On Tue, Oct 21, 2014 at 11:00:26AM -0400, Vince Weaver wrote: > On Mon, 20 Oct 2014, Andy Lutomirski wrote: > > > ISTM it would be a lot better to use the perf subsystem for this. You > > can probably pin an event to a pmu. > > No, you cannot pin an event to a counter with perf_event. > That's one of the big differences between perf_event and, say, perfmon2. > > With perf_event the kernel controls which events go in which counters and > the user has no say. That's part of why you need to check the mmap page > every time you want to use rdpmc because there's no other way of knowing > which counter to read to get the event you want. > > 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? > and mildly high overhead when doing a proper rdpmc call (due to > the required looking at mmap, and the fact that you need to do two rdpmc > calls before/after to get your value). This is why people really worried > about low-latency measurements bypass as much of perf_event as possible. 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? -- 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/
