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https://issues.apache.org/jira/browse/HDFS-14579?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16866659#comment-16866659
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Stephen O'Donnell commented on HDFS-14579:
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Both [~hexiaoqiao] and [~elgoiri] said they had seen DNS holding up the refresh
nodes call in the past, and that code path looked like the most likely
candidate. However, we should prove that DNS is the root cause before moving
this Jira any further forward.
Is there any way one of you could capture some jstacks at 1 second interval
during a refreshNodes command so we can see if DNS is dominating and if so what
the call stack is?
> In refreshNodes, avoid performing a DNS lookup while holding the write lock
> ---------------------------------------------------------------------------
>
> Key: HDFS-14579
> URL: https://issues.apache.org/jira/browse/HDFS-14579
> Project: Hadoop HDFS
> Issue Type: Improvement
> Affects Versions: 3.3.0
> Reporter: Stephen O'Donnell
> Assignee: Stephen O'Donnell
> Priority: Major
> Attachments: HDFS-14579.001.patch
>
>
> When refreshNodes is called on a large cluster, or a cluster where DNS is not
> performing well, it can cause the namenode to hang for a long time. This is
> because the refreshNodes operation holds the global write lock while it is
> running. Most of refreshNodes code is simple and hence fast, but
> unfortunately it performs a DNS lookup for each host in the cluster while the
> lock is held.
> Right now, it calls:
> {code}
> public void refreshNodes(final Configuration conf) throws IOException {
> refreshHostsReader(conf);
> namesystem.writeLock();
> try {
> refreshDatanodes();
> countSoftwareVersions();
> } finally {
> namesystem.writeUnlock();
> }
> }
> {code}
> The line refreshHostsReader(conf); reads the new config file and does a DNS
> lookup on each entry - the write lock is not held here. Then the main work is
> done here:
> {code}
> private void refreshDatanodes() {
> final Map<String, DatanodeDescriptor> copy;
> synchronized (this) {
> copy = new HashMap<>(datanodeMap);
> }
> for (DatanodeDescriptor node : copy.values()) {
> // Check if not include.
> if (!hostConfigManager.isIncluded(node)) {
> node.setDisallowed(true);
> } else {
> long maintenanceExpireTimeInMS =
> hostConfigManager.getMaintenanceExpirationTimeInMS(node);
> if (node.maintenanceNotExpired(maintenanceExpireTimeInMS)) {
> datanodeAdminManager.startMaintenance(
> node, maintenanceExpireTimeInMS);
> } else if (hostConfigManager.isExcluded(node)) {
> datanodeAdminManager.startDecommission(node);
> } else {
> datanodeAdminManager.stopMaintenance(node);
> datanodeAdminManager.stopDecommission(node);
> }
> }
> node.setUpgradeDomain(hostConfigManager.getUpgradeDomain(node));
> }
> }
> {code}
> All the isIncluded(), isExcluded() methods call node.getResolvedAddress()
> which does the DNS lookup. We could probably change things to perform all the
> DNS lookups outside of the write lock, and then take the lock and process the
> nodes. Also change or overload isIncluded() etc to take the inetAddress
> rather than the datanode descriptor.
> It would not shorten the time the operation takes to run overall, but it
> would move the long duration out of the write lock and avoid blocking the
> namenode for the entire time.
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