### Description

Today, [force merge is assigned one thread](https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-threadpool.html) only.  

> For [force merge](https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-forcemerge.html) operations. Thread pool type is fixed with a size of 1 and an unbounded queue size.

In a large cluster where there are many processors available, there could be a backlog of force merge queued up.  Perhaps we should look into changing the default of force merge thread_pool to a max of `min(1, (# of allocated processors) / 8)`
server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java
"timer thread slept for [{}/{}ms] on absolute clock which is above the warn threshold of [{}ms]",
                        delta,
                        deltaMillis,
                        thresholdMillis
                    );
                } else if (deltaMillis < 0) {
                    final TimeValue delta = TimeValue.timeValueMillis(-deltaMillis);
                    logger.warn("absolute clock went backwards by [{}/{}ms] while timer thread was sleeping", delta, -deltaMillis);
                }
                final long deltaNanos = newRelativeNanos - relativeNanos;
                if (deltaNanos > thresholdNanos) {
                    final TimeValue delta = TimeValue.timeValueNanos(deltaNanos);
                    logger.warn(
                        "timer thread slept for [{}/{}ns] on relative clock which is above the warn threshold of [{}ms]",
                        delta,
                        deltaNanos,
                        thresholdMillis
                    );
                } else if (deltaNanos < 0) {
                    final TimeValue delta = TimeValue.timeValueNanos(-deltaNanos);
                    logger.error("relative clock went backwards by [{}/{}ns] while timer thread was sleeping", delta, -deltaNanos);
                    assert false : "System::nanoTime time should be monotonic";
                }
            } finally {
                absoluteMillis = newAbsoluteMillis;
                relativeNanos = newRelativeNanos;
            }
        }
    }
    static class ExecutorHolder {