Today I came across #10271, and while reading the discussion #6358 was linked to. Linux systems have a `SCHED_BATCH` scheduler priority that is useful for threads like loadblk. You can find the full details at [sched(7)](http://man7.org/linux/man-pages/man7/sched.7.html), but I'll quote the relevant part of the man page below:

> ...this policy will cause the scheduler to always assume that the thread is
CPU-intensive. Consequently, the scheduler will apply a small scheduling penalty
with respect to wakeup behavior, so that this thread is mildly disfavored in
scheduling decisions.
>
> This policy is useful for workloads that are noninteractive, but do not want to
lower their nice value, and for workloads that want a deterministic scheduling
policy without interactivity causing extra preemptions (between the workload's
tasks).

I think this change is useful independently of #10271 and irrespective of whether that change is merged. Under normal operation the loadblk thread will just import `mempool.dat`. However, if Bitcoin is started with `-reindex` or `-reindex-chainstate` this thread will use a great deal of CPU while it rebuilds the chainstate database (and the block database in the case of `-reindex`). By setting `SCHED_BATCH` this thread is less likely to interfere with interactive tasks (e.g. the user's web browser, text editor, etc.).

I'm leaving the nice value unchanged (which also affects scheduling decisions) because I think that's better set by the user. Likewise I'm not using [ioprio_set(2)](http://man7.org/linux/man-pages/man2/ioprio_set.2.html) because it can cause the thread to become completely I/O starved (and knowledgeable users can use `ionice(1)` anyway).

make check
test_runner.py