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https://issues.apache.org/jira/browse/HDFS-6786?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14080397#comment-14080397
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Yi Liu commented on HDFS-6786:
------------------------------
Thanks [~cmccabe], I found I had a misunderstanding about the interval so later
I confirmed with you offline. :)
I still have concern as the front part of the description: if there are a lot
of HDFS catch, rescan will be expensive, right? (There are two much stuff in
the rescan, {{cachedBlocks}} is very large). The default interval value is 30s,
and it will subtract the rescan time and the time of ops if other threads jump
into. A worse case is if user set the interval to a more smaller value, then
the thread will fall into rescan loop? Rescan will hold FSN lock, won't that
affect NN performance?
> Fix potential issue of cache refresh interval
> ---------------------------------------------
>
> Key: HDFS-6786
> URL: https://issues.apache.org/jira/browse/HDFS-6786
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: caching
> Affects Versions: 2.4.0
> Reporter: Yi Liu
> Assignee: Yi Liu
>
> In {{CacheReplicationMonitor}}, following code is try to check whether needs
> to rescan every interval ms, if rescan takes n ms, then subtract n ms for the
> interval. But if the delta <=0, it breaks and start rescan, there will be
> potential issue: if user set the interval to a small value or rescan finished
> after a while exceeding the interval, then rescan will happen in loop.
> Furthermore, {{delta <= 0}} trigger the rescan should not be the intention,
> since if needs rescan, {{needsRescan}} will be set.
> {code}
> while (true) {
> if (shutdown) {
> LOG.info("Shutting down CacheReplicationMonitor");
> return;
> }
> if (needsRescan) {
> LOG.info("Rescanning because of pending operations");
> break;
> }
> long delta = (startTimeMs + intervalMs) - curTimeMs;
> if (delta <= 0) {
> LOG.info("Rescanning after " + (curTimeMs - startTimeMs) +
> " milliseconds");
> break;
> }
> doRescan.await(delta, TimeUnit.MILLISECONDS);
> curTimeMs = Time.monotonicNow();
> }
> {code}
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