bert hubert finally blogs

Optimisation: sometimes the best move is not to play

Posted by bert hubert Sun, 09 Apr 2006 22:18:00 GMT

Spent some time this weekend setting up my father’s holiday location, which was very good. Time flew by and we had to race to pick up my car, which was on a company property that would be locked up at night. His new place has Wifi, but he is too far removed from the access point. I ordered a very nice directional antenna from the Wifi Shop, hope it shows up soon. Aiming it should be hard work, as getting line of sight requires mounting the antenna on a large pole. Should be worthwhile though!

Movies & PowerDNS

As regular readers of this blog will know, I’m currently working hard on making the PowerDNS recursor the fastest and most ablest nameserver on this planet.

To this end, I’ve done a lot of micro-optimisation, which means trying to make individual functions as fast as possible. Time and time again however, I’ve discovered that gcc 4.1 does a very fine job already.

Until I started finding very major optimisations, which remind me of one of my favorite movies, War Games, which has a famous quote, ‘Sometimes the best move is not to play’. This has turned out to be very apt.

The best way to speed up a program on a modern platform.. is by making it do less. So I spent some time doing that. DNS at its core is case-insensitive, so www.powerdns.com is equal to WwW.PoWeRdNs.CoM. There are basically two ways do go about this. The first is to first lowercase everything, the second is to make your comparison functions case-insensitive.

The problem with the first solution is that, while allowed, many DNS clients will react badly when they receive an answer for ‘www.google.com’ when they asked for ‘WWW.GOOGLE.COM’. So you have to keep the original question around. This costs memory, and time.

Previously PowerDNS followed approach 1. I’ve now moved it to true case insensitvity. I also solved the strange and murky world of the trailing dot, ‘www.google.com’ versus ‘www.google.com.’.

Al this proved to be a small speedup, but it feels a lot better now lowercasing everything all the time.

Even bigger news in the ‘not to play’ department was short-circuiting the whole wonderful MOADNSParser (mother of all etc) structure. This system is wonderful, a joy to program, and about as efficient as it can get while remaining safe.

However - to decode and encode A, CNAME and NS records, it is overkill. So, large parts of PowerDNS now do those records by hand, saving a lot of malloc/free (new/delete) calls. Especially for A records, which constitute the bulk of all queries, this makes a big difference.

I wonder very much why I did not do this earlier, but I think it is because I love the MOADNSParser too much. I should be honest however and know that it is overkill to use so much code for parsing the 4 bytes of an IP address!

PowerDNS appears to be 20-30% faster because of the work today. On FreeBSD, a loaded PowerDNS now spends 25% of its time in the kernel, which tells me we are doing all right.