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Asked: 2026-06-18T13:12:39+00:00

Background: I have a Java application which does intensive IO on quite large memory

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Background:

I have a Java application which does intensive IO on quite large
memory mapped files (> 500 MB). The program reads data, writes data,
and sometimes does both.

All read/write functions have similar computation complexity.

I benchmarked the IO layer of the program and noticed strange
performance characteristics of memory mapped files:

  • It performs 90k reads per second (read 1KB every iteration at random position)
  • It performs 38k writes per second (write 1KB every iteration sequentially)
  • It performs 43k writes per second (write 4 bytes every iteration at random position)
  • It performs only 9k read/write combined operation per second (read 12 bytes then write 1KB every iteration, at random position)

The programs on 64-bit JDK 1.7, Linux 3.4.

The machine is an ordinary Intel PC with 8 threads CPU and 4GB physical memory. Only 1 GB was assigned to JVM heap when conducting the benchmark.

If more details are needed, here is the benchmark code: https://github.com/HouzuoGuo/Aurinko2/blob/master/src/test/scala/storage/Benchmark.scala

And here is the implementation of the above read, write, read/write functions: https://github.com/HouzuoGuo/Aurinko2/blob/master/src/main/scala/aurinko2/storage/Collection.scala

So my questions are:

  • Given fixed file size and memory size, what factors affect memory mapped file random read performance?
  • Given fixed file size and memory size, what factors affect memory mapped file random write performance?
  • How do I explain the benchmark result of read/write combined operation? (I was expecting it to perform over 20K iterations per second).

Thank you.

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1 Answer

  1. Editorial Team

    The memory mapped file performance depends on disk performance, file system type, free memory available for file system cache and read/write block size. The page size on the linux is 4K. So you should expect most performance with 4k read/writes. An access at random position causes page fault if page is not mapped and will pull a new page read. Usually, you want memory mapped file if you want to see the files as a one memory array ( or ByteBuffer in Java ).

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