// The optimistic mode does not converge.  Let's fall back to the
      // pessimistic mode.
      TF_RETURN_IF_ERROR(
          PopulateFrame(sub_topo, /*use_optimistic_mode=*/false, nullptr));
    }
    VLOG(2) << "Done populating frame " << cur_frame_name << " using the "
            << (success ? "optimistic" : "pessimistic") << " mode.";
  }

  return absl::OkStatus();
}

    // calls a function that has not yet been analyzed, we will not be able to
    // backtrack through that function call (our analysis will be correct but
    // pessimistic).
    for (CallGraphNode* node : scc) {
      if (node->isExternal()) continue;
      Region* region = node->getCallableRegion();
      GetOrCreateAnalysis(*region);
    }
  }

      // cluster that will be moved to the host at the same time since both
      // defining op and user op will be moved to host.
      if (cluster_ops.count(user)) {
        continue;
      }
      // This is pessimistic and in some cases will add extra communication.
      if (!HasOutsideCompilationAncestor(user) || has_dynamic_outputs ||
          HasDynamicOutputs(user)) {
        if (!user_set.insert(user).second) continue;

		// that the overheads of scavenging are better amortized, so better
		// scavenging throughput.
		//
		// The current value is chosen assuming a cost of ~10µs/physical page
		// (this is somewhat pessimistic), which implies a worst-case latency of
		// about 160µs for 4 KiB physical pages. The current value is biased
		// toward latency over throughput.
		const scavengeQuantum = 64 << 10