Torsten Bøgh Köster created SOLR-16497:
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Summary: Allow finer grained locking in SolrCores
Key: SOLR-16497
URL: https://issues.apache.org/jira/browse/SOLR-16497
Project: Solr
Issue Type: Bug
Security Level: Public (Default Security Level. Issues are Public)
Components: search
Affects Versions: 8.11.2, 9.0
Reporter: Torsten Bøgh Köster
Attachments: solrcores_locking.png, solrcores_locking_fixed.png
Access to loaded SolrCore instances is a synchronized read and write operation
in SolrCores#getCoreFromAnyList. This method is touched by every request as
every HTTP request is assigned the SolrCore it operates on.
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 modifyLock
in SolrCores. In our case this means that we can only utilize our machines up
to 25% cpu usage. With the fix applied, a utilization up to 80% is perfectly
doable.
In our case this specific locking problem was masked by another locking problem
in the SlowCompositeReaderWrapper. We'll submit our fix to the locking problem
in the SlowCompositeReaderWrapper in a following issue.
h3. Problem
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%.
!solrcores_locking.png!
During our 60s Java Flight Recorder snapshot, the ~2k Jetty acceptor threads
accumulated more than 12h locking time inside SolrCores on the modifyLock
instance used as synchronized lock (see screenshot). With this fix the locking
access is reduced to write accesses only. We validated this using another JFR
snapshot:
!solrcores_locking_fixed.png!
We ran this code for a couple of weeks in our live environment.
h3. Implementation
The synchronized modifyLock is replaced by a ReentrantReadWriteLock. This
allows concurrent reads from the internal SolrCore instance list (cores) but
grants exclusive access to write operations.
We need to ensure that only a single transientSolrCoreCache inside
TransientSolrCoreCacheFactoryDefault is created. As we now allow multiple read
threads, we call the the getTransientCacheHandler() method initially inside a
write lock inside the load() method. This ensures that a single instance of
transientSolrCoreCache is created.
The lock signaling between SolrCore and CoreContainer gets replaced by a
Condition that is tied to the write lock.
h3. Summary
This change allows for a finer grained access to the list of open SolrCores.
The decreased blocking read access is noticeable in decreased blocking times of
the Solr application (see screenshot).
This change has been composed together by Dennis Berger, Torsten Bøgh Köster
and Marco Petris.
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