In part I, part II and part III I talked about the changes to DNS resolution in curl and why we do them. In this post I cover the performance/resources related changes in the threaded resolver. This is the most common build option, deployed by many distros. Previously… In curl 8.19.0 and earlier, the “threaded resolver” means libcurl starts a new thread to call getaddrinfo(), as I explained in part II. In addition, it opens a socketpair (or eventfd on modern linux), so the resolver thread can notify the “main” thread it’s done.