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From "Remko Popma (JIRA)" <j...@apache.org>
Subject [jira] [Created] (LOG4J2-431) Create MemoryMappedFileAppender
Date Thu, 17 Oct 2013 02:05:41 GMT
Remko Popma created LOG4J2-431:

             Summary: Create MemoryMappedFileAppender
                 Key: LOG4J2-431
                 URL: https://issues.apache.org/jira/browse/LOG4J2-431
             Project: Log4j 2
          Issue Type: New Feature
          Components: Appenders
            Reporter: Remko Popma
            Priority: Minor

A memory-mapped file appender may have better performance than the ByteBuffer + RandomAccessFile
combination used by the RandomAccessFileAppender. 

* The drawback is that the file needs to be pre-allocated and only up to the file size can
be mapped into memory. When the end of the file is reached the appender would need to extend
the file and re-map.

* Remapping is expensive (I think single-digit millisecond-range, need to check). For low-latency
apps this kind of spike may be unacceptable so careful tuning is required.

* Memory usage: If re-mapping happens too often you lose the performance benefits, so the
memory-mapped buffer needs to be fairly large, which uses up memory.

* At roll-over and shutdown the file should be truncated to immediately after the last written
data (otherwise the user is left with a log file that ends in garbage).

Measuring on a Solaris box, the difference between flushing to disk (with {{RandomAccessFile.write(bytes[])}})
and putting data in a MappedByteBuffer is about 20x: around 600ns for a ByteBuffer put and
around 12-15 microseconds for a RandomAccessFile.write.
(Of course different hardware and OS may give different results...)

*Use cases*
The difference may be most visible if {{immediateFlush}} is set to {{true}}, which is only
recommended if async loggers/appenders are not used. If {{immediateFlush=false}}, the large
buffer used by RandomAccessFileAppender means you won't need to touch disk very often.

So a MemoryMappedFileAppender is most useful in _synchronous_ logging scenarios, where you
get the speed of writing to memory but the data is available on disk almost immediately. (MMap
writes directly to the OS disk buffer.)

In case of a application crash, the OS ensures that all data in the buffer will be written
to disk. In case of an OS crash the data that was most recently added to the buffer may not
be written to disk.

Because by nature this appender would occupy a fair amount of memory, it is most suitable
for applications running on server-class hardware with lots of memory available.

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