config: Use null memory for DRAM sweep script

Do not waste time when we do not care about the data.

configs/dram/sweep.py

@@ -93,6 +93,9 @@ system.clk_domain = SrcClockDomain(clock = '1.5GHz',
 mem_range = AddrRange('256MB')
 system.mem_ranges = [mem_range]
 
+# do not worry about reserving space for the backing store
+mmap_using_noreserve = True
+
 # force a single channel to match the assumptions in the DRAM traffic
 # generator
 options.mem_channels = 1
@@ -104,6 +107,9 @@ MemConfig.config_mem(options, system)
 if not isinstance(system.mem_ctrls[0], m5.objects.DRAMCtrl):
     fatal("This script assumes the memory is a DRAMCtrl subclass")
 
+# there is no point slowing things down by saving any data
+system.mem_ctrls[0].null = True
+
 # Set the address mapping based on input argument
 # Default to RoRaBaCoCh
 if options.addr_map == 0: