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Posted to issues@solr.apache.org by "Torsten Bøgh Köster (Jira)" <ji...@apache.org> on 2022/11/01 14:16:00 UTC
[jira] [Updated] (SOLR-16515) Remove synchronized access to cachedOrdMaps in SlowCompositeReaderWrapper
[ https://issues.apache.org/jira/browse/SOLR-16515?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Torsten Bøgh Köster updated SOLR-16515:
---------------------------------------
Description:
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|height=1024px!
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|height=1024px!
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.
was:
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.
> 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: 9.0, 8.11.2
> Reporter: Torsten Bøgh Köster
> Priority: Major
> 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|height=1024px!
> 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|height=1024px!
> 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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