On Tue, Oct 21, 2014 at 01:05:49PM -0400, Vince Weaver wrote: > On Tue, 21 Oct 2014, Peter Zijlstra wrote: > > > > perf_event is also fairly high overhead for setting up and starting > > > events, > > > > Which you only do once at the start, so is that really a problem? > > There are various reasons why you might want to start events at times > other than the beginning of the program. Some people don't like kernel > multiplexing so they start/stop manually if they want to switch eventsets.
I suppose you could pre-create all events and use ioctl()s to start/stop them where/when desired, this should be faster I think. But yes, this is not a use-case I've though much about. > But no, I suppose you could ask anyone wanting to use rdpmc to open some > sort of dummy event at startup just to get cr4 enabled. That's one work-around :-) > > I still don't get that argument, 2 rdpmc's is cheaper than doing wrmsr, > > not to mention doing wrmsr through a syscall. And looking at that mmap > > page is 1 cacheline. Is that cacheline read (assuming you miss) the real > > problem? > > Well at least by default the first read of the mmap page causes a > pagefault which adds a few thousand cycles of latency. Though you can > somewhat get around this by prefaulting it in at some point. MAP_POPULATE is your friend there, but yes manually prefaulting is perfectly fine too, and the HPC people are quite familiar with the concept, they do it for a lot of things. > Anyway I'm just reporting numbers I get when measuring the overhead of > the old perfctr interface vs perf_event on typical PAPI workloads. It's > true you can re-arrange calls and such so that perf_event behaves better > but that involves redoing a lot of existing code. OK agreed, having to change existing code is often subject to various forms of inertia/resistance. And yes I cannot deny that some of the features perf has come at the expense of various overheads, however hard we're trying to keep costs down. > I do appreciate the trouble you've gone through keeping self-monitoring > working considering the fact that I'm the only user admitting to using it. I have some code somewhere that uses it too, I've tried pushing it off to other people but so far there are no takers :-) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
