Alexander Motin wrote: > YAMAMOTO, Taku wrote: >> On Mon, 30 Aug 2010 13:07:38 +0300 >> Alexander Motin <m...@freebsd.org> wrote: >>> Gary Jennejohn wrote: >> (snip) >>>> So, what else did you do to reduce interrupts so much? >>>> >>>> Ah, I think I see it now. My desktop has only C1 enabled. Is that it? >>>> Unfortunately, it appears that only C1 is supported :( >>> Yes, as I have said, at this moment empty ticks skipped only while CPU >>> is in C2/C3 states. In C1 state there is no way to handle lost events on >>> wake up. While it may be not very dangerous, it is not very good. >> There's an alternative way to catch exit-from-C1 atomically: >> use MWAIT with bit0 of ECX set (``Treat masked interrupts as break events'' >> in Intel 64 and IA-32 Architecthres Software Developer's Manual). >> >> In this way we can put each core individually into deeper Cx state without >> additional costs (SMIs and the like) as a bonus. >> >> The problem is that it may be unavailable to earlier CPUs that support >> MONITOR/MWAIT instructions: >> we should check the presense of this feature by examining bit0 and bit1 of >> ECX >> that is returned by CPUID 5. > > Thank you for the hint. I will investigate it now. But it still help > only x86 systems. I have no idea how power management works on > arm/mips/ppc/..., but I assume that periodic wake up there also may be > not free.
I have looked on these MWAIT features. They indeed allow to wake up with interrupts disabled, but I am worrying about the C-state in which CPU goes in that case. MWAIT states are CPU C-STATES, not ACPI C-states. So I have doubts that using MWAIT instead of HLT on ACPI system is correct. Also I have found some comments that MWAIT on AMD 10h family CPUs does not allows CPU to go to C1E state, while HLT does. So it is not a complete (worse) equivalent. Later AMD CPUs just do not support MWAIT. -- Alexander Motin _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
