In addition to that a less aggressive (or lazy) mode of operation could detect rogue recursions and throw in such a situation to prevent deadlocks.
The last mode of operation could do no checks at all, but that mode would not prevent from deadlocks or infinite recursions at all. On 10 Sep 2017 4:06 p.m., "Dominik Psenner" <dpsen...@gmail.com> wrote: > I opt that everyone should use the library in a way that makes sure > deadlocks cannot happen. Trying to solve this by fancy "deadlock could > occur" mechanisms feels wrong. > > A very restrictive mode of operation could enforce that all arguments > passed into a log event must be limited to immutable primitive types. In > this mode deadlocks could no longer occur because rogue recursions would > actively prevented. > > On 10 Sep 2017 3:43 p.m., "Remko Popma (JIRA)" <j...@apache.org> wrote: > > > [ https://issues.apache.org/jira/browse/LOG4J2-2031?page=com. > atlassian.jira.plugin.system.issuetabpanels:comment-tabpane > l&focusedCommentId=16160342#comment-16160342 ] > > Remko Popma commented on LOG4J2-2031: > ------------------------------------- > > After thinking about this some more, I think it should be possible to > prevent messages from appearing out of order in most cases. > > The reason Log4j2 logs directly to the appender, bypassing the async queue > when the queue is full, is to prevent deadlock. > > However, when exactly is deadlock possible? Only when an application > object is logged that itself invokes a logging call in its {{toString}} > method. If the logging call from the {{toString}} method would block until > it was able to add a new LogEvent to the queue, then the object that is > being logged could never return from its {{toString}} method, resulting in > deadlock. > > For "normal" logging calls (not from a {{toString}}) it is fine to block > when the queue is full, because the background thread will eventually make > a slot available. > > Currently the [DefaultAsyncQueueFullPolicy|https://logging.apache.org/log4 > j/2.0/log4j-core/apidocs/org/apache/logging/log4j/core/asyn > c/DefaultAsyncQueueFullPolicy.html] is very conservative and logs > directly to the underlying appender for _all_ events when the queue is > full. This can be improved by trying to rule out cases where deadlock could > not occur, and in such cases, block until space in the queue becomes > available. > > Ways to detect potential deadlock situations when the queue is full: > * the logging thread is a background thread (of any of the Async > Appenders, or a Disruptor) - this is the original solution for LOG4J2-471 > * set a thread-local flag when logging is invoked (specifically, before > calling {{Disruptor.tryPublish}}). If the flag has already been set then > deadlock is possible - this should cover LOG4J2-1518 > > If deadlock is possible, the current behaviour (log directly to the > underlying appender) is the safe thing to do. Perhaps a suffix should be > appended to such log events to the effect that "(Log4j2) This message > appears out of order to prevent deadlock: logging inside the toString() > method is not recommended." > > Thoughts? > > > Messages appear out of order in log file (was: Log4j2 log file not > reflecting application log function calls) > > ------------------------------------------------------------ > ------------------------------------------------- > > > > Key: LOG4J2-2031 > > URL: https://issues.apache.org/jira/browse/LOG4J2-2031 > > Project: Log4j 2 > > Issue Type: Bug > > Components: Appenders > > Affects Versions: 2.8.2, 2.9.0 > > Environment: Windows, Sun Java 8. > > Reporter: Colin McDowell > > Assignee: Remko Popma > > Attachments: CapacityTest.java, log4j2.xml, pom2.xml > > > > Original Estimate: 672h > > Remaining Estimate: 672h > > > > Was hoping to move our numerous J2EE projects from Log4j to Log4j2 for > the performance improvements. I put together a small test case that writes > a string pattern to a Rolling File. There is a 6 digit sequence number at > the start of the log message. This allows me to quickly see if all the log > requests are making it into the log file. I attach the test case and > log4j2.xml. The log4j2.xml uses an asynchronous appender. > > What I observe in the output log file is that after a short interval > (120 or so entries) the logged are appearing in the wrong order, and > entries can be missing. The missing entries issues especially shows up > when rolling to the next log file. > > Perhaps there is a deliberate decision to not to guarantee log file > accurately for speed. However we need the logs to accurately reflect what > the application is logging. I have also noticed the performance is 25% > worse in Log4j2 than Log4j when not using the asynchronous appender. So > that rather kills us using Log4j2 at the moment. > > > > -- > This message was sent by Atlassian JIRA > (v6.4.14#64029) > > >
