Messages in this thread | | | Date | Fri, 25 Jan 2002 18:14:25 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [PATCH] syscall latency improvement #1 |
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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).
Linus
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