tree 9f2ebd9f2816e8120b0b0b198d5eca7d54c47d18
parent c35bcfcc2116b46e89cad5b34656ed3eb5378d3a
author Willy Tarreau <w@1wt.eu> 1593438995 +0200
committer Willy Tarreau <w@1wt.eu> 1593440950 +0200
encoding latin1

MEDIUM: server: improve estimate of the need for idle connections

Starting with commit 079cb9a ("MEDIUM: connections: Revamp the way idle
connections are killed") we started to improve the way to compute the
need for idle connections. But the condition to keep a connection idle
or drop it when releasing it was not updated. This often results in
storms of close when certain thresholds are met, and long series of
takeover() when there aren't enough connections left for a thread on
a server.

This patch tries to improve the situation this way:
  - it keeps an estimate of the number of connections needed for a server.
    This estimate is a copy of the max over previous purge period, or is a
    max of what is seen over current period; it differs from max_used_conns
    in that this one is a counter that's reset on each purge period ;

  - when releasing, if the number of current idle+used connections is
    lower than this last estimate, then we'll keep the connection;

  - when releasing, if the current thread's idle conns head is empty,
    and we don't exceed the estimate by the number of threads, then
    we'll keep the connection.

  - when cleaning up connections, we consider the max of the last two
    periods to avoid killing too many idle conns when facing bursty
    traffic.

Thanks to this we can better converge towards a situation where, provided
there are enough FDs, each active server keeps at least one idle connection
per thread all the time, with a total number close to what was needed over
the previous measurement period (as defined by pool-purge-delay).

On tests with large numbers of concurrent connections (30k) and many
servers (200), this has quite smoothed the CPU usage pattern, increased
the reuse rate and roughly halved the takeover rate.