On Friday 24 November 2006 21:46, Ingo Molnar wrote:
>
> * Andi Kleen <[email protected]> wrote:
>
> >
> > > yeah - the main new bit for x86-64 is the unconditional check for time
> > > warps. We shouldnt (and cannot) really trust the CPU and the board/BIOS
> > > getting it right. There were always some motherboards using Intel CPUs
> > > that had the TSCs wrong.
> >
> > In the 64bit capable generation I don't know of any run in spec
> > (except for multinode systems and there was one overclocked system
> > where the cores got unsync, but overclocking is an operator error)
>
> i have one (Intel based), 64-bit, fully in spec, which is off by
> ~3000-4000 cycles. So it happens.
More details?
> I was in fact surprised when i noticed that you removed the
> unconditional TSC check that i put there years ago
I removed it because you pointed out that it usually caused
trouble on Intel systems: we would always detect errors due to measurement errors
and then make things worse by trying to fix it.
But you're right it might have been better to keep
a check with a threshold to catch totally broken cases.
> but which apps are using RDTSC natively? Trapping isnt too good i agree
The only sure way would be to trap+printk -- but from previous
user complaints it's a substantial number.
> - if then we should remove it from the CPU features and hence apps wont
> (or shouldnt) use it.
I doubt the majority checks any cpu features first ...
>
> > > nor can the TSC really be synced up properly in the hotplug CPU
> > > case, after the fact - what if the app already read out an older TSC
> > > value and a new CPU is added. If the TSC isnt sync on SMP then it
> > > quickly gets pretty messy, and we should rather take a look at /why/
> > > these apps are using RDTSC.
> >
> > Because gettimeofday is too slow.
>
> as i indicated it in another discussion, i can fix that. Next patch will
> be that.
Well I hope it's not making it HZ resolution. As noted earlier we tried
that already and it didn't work (it violates the "forward monotonity"
that is commonly expected)
Ok I could imagine it making sense as a new CLOCK_FASTBUTLOUSYRESOLUTION timer in
clock_gettime() [together with the new vdso fastpath], but not as default.
-Andi
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