With https://reviews.llvm.org/D136627, now we have the metrics for profile staleness based on profile statistics, monitoring the profile staleness in real-time can help user quickly identify performance issues. For a production scenario, the build is usually incremental and if we want the real-time metrics, we should store/cache all the old object's metrics somewhere and pull them in a post-build time. To make it more convenient, this patch add an option to persist them into the object binary, the metrics can be reported right away by decoding the binary rather than polling the previous stdout/stderrs from a cache system.
For implementation, it writes the statistics first into a new metadata section(llvm.stats) then encode into a special ELF `.llvm_stats` section. The section data is formatted as a list of key/value pair so that future statistics can be easily extended. This is also under a new switch(`-persist-profile-staleness`)
In terms of size overhead, the metrics are computed at module level, so the size overhead should be small, measured on one of our internal service, it costs less than < 1MB for a 10GB+ binary.
Reviewed By: wenlei
Differential Revision: https://reviews.llvm.org/D136698
When a profile is stale and profile mismatch could happen, the mismatched samples are discarded, so we'd like to compute the mismatch metrics to quantify how stale the profile is, which will suggest user to refresh the profile if the number is high.
Two sets of metrics are introduced here:
- (Num_of_mismatched_funchash/Total_profiled_funchash), (Samples_of_mismached_func_hash / Samples_of_profiled_function) : Here it leverages the FunctionSamples's checksums attribute which is a feature of pseudo probe. When the source code CFG changes, the function checksums will be different, later sample loader will discard the whole functions' samples, this metrics can show the percentage of samples are discarded due to this.
- (Num_of_mismatched_callsite/Total_profiled_callsite), (Samples_of_mismached_callsite / Samples_of_profiled_callsite) : This shows how many mismatching for the callsite location as callsite location mismatch will affect the inlining which is highly correlated with the performance. It goes through all the callsite location in the IR and profile, use the call target name to match, report the num of samples in the profile that doesn't match a IR callsite.
This is implemented in a new class(SampleProfileMatcher) and under a switch("--report-profile-staleness"), we plan to extend it with a fuzzy profile matching feature in the future.
Reviewed By: hoy, wenlei, davidxl
Differential Revision: https://reviews.llvm.org/D136627
