Messages in this thread | | | Date | Tue, 22 Jan 2008 11:04:14 -0800 | From | Arjan van de Ven <> | Subject | Re: Strange interaction between latencytop and the scheduler |
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Török Edwin wrote: > Hi Arjan, Ingo,
> [for minor latencytop userspace specific issues see at end of mail] > > * if I compile with CONFIG_SCHED_DEBUG I see a 30-40msec latency from > the scheduler, always (Scheduler: waiting for cpu). > There is also a constant ~5% user, ~2% sys CPU usage on an idle system, > regardless if latencytop sysctl is on or off. > Is this normal? (is overhead really 40msec?)
I'm seeing similar, I would not rule out that this is actual scheduler behavior ;( at least it seems consistent. I also made a patch (to lkml yesterday) that counts the total and count of scheduler latencies to also show the cumulative effect of these latencies
> I was also seeing an unusually high number of context switches (as shown > by vmstat), I usually have 400-800 with non-patched kernels (while > running mplayer too), but I was getting steadily over 1100 with the > patch (on idle system).
that's weird; there's nothing blocking in the entire patch at all. (the only lock is a spinlock)
The performance aspect... collecting the data isn't cheap (which is why it's made a sysctl), I still plan to look at optimizing it but it won't ever be free.
> * I compile without CONFIG_SCHED_DEBUG, I no longer get *any* latency > from the scheduler, even if I run multi-threaded programs, etc. Is this > to be expected? (i.e. is this feature available only when enabling > CONFIG_SCHED_DEBUG?)
afaik yes.
> Some minor latencytop userspace issues: > * percentages: new feature (nice!), but the values seem all wrong > (global perc. are always wrong, per app perc. are usually ok, but see > second example below)
note that the percentages are percentage this entry had compared the total sum of latencies. The maximum entry is only loosely related to that; say you have 1 latency in "foo" for 100ms but 900 of 1ms in "bar", "foo" will show 10% and "bar" will show 90%
> * I miss the Average latency column. If it is too costly to keep account > of an overall average, can we have last N second average?
it's not costly to calculate, it's the screen space versus the value of the information :(
> * unknown reasons show a backtrace, but backtrace doesn't have enough > room on screen
still working on that; you can pass the --unknown option to dump these so that I can add them to the translation table. -- 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/
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