[
https://issues.apache.org/jira/browse/OAK-7259?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16365769#comment-16365769
]
Andrei Dulceanu commented on OAK-7259:
--------------------------------------
[~frm], while I agree with your concern about thread-safety for these newly
introduced methods, I think we should take into consideration the purpose of
{{SegmentNodeStoreStats}} which is monitoring. I guess the following excerpt
from {{ConcurrentHashMap}}'s JavaDoc applies here as well:
{quote}
Otherwise the results of these methods reflect transient states that may be
adequate for monitoring or estimation purposes, but not for program control.
{quote}
What I wanted to avoid was to introduce yet another lock contention point.
Suppose multiple threads access {{LockBasedScheduler#schedule}} while the
commit semaphore is already acquired by some other thread. If we go for a
synchronized version of {{SegmentNodeStoreStats#getCommitsCountPerWriter}}, as
found in [0], we'll basically have all those threads contend for the newly
introduced lock. Moreover, if the thread which acquired the semaphore modifies
(with the same lock held) the data structures used in
{{#getCommitsCountPerWriter}}, then we basically block all other threads.
Just writing this, I realized that {{SegmentNodeStoreStats#onCommit}} is only
called with the commit semaphore acquired so it won't need additional
synchronization (in the current implementation of {{LockBasedScheduler}}). As
for {{onCommitQueued}} and {{onCommitDequeued}}, going forward with the current
approach means that {{getQueuedWriters}} might return an incomplete view of the
currently queued threads, which should be ok, since this is only about
monitoring.
WDYT?
[0]
https://github.com/apache/jackrabbit-oak/blob/trunk/oak-store-composite/src/main/java/org/apache/jackrabbit/oak/composite/CompositeNodeStoreStats.java#L150
> Improve SegmentNodeStoreStats to include number of commits per thread and
> threads currently waiting on the semaphore
> --------------------------------------------------------------------------------------------------------------------
>
> Key: OAK-7259
> URL: https://issues.apache.org/jira/browse/OAK-7259
> Project: Jackrabbit Oak
> Issue Type: Improvement
> Components: segment-tar
> Reporter: Andrei Dulceanu
> Assignee: Andrei Dulceanu
> Priority: Major
> Labels: tooling
> Fix For: 1.9.0, 1.10
>
>
> When investigating the performance of {{segment-tar}}, the source of the
> writes (commits) is a very useful indicator of the cause.
> To better understand which threads are currently writing in the repository
> and which are blocked on the semaphore, we need to improve
> {{SegmentNodeStoreStats}} to:
> * expose the number of commits executed per thread
> * expose threads currently waiting on the semaphore
--
This message was sent by Atlassian JIRA
(v7.6.3#76005)
