Radim Kolar created LUCENE-4544:
-----------------------------------
Summary: possible bug in
ConcurrentMergeScheduler.merge(IndexWriter)
Key: LUCENE-4544
URL: https://issues.apache.org/jira/browse/LUCENE-4544
Project: Lucene - Core
Issue Type: Bug
Components: core/other
Affects Versions: 5.0
Reporter: Radim Kolar
from dev list:
¨i suspect that this code is broken. Lines 331 - 343 in
org.apache.lucene.index.ConcurrentMergeScheduler.merge(IndexWriter)
mergeThreadCount() are currently active merges, they can be at most
maxThreadCount, maxMergeCount is number of queued merges defaulted with
maxThreadCount+2 and it can never be lower then maxThreadCount, which means
that condition in while can never become true.
synchronized(this) {
long startStallTime = 0;
while (mergeThreadCount() >= 1+maxMergeCount) {
startStallTime = System.currentTimeMillis();
if (verbose()) {
message(" too many merges; stalling...");
}
try {
wait();
} catch (InterruptedException ie) {
throw new ThreadInterruptedException(ie);
}
}
While confusing, I think the code is actually nearly correct... but I
would love to find some simplifications of CMS's logic (it's really
hairy).
It turns out mergeThreadCount() is allowed to go higher than
maxThreadCount; when this happens, Lucene pauses
mergeThreadCount()-maxThreadCount of those merge threads, and resumes
them once threads finish (see updateMergeThreads). Ie, CMS will
accept up to maxMergeCount merges (and launch threads for them), but
will only allow maxThreadCount of those threads to be running at once.
So what that while loop is doing is preventing more than
maxMergeCount+1 threads from starting, and then pausing the incoming
thread to slow down the rate of segment creation (since merging cannot
keep up).
But ... I think the 1+ is wrong ... it seems like it should just be
mergeThreadCount() >= maxMergeCount().
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