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https://issues.apache.org/jira/browse/HADOOP-17313?focusedWorklogId=508555&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-508555
]
ASF GitHub Bot logged work on HADOOP-17313:
-------------------------------------------
Author: ASF GitHub Bot
Created on: 06/Nov/20 18:18
Start Date: 06/Nov/20 18:18
Worklog Time Spent: 10m
Work Description: steveloughran commented on a change in pull request
#2396:
URL: https://github.com/apache/hadoop/pull/2396#discussion_r518923935
##########
File path:
hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/FileSystem.java
##########
@@ -3517,33 +3546,86 @@ private FileSystem getInternal(URI uri, Configuration
conf, Key key)
if (fs != null) {
return fs;
}
-
- fs = createFileSystem(uri, conf);
- final long timeout = conf.getTimeDuration(SERVICE_SHUTDOWN_TIMEOUT,
- SERVICE_SHUTDOWN_TIMEOUT_DEFAULT,
- ShutdownHookManager.TIME_UNIT_DEFAULT);
- synchronized (this) { // refetch the lock again
- FileSystem oldfs = map.get(key);
- if (oldfs != null) { // a file system is created while lock is
releasing
- fs.close(); // close the new file system
- return oldfs; // return the old file system
- }
-
- // now insert the new file system into the map
- if (map.isEmpty()
- && !ShutdownHookManager.get().isShutdownInProgress()) {
- ShutdownHookManager.get().addShutdownHook(clientFinalizer,
- SHUTDOWN_HOOK_PRIORITY, timeout,
- ShutdownHookManager.TIME_UNIT_DEFAULT);
+ // fs not yet created, acquire lock
+ // to construct an instance.
+ try (DurationInfo d = new DurationInfo(LOGGER, false,
+ "Acquiring creator semaphore for %s", uri)) {
+ creatorPermits.acquire();
+ } catch (InterruptedException e) {
+ // acquisition was interrupted; convert to an IOE.
+ throw (IOException)new InterruptedIOException(e.toString())
+ .initCause(e);
+ }
+ FileSystem fsToClose = null;
+ try {
+ // See if FS was instantiated by another thread while waiting
+ // for the permit.
+ synchronized (this) {
+ fs = map.get(key);
}
- fs.key = key;
- map.put(key, fs);
- if (conf.getBoolean(
- FS_AUTOMATIC_CLOSE_KEY, FS_AUTOMATIC_CLOSE_DEFAULT)) {
- toAutoClose.add(key);
+ if (fs != null) {
+ LOGGER.debug("Filesystem {} created while awaiting semaphore", uri);
+ return fs;
}
- return fs;
+ // create the filesystem
+ fs = createFileSystem(uri, conf);
+ final long timeout = conf.getTimeDuration(SERVICE_SHUTDOWN_TIMEOUT,
+ SERVICE_SHUTDOWN_TIMEOUT_DEFAULT,
+ ShutdownHookManager.TIME_UNIT_DEFAULT);
+ // any FS to close outside of the synchronized section
+ synchronized (this) { // lock on the Cache object
+
+ // see if there is now an entry for the FS, which happens
+ // if another thread's creation overlapped with this one.
+ FileSystem oldfs = map.get(key);
+ if (oldfs != null) {
+ // a file system was created in a separate thread.
+ // save the FS reference to close outside all locks,
+ // and switch to returning the oldFS
+ fsToClose = fs;
+ fs = oldfs;
+ } else {
+ // register the clientFinalizer if needed and shutdown isn't
+ // already active
+ if (map.isEmpty()
+ && !ShutdownHookManager.get().isShutdownInProgress()) {
+ ShutdownHookManager.get().addShutdownHook(clientFinalizer,
+ SHUTDOWN_HOOK_PRIORITY, timeout,
+ ShutdownHookManager.TIME_UNIT_DEFAULT);
+ }
+ // insert the new file system into the map
+ fs.key = key;
+ map.put(key, fs);
+ if (conf.getBoolean(
+ FS_AUTOMATIC_CLOSE_KEY, FS_AUTOMATIC_CLOSE_DEFAULT)) {
+ toAutoClose.add(key);
+ }
+ }
+ } // end of synchronized block
+ } finally {
+ // release the creator permit.
+ creatorPermits.release();
}
+ if (fsToClose != null) {
+ LOGGER.debug("Duplicate FS created for {}; discarding {}",
+ uri, fs);
+ discardedInstances.incrementAndGet();
+ // close the new file system
+ // note this will briefly remove and reinstate "fsToClose" from
+ // the map. It is done in a synchronized block so will not be
+ // visible to others.
+ fsToClose.close();
Review comment:
That is an interesting thought. I just left that code as is, but yes, it
could fail
Looking at FileSystem.close, the removal of the entry from the cache should
be in a finally clause too, shouldn't it. ouch. Ignoring that for now.
How about I go to IOUtils.close() & catch and log on failures.
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Issue Time Tracking
-------------------
Worklog Id: (was: 508555)
Time Spent: 3h (was: 2h 50m)
> FileSystem.get to support slow-to-instantiate FS clients
> --------------------------------------------------------
>
> Key: HADOOP-17313
> URL: https://issues.apache.org/jira/browse/HADOOP-17313
> Project: Hadoop Common
> Issue Type: Sub-task
> Components: fs, fs/azure, fs/s3
> Affects Versions: 3.3.0
> Reporter: Steve Loughran
> Assignee: Steve Loughran
> Priority: Major
> Labels: pull-request-available
> Time Spent: 3h
> Remaining Estimate: 0h
>
> A recurrent problem in processes with many worker threads (hive, spark etc)
> is that calling `FileSystem.get(URI-to-object-store)` triggers the creation
> and then discard of many FS clients -all but one for the same URL. As well as
> the direct performance hit, this can exacerbate locking problems and make
> instantiation a lot slower than it would otherwise be.
> This has been observed with the S3A and ABFS connectors.
> The ultimate solution here would probably be something more complicated to
> ensure that only one thread was ever creating a connector for a given URL
> -the rest would wait for it to be initialized. This would (a) reduce
> contention & CPU, IO network load, and (b) reduce the time for all but the
> first thread to resume processing to that of the remaining time in
> .initialize(). This would also benefit the S3A connector.
> We'd need something like
> # A (per-user) map of filesystems being created <URI, FileSystem>
> # split createFileSystem into two: instantiateFileSystem and
> initializeFileSystem
> # each thread to instantiate the FS, put() it into the new map
> # If there was one already, discard the old one and wait for the new one to
> be ready via a call to Object.wait()
> # If there wasn't an entry, call initializeFileSystem) and then, finally,
> call Object.notifyAll(), and move it from the map of filesystems being
> initialized to the map of created filesystems
> This sounds too straightforward to be that simple; the troublespots are
> probably related to race conditions moving entries between the two maps and
> making sure that no thread will block on the FS being initialized while it
> has already been initialized (and so wait() will block forever).
> Rather than seek perfection, it may be safest go for a best-effort
> optimisation of the #of FS instances created/initialized. That is: its better
> to maybe create a few more FS instances than needed than it is to block
> forever.
> Something is doable here, it's just not quick-and-dirty. Testing will be
> "fun"; probably best to isolate this new logic somewhere where we can
> simulate slow starts on one thread with many other threads waiting for it.
> A simpler option would be to have a lock on the construction process: only
> one FS can be instantiated per user at a a time.
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