tree 173582527188927a501a5ed580fbfec712dfc969
parent b24086923c48767503a2d5d128e922794cd3e4c5
author Willy Tarreau <w@1wt.eu> 1592976068 +0200
committer Willy Tarreau <w@1wt.eu> 1592994086 +0200
encoding latin1

MEDIUM: tasks: apply a fair CPU distribution between tasklet classes

Till now in process_runnable_tasks() we used to reserve a fixed portion
of max_processed to urgent tasks, then a portion of what remains for
normal tasks, then what remains for bulk tasks. This causes two issues:

  - the current budget for processed tasks could be drained once for
    all by higher level tasks so that they couldn't have enough left
    for the next run. For example, if bulk tasklets cause task wakeups,
    the required share to run them could be eaten by other bulk tasklets.

  - it forces the urgent tasks to be run before scanning the tree so that
    we know how many tasks to pick from the tree, and this isn't very
    efficient cache-wise.

This patch changes this so that we compute upfront how max_processed will
be shared between classes that require so. We can then decide in advance
to pick a certain number of tasks from the tree, then execute all tasklets
in turn. When reaching the end, if there's still some budget, we can go
back and do the same thing again, improving chances to pick new work
before the global budget is depleted.

The default weights have been set to 50% for urgent tasklets, 37% for
normal ones and 13% for the bulk ones. In practice, there are not that
many urgent tasklets but when they appear they are cheap and must be
processed in as large batches as possible. Every time there is nothing
to pick there, the unused budget is shared between normal and bulk and
this allows bulk tasklets to still have quite some CPU to run on.