Verify that we have enough memory if cache_memory is set

fdbserver/fdbserver.actor.cpp

@@ -1488,6 +1488,11 @@ int main(int argc, char* argv[]) {
 		// evictionPolicyStringToEnum will throw an exception if the string is not recognized as a valid
 		EvictablePageCache::evictionPolicyStringToEnum(flowKnobs->CACHE_EVICTION_POLICY);
 
+		if (memLimit < FLOW_KNOBS->PAGE_CACHE_4K + (2LL<<30)) {
+			fprintf(stderr, "ERROR: --memory has to be at least 2GiB larger than --cache_memory\n");
+			flushAndExit(FDB_EXIT_ERROR);
+		}
+
 		if (role == SkipListTest) {
 			skipListTest();
 			flushAndExit(FDB_EXIT_SUCCESS);