Use concurrent sweeping instead of parallel sweeping by default.

src/flag-definitions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 526 matching lines @@
 DEFINE_bool(trace_code_flushing, false, "trace code flushing progress")
 DEFINE_bool(age_code, true,
             "track un-executed functions to age code and flush only "
             "old code (required for code flushing)")
 DEFINE_bool(incremental_marking, true, "use incremental marking")
 DEFINE_bool(incremental_marking_steps, true, "do incremental marking steps")
 DEFINE_bool(trace_incremental_marking, false,
             "trace progress of the incremental marking")
 DEFINE_bool(track_gc_object_stats, false,
             "track object counts and memory usage")
-DEFINE_bool(parallel_sweeping, true, "enable parallel sweeping")
-DEFINE_bool(concurrent_sweeping, false, "enable concurrent sweeping")
+DEFINE_bool(parallel_sweeping, false, "enable parallel sweeping")
+DEFINE_bool(concurrent_sweeping, true, "enable concurrent sweeping")
 DEFINE_int(sweeper_threads, 0,
            "number of parallel and concurrent sweeping threads")
 #ifdef VERIFY_HEAP
 DEFINE_bool(verify_heap, false, "verify heap pointers before and after GC")
 #endif
 
 
 // heap-snapshot-generator.cc
 DEFINE_bool(heap_profiler_trace_objects, false,
             "Dump heap object allocations/movements/size_updates")
@@ skipping 339 matching lines @@
 #undef DEFINE_ALIAS_float
 #undef DEFINE_ALIAS_args
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 
 #undef COMMA