To get the best out of the PowerDNS recursor, which is important if you are doing thousands of queries per second, please consider the following.
Limit the size of the caches to a sensible value. Cache hit rate does not improve meaningfully beyond 4 million max-cache-entries per thread, reducing the memory footprint reduces CPU cache misses. See below for more information about the various caches.
Compile using g++ 4.1 or later. This compiler really does a good job on PowerDNS, much better than 3.4 or 4.0.
On AMD/Intel hardware, wherever possible, run a 64-bit binary. This delivers a nearly twofold performance increase. On UltraSPARC, there is no need to run with 64 bits.
Consider performing a 'profiled build' as described in the README. This is good for a 20% performance boost in some cases.
When running with >3000 queries per second, and running Linux versions prior to 2.6.17 on some motherboards, your computer may spend an inordinate amount of time working around an ACPI bug for each call to gettimeofday. This is solved by rebooting with 'clock=tsc' or upgrading to a 2.6.17 kernel.
The above is relevant if dmesg shows Using pmtmr for high-res timesource
A busy server may need hundreds of file descriptors on startup, and deals with spikes better if it has that many available later on. Linux by default restricts processes to 1024 file descriptors, which should suffice most of the time, but Solaris has a default limit of 256. This can be raised using the ulimit command. FreeBSD has a default limit that is high enough for even very heavy duty use.
For older versions <3.2: If you need it, try --fork, this will fork the daemon into two halves, allowing it to benefit from a second CPU. This feature almost doubles performance, but is a bit of a hack.
for 3.2 and higher, set 'threads' to your number of CPUs.
For best PowerDNS Recursor performance, use a recent version of your operating system, since this generally offers the best event multiplexer implementation available (kqueue, epoll, ports or /dev/poll).
A Recursor under high load puts a severe stress on any stateful (connection tracking) firewall, so much so that the firewall may fail.
Specifically, many Linux distributions run with a connection tracking firewall configured. For high load operation (thousands of queries/second), It is advised to either turn off iptables completely, or use the 'NOTRACK' feature to make sure DNS traffic bypasses the connection tracking.
Sample Linux command lines would be:
# iptables -t raw -I OUTPUT -p udp --dport 53 -j NOTRACK # iptables -t raw -I PREROUTING -p udp --sport 53 -j NOTRACK # iptables -I INPUT -p udp --dport 53 -j ACCEPT # iptables -I INPUT -p udp --sport 53 -j ACCEPT # iptables -I OUTPUT -p udp --dport 53 -j ACCEPT # # optionally # ip6tables -t raw -I OUTPUT -p udp --dport 53 -j NOTRACK # ip6tables -t raw -I PREROUTING -p udp --sport 53 -j NOTRACK # ip6tables -I INPUT -p udp --dport 53 -j ACCEPT # ip6tables -I INPUT -p udp --sport 53 -j ACCEPT # ip6tables -I OUTPUT -p udp --dport 53 -j ACCEPT
Following the instructions above, you should be able to attain very high query rates.
The PowerDNS Recursor contains a number of caches, or information stores:
The "NSSpeeds" cache contains the average latency to all remote authoritative servers.
The "Negcache" contains all domains known not to exist, or record types not to exist for a domain.
The Recursor Cache contains all DNS knowledge gathered over time.
The Packet Cache contains previous answers sent to clients. If a question comes in that matches a previous answer, this is sent back directly.
The Packet Cache is consulted first, immediately after receiving a packet. This means that a high hitrate for the Packet Cache automatically lowers the cache hitrate of subsequent caches. This explains why releases 3.2 and beyond see dramatically lower DNS cache hitrates, since this is the first version with a Packet Cache.