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When to use memory-mapped files?

Asked 2009-12-07T10:41:18.943
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I have an application that receives chunks of data over the network, and writes these to disk. Once all chunks have been received, they can be decoded/recombined into the single file they actually represent.

I'm wondering if it's useful to use memory-mapped files or not - first for writing the single chunks to disk, second for the single file into which all of them are decoded.

My own feeling is that it might be useful for the second case only, anyone got some ideas on this?

Edit: It's a C# app, and I'm only planning an x64 version. (So running into the 'largest contigious free space' problem shouldn't be relevant)

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Memory-mapped files are primarily used for Inter-Process Communication or I/O performance improvement.

In your case, are you trying to get better I/O performance?

Hate to point out the obivious, but Wikipedia gives a good rundown of the situation... http://en.wikipedia.org/wiki/Memory-mapped_file

Specifically...

The memory mapped approach has its cost in minor page faults - when a block of data is loaded in page cache, but not yet mapped in to the process's virtual memory space. Depending on the circumstances, memory mapped file I/O can actually be substantially slower than standard file I/O.

It sounds like you're about to prematurely optimize for speed. Why not a regular file approach, and then refactor for MM files later if needed?

answered 2009-12-07T11:24:18.233

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