[389-devel] Nunc-stans status

[thierry bordaz]

Hi,

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/

      * /Event: New connections /
          o /Start all clients in parallel to establish connections
            (keeping them opened) take the duration to get 1000, 2000,
            ... 10000 connections and check there are drop or not/
          o /Establish 1000 connections and monitor during to open 100
            more, the same starting with 2000, 10000/
          o /Client should not run any operations during the monitoring/
      * /Event: New operations /
          o /Start all clients and when 1000 connections are
            established, launch simple operations (e.g. search -s base
            -b "" objectclass) and monitor how many of them can be
            handled. The same with 2000, ... 10000./
          o /response time and workqueue length could be monitored to be
            sure the bottleneck are not the worker./

[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