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    SubjectRe: [PATCH] syscall latency improvement #1
    On Fri, 25 Jan 2002, Linus Torvalds wrote:

    >
    > On Sat, 26 Jan 2002, Andi Kleen wrote:
    > > On Fri, Jan 25, 2002 at 05:53:57PM -0800, Linus Torvalds wrote:
    > > >
    > > > On 26 Jan 2002, Andi Kleen wrote:
    > > > >
    > > > > It doesn't explain the Athlon speedups. On athlon cli is ~4 cycles.
    > > >
    > > > .. and it probably serializes the instruction stream.
    > >
    > > I have word from AMD engineering that it doesn't stall the pipeline
    > > or serializes.
    >
    > Note that it may not be the "cli" itself - the "iret" may be slower if it
    > has to enable interrupts that were disabled before. Ie the iret microcode
    > may have the equivalent of
    >
    > /* Did eflags change? */
    > if ((new_eflags ^ old_eflags) & IF_MASK)
    > .. do sti/cli as appropriate ..
    >
    > which would mean that the "cli" itself may take 4 cycles, but the "sti"
    > implicit in the iret will _also_ take 4 cycles and is optimized away when
    > not needed.
    >
    > Which would add up to the 8 cycles needed for a ~3.4% speedup (this is
    > assuming the baseline is something like 250 cycles per system call, I've
    > not checked that assumption).

    guys, why don't you use #rdtsc to discover where perf improvement comes from ?




    - Davide


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