Re: 2.6.13-rt6, ktimer subsystem

[George Anzinger <george@mvista.com>]

Ingo Molnar wrote:
> i have released the 2.6.13-rt6 tree, which can be downloaded from the 
> usual place:
> 
>   http://redhat.com/~mingo/realtime-preempt/
> 
> there are lots of small updates all across and there's a big feature as 
> well in this release: a complete rework of the high-resolution timers 
> framework, from Thomas Gleixner, called 'ktimers'.
> 
> under the ktimer framework the HR (and posix) timers live in a separate 
> domain, have their own (per-CPU) rbtree to stay scalable and 
> deterministic even with a high number of timers. Another positive effect 
> of the introduction of separate ktimers is that kernel/timer.c is now 
> using preemptible locks again, removing the cascade() worst-case 
> latency. The cleanup factor is high as well: the ktimer framework 
> slashes 1300+ lines off the HRT code. See kernel/ktimer.c for details.
> 
> the end-effect of ktimers is a much more deterministic HRT engine. The 
> original merging of HR timers into the stock timer wheel was a Bad Idea 
> (tm). We intend to push the ktimer subsystem upstream as well.

Well, having spent a bit of time looking at the code it appears that a 
lot of the ideas we looked at and discarded (see 
high-res-timers-discourse@lists.sourceforge.net) are in this.  Shame it 
was all done with out reference or comment to that list, anyone on it or 
even the lkml.

I DO agree that it _looks_ nicer, cleaner and so on.  But there are a 
lot of things we rejected in here and they really do need, at least, a 
hard look.

A few of the top issues:

time in nanoseconds 64-bits, requires a divide to do much of anything 
with it.  Divides are slow and should be avoided if possible.  This is 
especially true in the embedded market.


The rbtree is a high overhead tree.  I suspect performance problems 
here.  If it is the right answer here, then why not use it for normal 
timers?  A list of timers is a rather unique thing and, I think, 
deserves a management structure that accounts for the fact that the 
elements in the tree are perishable.

It appears that the "monotonic_clock" is being used to drive ktimers. 
The "monotonic_clock" was NEVER meant to poke outside of the kernel.  It 
is a raw kernel clock that is only required to be monotonic with nothing 
said about accuracy.  It should NOT be confused with CLOCK_MONOTONIC 
which is directly tied to xtime and therefor is ntp corrected.

These are only the concerns I have from having a rather quick look at 
the code.  I am sure that there are other issues...



-- 
George Anzinger   george@mvista.com
HRT (High-res-timers):  http://sourceforge.net/projects/high-res-timers/