cvs commit: src/sys/amd64/amd64 cpu_switch.S machdep.c

[Andrew Gallatin]

David Xu [davidxu at FreeBSD.org] wrote:
> davidxu     2005-10-17 23:10:31 UTC
> 
>   FreeBSD src repository
> 
>   Modified files:
>     sys/amd64/amd64      cpu_switch.S machdep.c 
>   Log:
>   Micro optimization for context switch. Eliminate code for saving gs.base
>   and fs.base. We always update pcb.pcb_gsbase and pcb.pcb_fsbase
>   when user wants to set them, in context switch routine, we only need to
>   write them into registers, we never have to read them out from registers
>   when thread is switched away. Since rdmsr is a serialization instruction,
>   micro benchmark shows it is worthy to do.

Nice.  This reduces lmbench context switch latency by about 0.4us (7.2
-> 6.8us), and reduces TCP loopback latency by about 0.9us (36.1 ->
35.2) on my dual core 3800+

It is a shame we can't find a way to use the TSC as a timecounter on
SMP systems.  It seems that about 40% of the context switch time is
spent just waiting for the PIO read of the ACPI-fast or i8254 to
return.


Drew