Re: [Libunwind-devel] Testing time

[Lassi Tuura]

Hi,

> I also applied two of your patches. 01-performance-optimisations.patch
> is the only remaining patch in my inbox.

Thanks!

I refreshed the performance optimisation patch to reflect the current git tree; 
attached at the end.

There was also the trace-specific getcontext patch in another thread ("Another 
optimisation for x86-64 fast trace"). I've attached it here again, for 
completeness.

In the mean time I've experimented with yet another change to the fast trace, 
this time replacing the two-level hash table with a single-level one. This 
doesn't use mempool but instead grabs a slab of memory use GET_MEMORY (= mmap). 
Apart from the app-provided memory allocation hooks it's a bit more like what 
Jason Evans submitted.

This last one yields a fairly consistent ~5% improvement. I'm seeing this 
taking our application (387M stack walks to profile memory allocations) to 
50-51 clock cycles per stack level (was ~53), or ~1320 cycles per walk (was 
~1400). The gain is largely from avoiding the double indirect memory access in 
the hash code.

I hope the last improvement is still acceptable even with using less of the 
mempool stuff.

I experimented with a number of other changes, none of which yield consistent 
measurable improvement. I think with these changes the code is starting to be 
at the limit of what can be achieved - or at least what I am capable of :-) Per 
stack level time now mostly goes into a single hash table access plus reading 
the saved register values off the stack; everything else contributes little to 
the stack walk time (under heavy walking).

Regards,
Lassi

01-performance-optimisations.patch
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02-getcontext-lite.patch
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03-single-level-hash.patch
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