Skip to main content

Performance of Redis sorted set operations

I was working on a feature recently, which involved several Redis "remove from sorted set" operations. Target production environment sorted sets are expected to be small and most of these calls would be trying to remove items, which do not exist in the sets. Although ZREM operation has documented LON(N) time complexity, ZSCORE has documented constant time complexity. This led me to believe, that Redis might have constant time complecity for ZREM calls when values to be removed do not exist in the set (if ZSCORE "knows" if item is NOT in the set in constant time, ZREM can do the same, right?).

Not in this case, apparently. ZSCORE documented constant time complexity is actually misleading (as many cases of asymptotic time complexity documentation for small data sets). Redis stores small sorted sets (up to 128 items by default) as "ziplists", which are essentially linked lists, optimized for memory consumption. Browsing through Redis source code confirms the worst - there is no additional data structure hidden somewhere, which would make ZREM or ZSCORE operation on ziplists constant time or even LOG(N) - finding value in a set involves walking entire linked list! This (walking entire list) is exactly what happens, when you try to delete non-existing item in a small sorted set.

Following node.js script shows it in a small performance measurement (it is actually a good example of "callback hell" :) ). The script fills sorted set with certain number of values (5, 120, 1000, 10000) and then calls ZSCORE on this set in a loop. It may be surprising to see that calling ZSCORE on a set of 1000 items is faster than on a set of 5 items and much faster than on a set of 200 items. On the other hand, this script confirms that ZSCORE really becomes constant time operation on sets of more than 128 items.

Of course, real life is much more complex and ziplist use of CPU memory caches should be better on average. Still this example shows that one should take asymptotic time complexity documentation with a pinch of salt.


Anonymous said…
Types Of Baccarat | Baccarat | Legal Play
Play Baccarat Online, but 바카라 the rules of Baccarat are different. Each is dealt one single card and it betway is bet365 the same as the standard version. Baccarat has the

Popular posts from this blog

Nginx secure link with node.js

Serving static files is a natural task for web servers. They, especially ones, having asynchronous architecture (like Nginx), are very good at such tasks. However, usually there is an additional security logic, which should restrict access to files you've published. IIS, for example, offers deep integration with application layer, which allows custom .NET "middleware" logic injection into the request pipeline. Node.js applications are very often published behind Nginx for various reasons and, with the help of Nginx "Secure Link" module, it's possible to offload static file serving tasks from node.js to Nginx, even if the files are not public. This module uses "shared secret" string (known to Nginx and the application) and expects a hash, based on this secret, to be present in the request to decide whether to proceed or return an error. Secure Link module may work in 2 alternative modes (

Lost promises

I love Promises . I think they make modern JavaScript possible, especially on the server side. But promises are, you know, promises and some of them are literally lost! I would even say that lost promises are, to a certain degree, the buffer overflow of JavaScript. OK, it's not as widespread and it hasn't cost as many billions of dollars, but it still may be as subtle, as difficult to notice and just as devastating. At least I have encountered this issue a few times and it works like that: In the code above we simply forget to add "return" keyword before call to sideEffect3 function. This is totally OK, except when you rely on the fact that the Promise returned from giveMePromise is resolved after "side effect 3" can be observed. In our case, Promise was given, but it was lost. That sideEffect3  function is trying in vain, because it's work will never be used. I think this is just a danger of asynchronous code and such errors can only be detecte