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Posted to issues@solr.apache.org by GitBox <gi...@apache.org> on 2022/11/01 14:20:45 UTC

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

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%.

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:

### 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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