[llvm-bugs] [Bug 35964] New: Profile Instruction Generate/Coverage mapping: High overhead at runtime with many threads

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35964

            Bug ID: 35964
           Summary: Profile Instruction Generate/Coverage mapping: High
                    overhead at runtime with many threads
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: dennis.felsing at sap.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 19681
  --> https://bugs.llvm.org/attachment.cgi?id=19681&action=edit
Source code

Creating coverage information with -fprofile-instr-generate -fcoverage-mapping
usually doesn't create a high overhead. When using dozens of threads the
overhead seems to grow significantly in the end running 24 times as slow with
40 threads. See the following table created on a 2x Intel E5-2660v3 system:

Threads  No Coverage [s]  Coverage [s]  Factor
1                  0.011         0.011     1.0
2                  0.071         0.257     3.6
3                  0.216         1.419    6.57
4                  0.377         4.735   12.56
5                  0.678         8.097   11.94
10                 3.250        37.037   11.40
20                17.223       392.021   22.76
40                84.129      2087.047   24.81

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