[jira] [Commented] (SOLR-16515) Remove synchronized access to cachedOrdMaps in SlowCompositeReaderWrapper

["ASF subversion and git services (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SOLR-16515?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17629000#comment-17629000 ] 

ASF subversion and git services commented on SOLR-16515:
--------------------------------------------------------

Commit 53d90030d0c432a4b1288100ced527615ed71d59 in solr's branch refs/heads/main from Torsten Bøgh Köster
[ https://gitbox.apache.org/repos/asf?p=solr.git;h=53d90030d0c ]

[SOLR-16515] Remove synchronized access to cachedOrdMaps in SlowCompositeReaderWrapper (#1156)

By using a ConcurrentHashMap underlying implementation and the {{ConcurrentHashMap#computeIfAbsent}} method to compute cache values, we were able to reduce locking contention significantly.

Co-authored-by: Dennis Berger <db...@knownhosts.org>
Co-authored-by: Torsten Bøgh Köster <tb...@thiswayup.de>
Co-authored-by: Marco Petris <ma...@web.de>

> 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
>
>          Time Spent: 1h
>  Remaining Estimate: 0h
>
> 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|width=1024! 
> 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|width=1024! 
> 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&#39;s name it&#39;s now capable of a much higher request throughput.
> This change has been composed together by Dennis Berger, Torsten Bøgh Köster and Marco Petris.



--
This message was sent by Atlassian Jira
(v8.20.10#820010)

---------------------------------------------------------------------
To unsubscribe, e-mail: issues-unsubscribe@solr.apache.org
For additional commands, e-mail: issues-help@solr.apache.org