Improve CLI parallelism

.github/workflows/ci.yaml

@@ -24,6 +24,7 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
+          cache-version: 2
       - run: bundle exec rake
 
   rubocop:
@@ -36,6 +37,7 @@ jobs:
         with:
           ruby-version: '3.3'
           bundler-cache: true
+          cache-version: 2
       - run: bundle exec rubocop
 
   rubies:
@@ -51,6 +53,7 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
+          cache-version: 2
       - run: bundle exec rake
 
   psych4:
@@ -68,6 +71,7 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
+          cache-version: 2
       - run: bundle exec rake
 
   minimal:
@@ -83,4 +87,5 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
+          cache-version: 2
       - run: bin/test-minimal-support

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Unreleased
 
+* Attempt to detect a QEMU bug that can cause `bootsnap precompile` to hang forever when building ARM64 docker images
+  from x86_64 machines. See #495.
+* Improve CLI to detect cgroup CPU limits and avoid spawning too many worker processes.
+
 # 1.18.4
 
 * Allow using bootsnap without bundler. See #488.

lib/bootsnap/cli.rb

@@ -4,7 +4,6 @@
 require "bootsnap/cli/worker_pool"
 require "optparse"
 require "fileutils"
-require "etc"
 
 module Bootsnap
   class CLI
@@ -29,7 +28,7 @@ def initialize(argv)
       self.compile_gemfile = false
       self.exclude = nil
       self.verbose = false
-      self.jobs = Etc.nprocessors
+      self.jobs = nil
       self.iseq = true
       self.yaml = true
       self.json = true

lib/bootsnap/cli/worker_pool.rb

@@ -1,16 +1,88 @@
 # frozen_string_literal: true
 
+require "etc"
+require "rbconfig"
+require "io/wait" unless IO.method_defined?(:wait_readable)
+
 module Bootsnap
   class CLI
     class WorkerPool
       class << self
         def create(size:, jobs:)
+          size ||= default_size
           if size > 0 && Process.respond_to?(:fork)
             new(size: size, jobs: jobs)
           else
             Inline.new(jobs: jobs)
           end
         end
+
+        def default_size
+          nprocessors = Etc.nprocessors
+          size = [nprocessors, cpu_quota || nprocessors].min
+          case size
+          when 0, 1
+            0
+          else
+            if fork_defunct?
+              $stderr.puts "warning: faulty fork(2) detected, probably in cross platform docker builds. " \
+                           "Disabling parallel compilation."
+              0
+            else
+              size
+            end
+          end
+        end
+
+        def cpu_quota
+          if RbConfig::CONFIG["target_os"].include?("linux")
+            if File.exist?("/sys/fs/cgroup/cpu.max")
+              # cgroups v2: https://docs.kernel.org/admin-guide/cgroup-v2.html#cpu-interface-files
+              cpu_max = File.read("/sys/fs/cgroup/cpu.max")
+              return nil if cpu_max.start_with?("max ") # no limit
+
+              max, period = cpu_max.split.map(&:to_f)
+              max / period
+            elsif File.exist?("/sys/fs/cgroup/cpu,cpuacct/cpu.cfs_quota_us")
+              # cgroups v1: https://kernel.googlesource.com/pub/scm/linux/kernel/git/glommer/memcg/+/cpu_stat/Documentation/cgroups/cpu.txt
+              max = File.read("/sys/fs/cgroup/cpu,cpuacct/cpu.cfs_quota_us").to_i
+              # If the cpu.cfs_quota_us is -1, cgroup does not adhere to any CPU time restrictions
+              # https://docs.kernel.org/scheduler/sched-bwc.html#management
+              return nil if max <= 0
+
+              period = File.read("/sys/fs/cgroup/cpu,cpuacct/cpu.cfs_period_us").to_f
+              max / period
+            end
+          end
+        end
+
+        def fork_defunct?
+          return true unless ::Process.respond_to?(:fork)
+
+          # Ref: https://github.com/Shopify/bootsnap/issues/495
+          # The second forked process will hang on some QEMU environments
+          r, w = IO.pipe
+          pids = 2.times.map do
+            ::Process.fork do
+              exit!(true)
+            end
+          end
+          w.close
+          r.wait_readable(1) # Wait at most 1s
+
+          defunct = false
+
+          pids.each do |pid|
+            _pid, status = ::Process.wait2(pid, ::Process::WNOHANG)
+            if status.nil? # Didn't exit in 1s
+              defunct = true
+              Process.kill(:KILL, pid)
+              ::Process.wait2(pid)
+            end
+          end
+
+          defunct
+        end
       end
 
       class Inline