On Fri, 12 May 2023 13:19:44 GMT, Chen Liang <[email protected]> wrote:
>> Adam Sotona has updated the pull request incrementally with one additional
>> commit since the last revision:
>>
>> fixed jmh benchmark parameters
>
> test/micro/org/openjdk/bench/jdk/classfile/RebuildMethodBodies.java line 57:
>
>> 55: public void setup() throws IOException {
>> 56: shared = new LinkedList<>();
>> 57: unshared = new LinkedList<>();
>
> LinkedList should be replaced by ArrayList, as:
> 1. LinkedList's node wrapper around each object is too heavy, ArrayList has
> smaller wrapper
> 2. LinkedList iteration needs to follow links while ArrayList access can be
> machine optimized
> 3. ArrayList addition is amortized O(1), not really worse than LinkedList
> additions.
I have to side with @liach here: `LinkedList` iteration is significantly slower
than `ArrayList` even though the computational complexity is identical. Often
by an integer factor. This is due to the sparseness of the linked list data
structure on heap and the pointer chasing that entails.
For minimum overhead of iteration over an immutable list then turning the list
into an immutable via `List.copyOf` might be preferable due the JVM's ability
to optimize over `@Stable` arrays.
Adhoc [JMH
benchmark](https://gist.github.com/cl4es/1f21812241c47f32a9deaffcc996e8b3):
Benchmark Mode Cnt Score Error Units
ListIteration.iterateArrayList thrpt 15 188,724 ± 10,903 ops/ms
ListIteration.iterateImmutableList thrpt 15 230,901 ± 6,513 ops/ms
ListIteration.iterateLinkedList thrpt 15 58,289 ± 0,497 ops/ms
Is it important to fix this? Perhaps not, but in a microbenchmark like this the
fewer cycles we spend on "stuff" that isn't really part of the thing we're
testing, the better. Increasingly so as the thing we're testing is getting more
and more optimized.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/13671#discussion_r1193858864
