Messages in this thread | | | Subject | Re: [Jackit-devel] Re: Statistical methods for latency profiling | From | Lee Revell <> | Date | Sun, 01 Aug 2004 07:53:50 -0400 |
| |
On Sun, 2004-08-01 at 07:21, Ingo Molnar wrote: > On Sun, 1 Aug 2004, Lee Revell wrote: > > > So stressing the filesystem moves the center to the right a bit, from > > 6-7 to 9-10, and *drastically* lengthens the 'tail'. > > basically each codepath has a typical latency distribution, and when a > workload uses multiple codepaths then the latencies get intermixed almost > linearly. >
I noticed several distinct spikes with 1M samples, that blend into smooth Erlang/gamma type distribution at 5M. I posted some more results to jackit-dev. It seems like each of these would represent a common code path out of a non-preemptible region. I suspect the spike at 70-80 usecs is a bug in my code, from updating the histogram every 1024 cycles. I will start posting results on the web soon, it's getting big.
> > These numbers suggest to me that a lot of the latencies from 47 usecs > > and up are caused by one code path, because they are so uniformly > > distributed over the upper part of the histogram. The prime suspect of > > course being the ide io completions. I tested this theory by lowering > > max_sectors_kb from 64 to 32: > > > These numbers all point to the ide sg completion code as the only thing > > on the system generating latencies over ~42 usecs. > > yep, that's a fair assumption. Once the IO-APIC irq-redirection problems > are solved i'll try to further thread the IDE completion IRQ to remove > that ~100 usecs latency. >
It would be interesting to identify the code paths corresponding to the other peaks. It occurred to me that if you suspect a peak in the histogram is related to a certain code path, you could stick a udelay in there, and see if the spike moves up by the same amount.
Lee
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |