[GitHub] [solr] tboeghk opened a new pull request, #1156: [SOLR-16515] Remove synchronized access to cachedOrdMaps in SlowCompositeReaderWrapper

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tboeghk opened a new pull request, #1156:
URL: https://github.com/apache/solr/pull/1156

   https://issues.apache.org/jira/browse/SOLR-16515
   
   The `SlowCompositeReaderWrapper` uses synchronized read and write access to 
its internal `cachedOrdMaps`. By using a `ConcurrentHashMap` instead of a 
`LinkedHashMap` as the  underlying `cachedOrdMaps` implementation and the 
`ConcurrentHashMap#computeIfAbsent` method to compute cache values, we were 
able to reduce locking contention significantly.
   
   ### Background
   
   Under heavy load we discovered that application halts inside of Solr are 
becoming a serious problem in high traffic environments. Using Java Flight 
Recordings we discovered high accumulated applications halts on the 
`cachedOrdMaps` in `SlowCompositeReaderWrapper`. Without this fix we were able 
to utilize our machines only up to 25% cpu usage. With the fix applied, a 
utilization up to 80% is perfectly doable.
   
   ### Description
   
   Our Solr instances utilizes the `collapse` component heavily. The instances 
run with 32 cores and 32gb Java heap on a rather small index (4gb). The 
instances scale out at 50% cpu load. We take Java Flight Recorder snapshots of 
60 seconds
   as soon the cpu usage exceeds 50%.
   
   <img width="997" alt="solr-issues-scrw-locking" 
src="https://user-images.githubusercontent.com/557264/196221280-7eeac492-d204-497a-907e-bc261cd90530.png";>
   
   During our 60s Java Flight Recorder snapshot, the ~2k Jetty threads 
accumulated more than 16h locking time inside the `SlowCompositeReaderWrapper` 
(see screenshot). With this fix applied, the locking access is reduced to cache 
write accesses only. We validated this using another JFR snapshot:
   
   <img width="983" alt="solr-issues-scrw-after" 
src="https://user-images.githubusercontent.com/557264/196221323-3ab53d23-cf23-4aee-a395-b518d4b42e54.png";>
   
   ### Solution
   
   We propose the following improvement inside the `SlowCompositeReaderWrapper` 
removing blocking `synchronized` access to the internal `cachedOrdMaps`. The 
implementation keeps the semantics of the `getSortedDocValues` and 
`getSortedSetDocValues` methods but moves the expensive part of 
`OrdinalMap#build` into a producer. We use the producer to access the 
`ConcurrentHashMap` using the `ConcurrentHashMap#computeIfAbsent` method only.
   
   The current implementation uses the `synchronized` block not only to lock 
access to the `cachedOrdMaps` but also to protect the critical section between 
getting, building and putting the `OrdinalMap` into the cache. Inside the 
critical section the decision is formed, whether a cacheable value should be 
composed and added to the cache. 
   
   To support non-blocking read access to the cache, we move the building part 
of the critical section into a producer `Function`. The check whether we have a 
cacheable value is made upfront. To properly make that decision we had to take 
logic from `MultiDocValues#getSortedSetValues` and 
`MultiDocValues#getSortedValues` (the `SlowCompositeReaderWrapper` already 
contained duplicated code from those methods).
   
   ### Summary
   
   This change removes most blocking access inside the 
`SlowCompositeReaderWrapper` and despite it's name it's now capable of a much 
higher request throughput.
   
   This change has been composed together by Dennis Berger, Torsten Bøgh Köster 
and Marco Petris.
   
   # Checklist
   
   Please review the following and check all that apply:
   
   - [x] I have reviewed the guidelines for [How to 
Contribute](https://wiki.apache.org/solr/HowToContribute) and my code conforms 
to the standards described there to the best of my ability.
   - [x] I have created a Jira issue and added the issue ID to my pull request 
title.
   - [x] I have given Solr maintainers 
[access](https://help.github.com/en/articles/allowing-changes-to-a-pull-request-branch-created-from-a-fork)
 to contribute to my PR branch. (optional but recommended)
   - [x] I have developed this patch against the `main` branch.
   - [x] I have run `./gradlew check`.
   - [x] I have added tests for my changes.
   - [ ] I have added documentation for the [Reference 
Guide](https://github.com/apache/solr/tree/main/solr/solr-ref-guide)
   


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