Messages in this thread |  | | Date | Fri, 25 Jan 2002 18:53:16 -0800 (PST) | | From | Davide Libenzi <> | | Subject | Re: [PATCH] syscall latency improvement #1 |
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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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