tree 2e9ca8275cb796663ad84ebf96e91f715c02b880
parent d87a67f9bc422bf41b6b81c1e99d9aebbbc18d8e
author Willy Tarreau <w@1wt.eu> 1551771988 +0100
committer Willy Tarreau <w@1wt.eu> 1551963453 +0100
encoding latin1

MEDIUM: listener: change the LB algorithm again to use two round robins instead

At this point, the random used in the hybrid queue distribution algorithm
provides little benefit over a periodic scan, can even have a slightly
worse worst case, and it requires to establish a mapping between a
discrete number and a thread ID among a mask.

This patch introduces a different approach using two indexes. One scans
the thread mask from the left, the other one from the right. The related
threads' loads are compared, and the least loaded one receives the new
connection. Then one index is adjusted depending on the load resulting
from this election, so that we start the next election from two known
lightly loaded threads.

This approach provides an extra 1% peak performance boost over the previous
one, which likely corresponds to the removal of the extra work on the
random and the previously required two mappings of index to thread.

A test was attempted with two indexes going in the same direction but it
was much less interesting because the same thread pairs were compared most
of the time with the load climbing in a ladder-like model. With the reverse
directions this cannot happen.