NSURLSession 缓存问题

如题，我的代码如下。

    NSURL *url = [NSURL URLWithString:@"http://www.baidu.com"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
    configuration.requestCachePolicy = NSURLRequestReturnCacheDataElseLoad;
    [configuration setURLCache:[NSURLCache sharedURLCache]];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration];

    NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {

        if (error) {
            NSLog(@"%@",error);
        }
        NSString *responseStr = [[NSString alloc]initWithData:data encoding:NSUTF8StringEncoding];
        NSLog(@"%@",responseStr);

    }];

[task resume];

不能从缓存当中读到数据。手机在2G网下，每次还是要读几秒钟才出来。

为什么啊？问题在哪啊