reduce cache mishits #2583
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reduce cache mishits #2583
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shyouhei
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Excellent write up 🤤 |
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@ko1 any idea? |
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Conflicts with #2564, going to resolve... |
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Can I merge this? |
shyouhei
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Prior to this changeset, majority of inline cache mishits resulted
into the same method entry when rb_callable_method_entry() resolves
a method search. Let's not call the function at the first place on
such situations.
In doing so we extend the struct rb_call_cache from 44 bytes (in
case of 64 bit machine) to 64 bytes, and fill the gap with
secondary class serial(s). Call cache's class serials now behavies
as a LRU cache.
Calculating -------------------------------------
ours 2.7 2.6
vm2_poly_same_method 2.339M 1.744M 1.369M i/s - 6.000M times in 2.565086s 3.441329s 4.381386s
Comparison:
vm2_poly_same_method
ours: 2339103.0 i/s
2.7: 1743512.3 i/s - 1.34x slower
2.6: 1369429.8 i/s - 1.71x slower
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While running discourse benchmark under linux perf, I noticed one of the most frequent operations that ruby does is the method lookup.
That was kind of known, but something didn't smell right. I pushed 3ffd98c and ran the benchmark again, to get this output:
So 65,039,079 out of 94,613,117 inline cache mishits are spurious; they resulted in method lookups that end up exactly the same method entry the cache already stored. This is not a buggy behaviour and your program works as expected (apart from unnecessarily consuming our precious environmental resources to generate electricity for the computation). However there definitely is a room of improvements.
Let's use the cache more efficiently. We are facing the fact that several classes share the identical method entry for a method name; possibly due to inheritance, inclusions, and so on. We can use this. A call cache is valid for multiple classes. So in order to express the info, this changeset expands
struct rb_call_cachefrom 44-ish bytes to cache line width. The space we add is used for second and later class serials.By this changeset the debug counter output for the same benchmark is now like this:
The
mc_miss_spuriouscounter dropped down to 23,344,738.