* 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/