Tomasz Grabiec d61002cc33 lsa: Reduce reclamation latency ...

Currently eviction is performed until occupancy of the whole region
drops below the 85% threshold. This may take a while if region had
high occupancy and is large. We could improve the situation by only
evicting until occupancy of the sparsest segment drops below the
threshold, as is done by this change.

I tested this using a c-s read workload in which the condition
triggers in the cache region, with 1G per shard:

 lsa-timing - Reclamation cycle took 12.934 us.
 lsa-timing - Reclamation cycle took 47.771 us.
 lsa-timing - Reclamation cycle took 125.946 us.
 lsa-timing - Reclamation cycle took 144356 us.
 lsa-timing - Reclamation cycle took 655.765 us.
 lsa-timing - Reclamation cycle took 693.418 us.
 lsa-timing - Reclamation cycle took 509.869 us.
 lsa-timing - Reclamation cycle took 1139.15 us.

The 144ms pause is when large eviction is necessary.

The change improves worst case latency. Reclamation time statistics
over 30 second period after cache fills up, in microseconds:

Before:

  avg = 1524.283148
  stdev = 11021.021118
  min = 12.934000
  max = 144356.000000
  sum = 257603.852000
  samples = 169

After:

  avg = 1317.362414
  stdev = 1913.542802
  min = 263.935000
  max = 19244.600000
  sum = 175209.201000
  samples = 133

Refs #1634.

Message-Id: <1484730859-11969-1-git-send-email-tgrabiec@scylladb.com>

2017-01-19 17:35:36 +02:00

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