time issues and ZFS

[Andriy Gapon]

on 22/01/2013 20:42 Adrian Chadd said the following:
> Hi!
> 
> As I said before, the problem with non-HLT loops with event-timer in
> -9 and -head is that it calls the idle function inside a critical
> section (critical_enter and critical_exit) which blocks interrupts
> from occuring.
> 
> The EI;HLT instruction pair on i386/amd64 atomically and correctly
> handles things from what I've been told.
> 
> However, there's no atomic way to do this using ACPI sleeping, so
> there's a small window where an interrupt may come in but it isn't
> handled; waiting for the next interrupt to occur before it'll wake up
> and respond to that interrupt.

I don't think that this is true of x86 hardware in general.
You might have hit some limitation or a quirk or a bug or an erratum for some
particular hardware.

E.g. a chipset on this machine has a bit described as such:
"Set to 1 to skip the C state transition if there is break event
when entering C state."
The bit is set indeed and as far as I can tell the behavior matches the description.

Most modern (non-embedded) machines seem to behave this way. Attempt to enter a
deeper C state while a break event is pending still incurs some overhead, but it's
not as bad as waiting for the next break event.

> I kept hitting my head against this when doing network testing. :(
> 
> Now - specifically for timekeeping it shouldn't matter; that's to do
> with whether the counters are reliable or not (and heck, are even in
> lock-step on CPUs.) But extra latency could show up weirdly, hence why
> I was asking for you to try different timer configurations and idle
> loops.

-- 
Andriy Gapon

-- 
Andriy Gapon