Re: [patch 09/17] LTTng instrumentation - filemap

[Mathieu Desnoyers]

* Nick Piggin (nickpiggin@xxxxxxxxxxxx) wrote:
> On Wednesday 16 July 2008 08:26, Mathieu Desnoyers wrote:
> > Instrumentation of waits caused by memory accesses on mmap regions.
> >
> > Those tracepoints are used by LTTng.
> >
> > About the performance impact of tracepoints (which is comparable to
> > markers), even without immediate values optimizations, tests done by Hideo
> > Aoki on ia64 show no regression. His test case was using hackbench on a
> > kernel where scheduler instrumentation (about 5 events in code scheduler
> > code) was added. See the "Tracepoints" patch header for performance result
> > detail.
> 
> BTW. this sort of test is practically useless to measure overhead. If
> a modern CPU is executing out of primed insn/data and branch prediction
> cache, then yes this sort of thing is pretty well free.
> 
> I see *real* workloads that have got continually and incrementally slower
> eg from 2.6.5 to 2.6.20+ as "features" get added. Surprisingly, none of
> them ever showed up individually on a microbenchmark.
> 
> OK, for this case if you can configure it out, I guess that's fine. But
> let's not pretend that adding code and branches and function calls are
> ever free.

I never pretended anything like that. Actually, that's what the
"immediate values" are for : they allow to patch load immediate value
instead of a memory read to decrease d-cache impact. They now allow to
patch a jump instead of the memory read/immediate value read + test +
conditional branch to skip the function call with fairly minimal impact.
I agree with you that eating precious d-cache and jump prediction buffer
entries can eventually slow down the system. But this will be _hard_ to
show on a single macro benchmark, and the microbenchmark showing it will
have to be taken in conditions which will exacerbate the d-cache and BPB
impact.

Mathieu

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Mathieu Desnoyers
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