alloc profile: print percentage of allocs missed

stdlib/Profile/src/Allocs.jl

@@ -39,7 +39,7 @@ A sample rate of 1.0 will record everything; 0.0 will record nothing.
 julia> Profile.Allocs.@profile sample_rate=0.01 peakflops()
 1.03733270279065e11
 
-julia> results = Profile.Allocs.fetch();
+julia> results = Profile.Allocs.fetch()
 
 julia> last(sort(results.allocs, by=x->x.size))
 Profile.Allocs.Alloc(Vector{Any}, Base.StackTraces.StackFrame[_new_array_ at array.c:127, ...], 5576)
@@ -52,6 +52,12 @@ macro profile(ex)
     _prof_expr(ex, :(sample_rate=0.0001))
 end
 
+# globals used for tracking how many allocs we're missing
+# vs the alloc counters used by @time
+const _g_gc_num_before = Ref{Base.GC_Num}()
+const _g_sample_rate = Ref{Real}()
+const _g_expected_sampled_allocs = Ref{Float64}(0)
+
 function _prof_expr(expr, opts)
     quote
         $start(; $(esc(opts)))
@@ -69,6 +75,9 @@ A sample rate of 1.0 will record everything; 0.0 will record nothing.
 """
 function start(; sample_rate::Real)
     ccall(:jl_start_alloc_profile, Cvoid, (Cdouble,), Float64(sample_rate))
+
+    _g_sample_rate[] = sample_rate
+    _g_gc_num_before[] = Base.gc_num()
 end
 
 """
@@ -78,6 +87,15 @@ Stop recording allocations.
 """
 function stop()
     ccall(:jl_stop_alloc_profile, Cvoid, ())
+
+    # increment a counter of how many allocs we would expect
+    # the memory profiler to see, based on how many allocs
+    # actually happened.
+    gc_num_after = Base.gc_num()
+    gc_diff = Base.GC_Diff(gc_num_after, _g_gc_num_before[])
+    alloc_count = Base.gc_alloc_count(gc_diff)
+    expected_samples = alloc_count * _g_sample_rate[]
+    _g_expected_sampled_allocs[] += expected_samples
 end
 
 """
@@ -87,6 +105,8 @@ Clear all previously profiled allocation information from memory.
 """
 function clear()
     ccall(:jl_free_alloc_profile, Cvoid, ())
+
+    _g_expected_sampled_allocs[] = 0
 end
 
 """
@@ -98,8 +118,13 @@ objects which can be analyzed.
 function fetch()
     raw_results = ccall(:jl_fetch_alloc_profile, RawAllocResults, ())
     decoded_results = decode(raw_results)
-    @warn("This allocation profiler currently misses some allocations. " *
-    "For more info see https://github.com/JuliaLang/julia/issues/43688")
+
+    missed_allocs = _g_expected_sampled_allocs[] - length(decoded_results.allocs)
+    missed_percentage = round(Int, missed_allocs / _g_expected_sampled_allocs[] * 100)
+    @warn("The allocation profiler is not fully implemented, and missed $(missed_percentage)% " *
+            "($(round(Int, missed_allocs)) / $(round(Int, _g_expected_sampled_allocs[]))) " *
+            "of allocs in the last run. " *
+            "For more info see https://github.com/JuliaLang/julia/issues/43688")
     return decoded_results
 end