[jira] [Reopened] (LOG4J2-1080) Drop events when the RingBuffer is full

["Remko Popma (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/LOG4J2-1080?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Remko Popma reopened LOG4J2-1080:
---------------------------------

I am currently investigating a performance regression with Async Loggers and there is a chance that the way this issue was addressed is related to the reduced throughput.

The current implementation calls RingBuffer.size() to see if the buffer is full, and makes a separate call to RingBuffer.publishEvent() enqueue the log event.

A better alternative is to omit the call to size() and use the RingBuffer.tryPublishEvent() instead. In the vast majority of cases the buffer will have remaining capacity and we are done at that point. If the buffer is full, this method returns false, and we can either drop the event, or block until capacity is available. (Or decide based on the level of the event.)

This means we lose the ability to start dropping very low-level events when the queue is 80% full. However, I believe performance is more important than this feature.

> Drop events when the RingBuffer is full
> ---------------------------------------
>
>                 Key: LOG4J2-1080
>                 URL: https://issues.apache.org/jira/browse/LOG4J2-1080
>             Project: Log4j 2
>          Issue Type: New Feature
>            Reporter: tzachi
>            Assignee: Remko Popma
>             Fix For: 2.6
>
>         Attachments: AsyncLogger.dropEvents.patch
>
>
> I am running into performance issue with an appender, in a certain scenario (attached at the bottom), that causes RingBuffer to reach its full capacity. When that happens I can see that my app throughput drops significantly.
> I think it will be really useful to be able to configure the RingBuffer handler to be able to drop events whenever the buffer reaches its capacity, instead of what seems currently as blocking, as I don't want the logging to affect the main application.
> ---------------------------------------------------------------------
> Here is the scenario that led me to this request:
> I am currently testing the log4j-flume-ng appender and running into some issues. It seems like whenever log4j appender fails to log an event it causes the disruptor ring buffer to get full which slows down the whole system.
> My setup looks more or less like that: 
> process 1: Java app which uses log4j2 (with flume-ng’s Avro appender)
> process 2: local flume-ng which gets the logs on using an Avro source and process them 
> Here are my findings:
> When Flume (process 2) is up and running, everything actually looks really good. The ring buffer capacity is almost always full and there are no performance issues. The problem starts when I shut down process 2 - I am trying to simulate a case in which this process crashes, as I do not want it to effect process 1. As soon as I shut down flume I start getting exceptions produced by log4j telling me they cannot append the log - so far it makes sense. The thing is, that at the same time I can see that the ring buffer starts to fill up. As long as it’s not totally full process’s 1 throughput stays the same. The problem gets serious as soon as the buffer reaches full capacity. When that happens the throughput drops in 80% and it does not seem to recover from this state. But, as soon as I restart process 2, things get back to normal pretty quick - the buffer gets emptied, and the throughput climbs back to what it was before. I assume that from some reason a fail to append makes the RingBuffer consumer thread significantly slower.
> Besides checking why the flume appender preform slower when an exception is thrown, I wish I could just discard the log events when the buffer gets full.



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