// Calculate the total memory chunks of the segment
static void
decideTotalChunks(int32 *totalChunks, int32 *chunkSizeInBits)
{
  ……
	nsegments = Gp_role == GP_ROLE_EXECUTE ? host_segments : pResGroupControl->segmentsOnMaster;
  // ResGroupOps_GetTotalMemory获得系统可用内存：
  // RAM * overcommit_ratio + Swap
	tmptotalChunks = ResGroupOps_GetTotalMemory() * gp_resource_group_memory_limit / nsegments;

	/* If vmem is larger than 16GB (i.e., 16K MB), we make the chunks bigger
	 * so that the vmem limit in chunks unit is not larger than 16K.*/
	tmpchunkSizeInBits = BITS_IN_MB;
	while(tmptotalChunks > (16 * 1024))
	{
		tmpchunkSizeInBits++;
		tmptotalChunks >>= 1;
	}
  ……
}