Limit pool size

[ntkme]

Currently we limit the isolate pool size to 15 as we know 16 isolates would deadlock. However, it turns out on 32-bit Dart SDK it would deadlock with 8 isolates.

The issue is that in Dart SDK having more than MaxMutatorThreadCount number of isolates would deadlock the whole Dart VM:

• Spawning 16 (or more) [Isolate]s with [sleep] freeze the application dart-lang/sdk#51254

The number of MaxMutatorThreadCount is determined through the following logic:

https://github.com/dart-lang/sdk/blob/3.0.5/runtime/bin/main_impl.cc#L1174-L1181

  // When running from the command line we assume that we are optimizing for
  // throughput, and therefore use a larger new gen semi space size and a faster
  // new gen growth factor unless others have been specified.
  if (kWordSize <= 4) {
    vm_options.AddArgument("--new_gen_semi_max_size=16");
  } else {
    vm_options.AddArgument("--new_gen_semi_max_size=32");
  }

https://github.com/dart-lang/sdk/blob/3.0.5/runtime/vm/heap/scavenger.h#L121

  static const intptr_t kTLABSize = 512 * KB;

https://github.com/dart-lang/sdk/blob/3.0.5/runtime/vm/heap/scavenger.h#L222-L231

  // The maximum number of Dart mutator threads we allow to execute at the same
  // time.
  static intptr_t MaxMutatorThreadCount() {
    // With a max new-space of 16 MB and 512kb TLABs we would allow up to 8
    // mutator threads to run at the same time.
    const intptr_t max_parallel_tlab_usage =
        (FLAG_new_gen_semi_max_size * MB) / Scavenger::kTLABSize;
    const intptr_t max_pool_size = max_parallel_tlab_usage / 4;
    return max_pool_size > 0 ? max_pool_size : 1;
  }

For current Dart SDK:

• 32-bit: 16MB / 512KB / 4 = 8
• 64-bit: 32MB / 512KB / 4 = 16

Main isolate counts as one so the limit is really 7 for 32-bit and 15 for 64 bit.

Unfortunately, there is no programatic way to get MaxMutatorThreadCount from Dart code, so we can only hard code for whatever is the default today.