RFR: 8269004 Implement ResizableResourceHashtable [v3]

Ioi Lam iklam at openjdk.java.net
Tue Jun 29 03:45:41 UTC 2021


> In HotSpot we have (at least) two hashtable designs in the C++ code:
> 
> - share/utilities/hashtable.hpp
> - share/utilities/resourceHash.hpp
> 
> Of the two, the `ResourceHashtable` API is much cleaner and most new code has been written with it. However, one issue is that the `SIZE` of `ResourceHashtable` is a compile-time constant. This makes the hash-to-index computation very fast on x64 (gcc can avoid using the slow divq instruction for modulo). However, the downside is we cannot use `ResourceHashtable` when we need a hashtable whose size is determined at run time (and, optionally, resizeable).
> 
> This PR refactors `ResourceHashtable` into a base template class `ResourceHashtableBase`, whose `size()` function can be configured by a subclass to be either constant or runtime-configurable. 
> 
> Note: since we want to preserve the performance of `hash % SIZE`, we can't make `size()` a virtual function.
> 
> Preliminary benchmark shows that this refactoring has no impact on the performance of the constant `ResourceHashtable`. See https://github.com/iklam/tools/tree/main/bench/resourceHash:
> 
> *before*
> ResourceHashtable: 2.70 sec
> 
> *after*
> ResourceHashtable: 2.72 sec
> ResizableResourceHashtable: 5.29 sec
> 
> To make sure `ResizableResourceHashtable` works, I rewrote some CDS code to use `ResizableResourceHashtable` instead of `KVHashtable`

Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:

  @kimbarrett feedback to move the storage code to a base class

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4536/files
  - new: https://git.openjdk.java.net/jdk/pull/4536/files/cad6e8e6..e5f9c16f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4536&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4536&range=01-02

  Stats: 178 lines in 4 files changed: 95 ins; 42 del; 41 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4536.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4536/head:pull/4536

PR: https://git.openjdk.java.net/jdk/pull/4536


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