware sucks.
i will measure how expensive the SMP IPI interrupts are, from the hardware point of view. Maybe it makes sense to bombard one CPU with cross-CPU interrupts, generating profiling irqs. They should be much cheaper, theoretically, and if you control the bombardment, they can be rather random and irrational compared to the timer IRQ on the first CPU.
curious how the typical hardware irq latency numbers look like on the Sparc :)
-- mingo
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