[noamhacker]

How do you test a system like this for accuracy? Is this done by simulating millions of unique requests?

[andreareina]

The algorithm's accuracy is known. From the wiki[1]:

    The HyperLogLog algorithm is able to estimate 
    cardinalities of > 10^9 with a typical error rate of 2%

[1] https://en.wikipedia.org/wiki/HyperLogLog

[federicoponzi]

But what about the implementation accuracy? :)

[zeroxfe]

Tests against both historical and synthetic datasets.

[GhostVII]

Reddit probably has enough analytics to be able to show mathematically that it will be accurate without simulating any requests.

[icelancer]

Can't you just use Apache Benchmark and some proxies?