On Tue, 27 May 2025 at 12:45, Потапов Александр <a.pota...@postgrespro.ru> wrote: > > Hello! > > I ran some experiments with pgbench to measure the initialization time and > found that the time increases quadratically with the number of clients. It > was surprising to me and I would like to understand a reason of such behavior. > > Some details on how it was done: > [...] > > It turned out that the results correspond to a quadratic dependence like y ~ > 0.0002x^2 where x is a number of clients and y is init time (ms). > Here there is a question: is it expected behavior or a bug? What do you > think? I appreciate any comments and opinions.
Note that the value of "initial connection time" is based on the time it takes from about the start of the pg_bench process until the moment all N expected connections have been established, *not* the average time it took pg_bench to connect to PostgreSQL. This does also not exclude other known measurable delays (like spawning threads, synchronization, etc), so the actual per-connection connection time is probably closer to O(n) than O(n^2). Q: Did you check that pgbench or the OS does not have O(n_active_connections) or O(n_active_threads) overhead per worker during thread creation or connection establishment, e.g. by varying the number of threads used to manage these N clients? I wouldn't be surprised if there are inefficiencies in e.g. the threading- or synchronization model that cause O(N) per-thread overhead, or O(N^2) overall when you have one thread per connection. Kind regards, Matthias van de Meent Neon (https://neon.tech)
