Introduce a PassthroughLoadingCache for disabled caches

Gerrit documentation states that when the memoryLimit of a cache
is zero then it is disabled. However, the code told a different
story because there was always a Guava/Caffeine cache created
with a potential impact on performance due to the inherent locking
of the underlying ConcurrentHashMap.

The ConcurrentHashMap would always lock additions and removals on
the same key, and the only way to avoid locking is to avoid
the instantiation of the ConcurrentHashMap.

Why should someone want to ever disable a cache?

Disabling the cache typically isn't a good idea, however, there
could be situations where:
1. The rate of the access of the cached value for K is low and the
   load time is high
2. The rate of the eviction of the cached value for K is high

The Issue 16379 is one example of the above condition, however,
there are more instances of the same problem.

The eviction done at 2. for K would then cause all the incoming threads
to wait until the load at 1. for K is done.

Introduce a simple LoadingCache implementation called
PassthroughLoadingCache that is *really* just delegating the
calls to the underlying loader without involving any caching
or locking.

This change also makes the memoryLimit behaviour consistent with the
diskLimit set to zero means disabling the implementation of the cache.

Bug: Issue 16379
Release-Notes: Increase overall performance of disabled caches by avoiding any locking due to Guava/Caffeine caches with zero weight.
Change-Id: I5906a1f250014d96aaadcda0f64a37a78030d047
5 files changed
tree: 393ea09bbd4e434cf7f86726627c60654e41f1cf
  1. .settings/
  2. .ts-out/
  3. antlr3/
  4. contrib/
  5. Documentation/
  6. e2e-tests/
  7. java/
  8. javatests/
  9. lib/
  10. modules/
  11. plugins/
  12. polygerrit-ui/
  13. prolog/
  14. prologtests/
  15. proto/
  16. resources/
  17. tools/
  18. webapp/
  19. .bazelignore
  20. .bazelproject
  21. .bazelrc
  22. .bazelversion
  23. .editorconfig
  24. .git-blame-ignore-revs
  25. .gitignore
  26. .gitmodules
  27. .gitreview
  28. .mailmap
  29. .pydevproject
  30. .zuul.yaml
  31. BUILD
  32. COPYING
  33. INSTALL
  34. Jenkinsfile
  35. package.json
  36. README.md
  37. SUBMITTING_PATCHES
  38. version.bzl
  39. WORKSPACE
  40. yarn.lock
README.md

Gerrit Code Review

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