C10K is a scalability problem that a server can face when dealing with events of thousands of connections (i.e. clients) at the same time. Events can be new connections, new operations on the established connections, closure of connection (from client or server)

For 389-ds, C10K problem was resolved with a new framework Nunc-Stans [1]. Nunc-stans was first enabled in RHDS 7.4 and improved/fixed in 7.5. Robustness issues [2] and [3] were reported in 7.5 and it was decided to disable Nunc-stans. It is not known if those issues exist or not in 7.4.

William posted a PR to fix those two issues [4]. Nunc-stans is a complex framework, with its own dynamic. Review of this PR is not easy and even a careful review may not guaranty it will fix [2] and [3] and may not introduce others unexpected side effects.

From there we discussed two options (but there may be others):

  1. Review and merge the PR [4], then later run some intensive tests aiming to verify [2],[3] and checking the robustness in order to reenable NS
  2. Build some tests for
    1. measure the benefit of NS as [2] and [3] do not prevent some performance tests
    2. identify possible reproducers for [2] and [3]
    3. create robustness and long duration NS specific tests
    4. review and merge the PR [4]

As PR [4] is not intended for perf improvement, the step 2.1 will impact the priority according to the performance benefits.

Comments are welcomed

Regarding 2.1 plan we made the following notes for the test plan:

The benefit of Nunc-Stans can only be measure with a large number of connections (i.e. client) above 1000. That means a set of clients (sometime all) should keep their connection opened. Clients should run on several hosts so that clients are not the bootleneck.

For the two types of events (new connection and new operations), the measurement could be

[1] http://www.port389.org/docs/389ds/design/nunc-stans.html

[2] https://bugzilla.redhat.com/show_bug.cgi?id=1608746 deadlock
[3] https://bugzilla.redhat.com/show_bug.cgi?id=1605554 connection leaks

[4] https://pagure.io/389-ds-base/pull-request/49636