Re: top/ps incorrectly reporting process execution times

[Karim Yaghmour]

Dimitris Michailidis wrote:
> 
> That behavior is due to a long-standing bug in Linux's accounting of idle
> time.  A patch has been sent to Linus to fix it.

I'm glad this problem is being fixed, but, as I said, this isn't the
only problem and I haven't seen a solution that solves so many problems
in the same time and provides so many possibilities without having
a horrible impact on the system's performance.
 
> LTT doesn't solve the problem, rather it uses the solution (namely, exact
> accounting).

Interesting, a solution that solves the problem without solving the
problem. That's a first!

> The kernel could do the same independently of LTT (and more
> cheaply). 

Hmm... How do you suggest doing that? How can the kernel do "exact
accounting" without doing exact accounting?

Furthermore, are you aware of the overhead caused by LTT?

Just for info, running a completely instrumented kernel without
activated tracing bares less that 0.5% on most workloads. Adding
the computation for exact accounting might make this climb up to 1% or
even, to really be generous, 2%. So what??? If your system really
has to depend on 2% extra workload then there's a problem somewhere
else and chances are that the 2% you pay to have more accurate data
will be rewarded when you've found and solved other bottlenecks.

I'll send you the paper that I just presented at Usenix if you're
interested.

> Traditionally Unix systems rely on statistical accounting and they
> pay some price in accuracy for this choice.  They could adopt exact
> accounting and pay some price in performance for that choice.

Or it could be completely configurable. Therefore the administrator
compiling the kernel could choose to have exact accounting or
statistical accounting, whichever fits his needs. In any case
it's better than no choice at all.

Cheers.

===================================================
                 Karim Yaghmour
               [e-mail]
          Operating System Consultant
 (Linux kernel, real-time and distributed systems)
===================================================