module app;

import vibe.core.concurrency : async, Future;
import vibe.core.core;
import vibe.core.log;
import vibe.db.redis.redis;

import std.exception : assertThrown;
import std.format;
import std.getopt;
import std.range; //chunks, iota;

int main (string[] args)
{
    auto num_records = 1_000_000UL;
    getopt(args, "records|n", &num_records);

	RedisClient redis;
	try
		redis = new RedisClient();
	catch (Exception)
	{
		logInfo("Failed to connect to local Redis server.");
		return 1;
	}

	auto processed_records = 0UL;
	int ret = 0;

	runTask({
		auto num_chunks = 32;
		auto max_items_x_chunk = num_records / num_chunks;

		auto chunk_records = chunks(iota(0, num_records), max_items_x_chunk);
		logInfo("Number of chunks: %d", chunk_records.length);
		logInfo("Maximum number of items per chunk: %d",
			chunk_records[0].length);

		Future!(ulong)[] workers;
		foreach (chunk; chunk_records)
		{
			workers ~= async({
				try
				{
					foreach (item; chunk)
					{
						redis.getDatabase(0).set(
							"Key%d".format(item), "Value%d".format(item));
					}
				}
				catch (Throwable th)
		        {
					logError("Set operation failed: %s", th.msg);
					logDiagnostic("Full error: %s", th);
					ret = 1;
				}
				return chunk.length;
			});
		}

		foreach (worker; workers)
			processed_records += worker.getResult();

		exitEventLoop();
	});
	runEventLoop();

	logInfo("Processed records: %d", processed_records);

	return ret;
}

Running that version, the elapsed time and the context switches were considerable reduced 

$ /usr/bin/time -v ./bulktest
[main(k1sG) INF] Number of chunks: 32
[main(k1sG) INF] Maximum number of items per chunk: 31250
[main(----) INF] Processed records: 1000000
	Command being timed: "./bulktest"
	User time (seconds): 8.42
	System time (seconds): 6.23
	Percent of CPU this job got: 99%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 0:14.67
	Average shared text size (kbytes): 0
	Average unshared data size (kbytes): 0
	Average stack size (kbytes): 0
	Average total size (kbytes): 0
	Maximum resident set size (kbytes): 12400
	Average resident set size (kbytes): 0
	Major (requiring I/O) page faults: 0
	Minor (reclaiming a frame) page faults: 832
	Voluntary context switches: 37
	Involuntary context switches: 1190
	Swaps: 0
	File system inputs: 0
	File system outputs: 0
	Socket messages sent: 0
	Socket messages received: 0
	Signals delivered: 0
	Page size (bytes): 4096
	Exit status: 0