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    SubjectRe: HZ=1000 & kernel profiling?

    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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