Hash prefetching

[xbasel]

Prefetches hash type values.

This is 1 hash with 30 million fields, 70 bytes each

No hash prefetching (hget)
Summary:
  throughput summary: 974468.94 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
        0.295     0.112     0.295     0.343     0.391    33.151
With hash prefetching:
Summary:
  throughput summary: 1210800.38 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
        0.240     0.104     0.239     0.311     0.407     7.271
~%20 improvement.

[ranshid]

This is great!. I wonder if we went all the way though? in some cases we might also want to add another state to check if the hashtable has value pointer and prefetch it as well.