tcmalloc_sample_parameter is not zero by default, leading to terrible performance

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In our codebase, we see a significant amount of runtime being wasted in 
DoSampledAllocation. We're just using tcmalloc as an allocator, with no heap 
profiling or heap checking or any of that other fancy stuff.

Forcing tcmalloc_sample_parameter to zero gives us a 3.5% overall speedup 
when running sequential code, and a 1.5x speedup when running parallel code 
across 4 cores.

We're using tcmalloc 0.8 (yes, that's very old, but the sampled allocation 
stuff still seems to work the same way in current releases).

Original issue reported on code.google.com by meta...@gmail.com on 2 Jun 2010 at 5:30