[GSoc] Timeconter Performance Improvements

[John Baldwin]

On Thursday, March 24, 2011 9:34:35 am Jing Huang wrote:
> Hi,
> 
>    Thanks for your replay. That is just my self-introduction:) I want
> to borrow the shared memory idea from KVM, I am not want to port a
> whole KVM:)  But for this project, there are some basic problems.
> 
>     As I know, tsc counter is CPU specific. If the process running on
> a multi-core platform, we must consider switching problem. The one
> way, we can let the kernel to take of this. When switching to another
> CPU, the kernel will reset the shared memory according to the new CPU.
> The second way, we can use CPUID instruction to get the info of
> current CPU, which can be executed in user mode ether. At the same
> time, the kernel maintains shared memory for each CPU. When invoke
> gettimeofday, the function will calculate precise time with current
> CPU's shared memory.
> 
>    I don't know which is better? Could I need to deal other problems?

For modern Intel CPUs you can just assume that the TSCs are in sync across 
packages.  They also have invariant TSC's meaning that the frequency doesn't 
change.  You can easily export a copy of the current 'timehands' structure 
when the TSC is used as the timecounter and then just reimplement bintime() in 
userland.  This assumes you use the TSC as the kernel's timecounter, but you 
really need to do that so that ntpd_adjtime() is taken into account, etc.

That will give a very fast and very cheap timecounter.

I believe we already have a shared page (it holds the signal trampoline now)
for at least the x86 platform (probably some others as well).

-- 
John Baldwin