Is gc.log writing asynchronous? safe to put gc.log

问题:

I heard multiple times that putting gc.log on NFS volume is terrible idea because it might cause gc pauses to be longer. Is it still the case with current jdk (8u25)?

回答1:

So I checked - it's not asynchronous and uses regular fopen/fwrite. Relevant code from jdk8u:

gcLogFileStream::gcLogFileStream(const char* file_name) {
  _cur_file_num = 0;
  _bytes_written = 0L;
  _file_name = make_log_name(file_name, NULL);

  // gc log file rotation
  if (UseGCLogFileRotation && NumberOfGCLogFiles > 1) {
    char tempbuf[FILENAMEBUFLEN];
    jio_snprintf(tempbuf, sizeof(tempbuf), "%s.%d" CURRENTAPPX, _file_name, _cur_file_num);
    _file = fopen(tempbuf, "w");
  } else {
    _file = fopen(_file_name, "w");
  }
  if (_file != NULL) {
    _need_close = true;
    dump_loggc_header();
  } else {
    warning("Cannot open file %s due to %s\n", _file_name, strerror(errno));
    _need_close = false;
  }
}

void gcLogFileStream::write(const char* s, size_t len) {
  if (_file != NULL) {
    size_t count = fwrite(s, 1, len, _file);
    _bytes_written += count;
  }
  update_position(s, len);
}