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How big should my input stream buffer be?

In Sun's current implementation, the default buffer size for a BufferedInputStream is 8K. On certain systems at least, this appears to be about the "point of diminishing return" at which increasing the buffer size does not improve either overall read time or CPU usage. (And after all, Sun probably chose this size for a reason!) In other words, unless you've some reason not to, you may as well stick to the default.

The size of the read buffer affects both overall time taken to read the data plus the CPU time used. As a rule, both diminish as buffer size is increased, but overall time tends to plateau quickly as buffer size is increased. Even at the point where increasing the buffer size gives no gain in overall time to read the data, CPU time can continue to diminish. In other words, a large buffer size will probably benefit applications that are performing simultaneous reading and processing of data, but won't bring much benefit to single-threaded applications that must wait for the data before proceeding.

The graph below shows some example measurements of elapsed and CPU time as buffer size is increased. In each run of this test, the contents of 10 files each of 50MB in size were read in via a BufferedInputStream of the given size1. (In this case, the test system was a 2GHz uniprocessor Windows system with a standard 7,200 RPM hard drive.)

In this test case, overall read time plateaus at a buffer size of around 192 bytes. However, at this buffer size, we burn aroun 25% of the CPU. Beyond this buffer size, CPU usage continues to decrease even though overall read time remains more or less constant. At a buffer size of around 1K, we use in the order of 10% of the CPU while reading through the file.

Effect of larger buffer sizes on CPU usage

In our test case here, there appears to be a slight benefit in terms of CPU usage when the buffer size is made a little larger than 1K. However, beyond around 8-10K, CPU usage settles to around 4.5-5% (although the measured CPU usage fluctuates a little from run to run).


1. This size was chosen to try and minimise the effect of OS buffering on read performance. In the test configuration, the OS appeared to buffer up to around 100MB of data. So the overall 500MB of data meant that at the start of a run, there was probably little or no useful data in the file system caches left behind by the previous run.

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Written by Neil Coffey. Copyright © Javamex UK 2012. All rights reserved.