Torsten Bøgh Köster created SOLR-16515:
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Summary: Remove synchronized access to cachedOrdMaps in
SlowCompositeReaderWrapper
Key: SOLR-16515
URL: https://issues.apache.org/jira/browse/SOLR-16515
Project: Solr
Issue Type: Improvement
Security Level: Public (Default Security Level. Issues are Public)
Components: search
Affects Versions: 8.11.2, 9.0
Reporter: Torsten Bøgh Köster
Attachments: slow-composite-reader-wrapper-after.jpg,
slow-composite-reader-wrapper-before.jpg
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.
h3. 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.
h3. 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%.
!slow-composite-reader-wrapper-before.jpg!
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:
!slow-composite-reader-wrapper-after.jpg!
h3. 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).
h3. 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.
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