Re: Concurrent file read approaches?

"Karl Uppiano" <>
Sun, 08 Oct 2006 20:25:53 GMT
"Mark Thornton" <> wrote in message

Karl Uppiano wrote:

"Chris" <> wrote in message

What's the best approach for heavily concurrent read access to a file?

I have a file that needs to be read by many threads. The file is much too
big to fit in memory. I'll do some caching, but access is mostly random
so there will be a large number of cache misses.

Unfortunately, RandomAccessFile is single threaded. To read data, you
much call seek() and then read(), and this can only be done one thread at
a time.

I see three possible approaches to the problem:
1. Wrap the calls to seek() and read() in a synchronized method. This
will be slow.
2. Have a pool of RandomAccessFile objects all pointing to the same file.
Have each thread grab and release objects from the pool as needed. The
downside here is that many file handles will be required.
3. Do something fancy with NIO and selectors. I haven't looked into this
deep enough to know if it's an option.

What's the best approach?

I don't know the "best" approach, but NIO is very powerful and scalable.
It is about as close as you can get to overlapped I/O in the OS.

Unfortunately the implementation of NIO doesn't allow concurrent read (or
write) on a single file channel. See the evaluation and comments on bug

Mark Thornton


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