IA64-Linux perf tools for IO dorks

Itanium processors have very sophisticated performance monitoring tools integrated into the CPU. McKinley and Madison Itanium CPUs have over three hundred different types of events they can filter, trigger on, and count. The restrictions on which combinations of triggers are allowed is daunting and varies across CPU implementations. Fortunately, the tools hide this complicated mess. While the tools prevent us from shooting ourselves in the foot, it's not obvious how to use those tools for measuring kernel device driver behaviors.

IO driver writers can use pfmon to measure two key areas generally not obvious from the code: MMIO read and write frequency and precise addresses of instructions regularly causing L3 data cache misses. Measuring MMIO reads has some nuances related to instruction execution which are relevant to understanding ia64 and likely ia32 platforms. Similarly, the ability to pinpoint exactly which data is being accessed by drivers enables driver writers to either modify the algorithms or add prefetching directives where feasible. I include some examples on how I used pfmon to measure NIC drivers and give some guidelines on use.

q-syscollect is a "gprof without the pain" kind of tool. While q-syscollect uses the same kernel perfmon subsystem as pfmon, the former works at a higher level. With some knowledge about how the kernel operates, q-syscollect can collect call-graphs, function call counts, and percentage of time spent in particular routines. In other words, pfmon can tell us how much time the CPU spends stalled on d-cache misses and q-syscollect can give us the call-graph for the worst offenders.

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