Add ast_churn: port of Go's garbage benchmark

benches/multithreaded/ast_churn/Project.toml

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+[deps]

benches/multithreaded/ast_churn/ast_churn.jl

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+include(joinpath("..", "..", "..", "util", "utils.jl"))
+
+# Port of Go's `garbage` benchmark (golang/benchmarks/garbage), the GC-heavy
+# workload used to motivate the Green Tea collector.
+#
+# The original repeatedly parses net/http with go/parser and discards the result,
+# keeping an array of parsed ASTs alive and overwriting only the first half each
+# iteration, so the second half behaves as an old generation. Parsing is done by
+# 2*GOMAXPROCS concurrent workers.
+#
+# This version parses Julia source from Base with `Meta.parseall`. An AST is a
+# deep, pointer-rich graph of many *different* small types (Expr, Symbol,
+# LineNumberNode, Vector{Any}, literals), which makes this a heterogeneous-heap
+# stress test for the mark phase - unlike the binary-tree benchmarks, no two
+# neighbouring objects can be assumed to share a layout.
+
+module ASTChurn
+
+const SRC_NAMES = ("abstractarray.jl", "array.jl", "dict.jl", "strings/basic.jl",
+                   "regex.jl", "iterators.jl", "set.jl", "sort.jl", "show.jl",
+                   "reduce.jl")
+
+function load_source()
+    io = IOBuffer()
+    for name in SRC_NAMES
+        path = Base.find_source_file(name)
+        (path === nothing || !isfile(path)) && continue
+        write(io, read(path, String), "\n")
+    end
+    s = String(take!(io))
+    isempty(s) && error("no Julia source found to parse")
+    return s
+end
+
+const SRC = load_source()
+
+parse_package() = Meta.parseall(SRC)
+
+# Total parses and the size of the live window. The window is deliberately a
+# fixed count rather than derived from a heap-size measurement: deriving it made
+# the live heap differ between runs, which made mark times incomparable.
+const NITER = 200
+const NPKG = 64
+
+function churn()
+    parsed = Vector{Any}(undef, NPKG)
+    for i in 1:NPKG                          # warm up, and populate the live set
+        parsed[i] = parse_package()
+    end
+    GC.gc(true)
+
+    half = max(NPKG ÷ 2, 1)
+    pos = Threads.Atomic{Int}(0)
+    remain = Threads.Atomic{Int}(NITER)
+    lk = ReentrantLock()
+
+    # 2 * nthreads concurrent parsers, as in the Go version
+    tasks = map(1:(2 * Threads.nthreads())) do _
+        Threads.@spawn while Threads.atomic_sub!(remain, 1) > 0
+            p = parse_package()
+            # Overwrite only the first half; the rest stays as the old generation
+            i = Threads.atomic_add!(pos, 1)
+            @lock lk begin
+                parsed[i % half + 1] = p
+            end
+        end
+    end
+    foreach(wait, tasks)
+    return pos[]
+end
+
+end # module ASTChurn
+
+@gctime ASTChurn.churn()