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Perfcounters added to the mainline

Perfcounters added to the mainline

Posted Jul 3, 2009 23:24 UTC (Fri) by mingo (subscriber, #31122)
In reply to: Perfcounters added to the mainline by deater
Parent article: Perfcounters added to the mainline

* Not all Intel chips have full hardware counter support under perf. Most notably, Pentium Pro/II/III or Pentium 4. One might argue that these are old and don't matter, but they are supported by perfmon2. Pentium 4 is the troublesome one, because the performance counters for that architecture don't map well at all to the abstraction chosen by the perf developers.

Those chips are indeed old, abandoned and dont matter. perfmon has support for them partly because perfmon was started when those chips were still relevant. Alas, IMO, this also created a wrong design and the wrong mindset for perfmon.

So in a sense, perfcounters was lucky to have come later, when saner PMUs emerged on x86.

The central concept of perfmon is to expose the PMU to user-space and to push all the complexity to user-space.

The central concept of perfcounters is to provide rich, kernel-based abstractions to measure performance characteristics of a Linux system in a coherent, unified framework - regardless of whether the information comes from a PMU, a software counter, a tracepoint or some data field somewhere.

Those are two wildly different and fundamentally conflicting sets of design goals.

But you would be wrong to suggest that P4 support is not possible under perfcounters. For example PowerPC support (which was cited as the primary counter argument against perfcounters in the perfmon vs. perfcounters discussions) is alive, well and kicking under perfcounters.

The reason why people are not rushing to implement perfcounters for P4 is probably that Core2 and later CPUs are just so much more different from a performance profile than the abandoned Netburst architecture. They are also a lot more pleasant CPUs from many perspectives.

It also makes little sense to profile on too old CPUs, as any performance optimization would have to be re-validated on more recent CPUs as well. People who care about performance tend to try to stay on the hardware edge as well, and dont tend to use obsolete systems.

As the years advance, so will perfcounter's currently 'cutting edge' PMU support create the same kind of backwards-pointing trail of CPU models. Or, if anyone cares about P4 PMU support, it can be implemented just fine as well - patches are certainly welcome.

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