JIT: don't set edge weights to zero for inconsistent profiles #87947
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With the advent of scalable profile counters (or even with the old racing counters) counts might be approximate or inconsistent. When we run across a negative count during reconstruction, set the afflicted count to a small positive value instead of to zero, since zero has special meaning to the JIT. The aim is to reduce some of the benchmark instability we are seeing in dotnet#87324. Depending on exact counter values, we can see different sets of edge weights (and hence block weights) from run to run.
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Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch Issue DetailsWith the advent of scalable profile counters (or even with the old racing counters) counts might be approximate or inconsistent. When we run across a negative count during reconstruction, set the afflicted count to a small positive value instead of to zero, since zero has special meaning to the JIT. The aim is to reduce some of the benchmark instability we are seeing in #87324. Depending on exact counter values, we can see different sets of edge weights (and hence block weights) from run to run.
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@EgorBo PTAL Small number of diffs/missed contexts expected. |
src/coreclr/jit/fgprofile.cpp
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| // If we end up with negative edge weights because of inconsistent counts, we use this factor to | ||
| // substitute a small positive one. | ||
| // | ||
| weight_t const smallFactor = 0.001; |
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there is ProfileSynthesis.epsilon that probably can be used here?
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Yea, good idea. Updated.
With the advent of scalable profile counters (or even with the old racing counters) counts might be approximate or inconsistent. When we run across a negative count during reconstruction, set the afflicted count to a small positive value instead of to zero, since zero has special meaning to the JIT.
The aim is to reduce some of the benchmark instability we are seeing in #87324. Depending on exact counter values, we can see different sets of edge weights (and hence block weights) from run to run.