* Lee Revell <[email protected]> wrote:
> > could you run a bit with tracing disabled (in the .config) on the C3?
> > (but wakeup timing still enabled) It may very well be tracing overhead
> > that makes those latencies that high. Also, we'd thus have some hard
> > data on how much overhead tracing is in such a situation, on that CPU.
>
> I have not left it to run overnight yet with the swappiness set to
> 100, which triggers the biggest latencies as my entire desktop is
> swapped out, but so far it looks like the problem was tracing
> overhead. With timing enabled but tracing disabled the longest
> latency on the C3 so far is 270 usecs.
>
> An important giveaway is that with tracing enabled the same code path
> only triggers ~200 usec latencies on the K7 but ~2ms on the C3. Since
> the longest latency with PREEMPT_DESKTOP is normally more a function
> of memory bandwidth than processor speed, and the machines differ much
> more in the latter, this agrees with the theory that the overhead is
> the problem.
besides cycle overhead, function tracing increases cache footprint - and
with a CPU that has smaller caches (such as the C3) it can tip a loop
over the edge, and can make it cache-trashing, while it would fit into
the cache before. In such a situation the difference can be dramatic.
(on CPUs with larger caches similar artifacts can happen too, but it
needs a 'fatter' loop, which are apparently rarer.)
Ingo
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]