CODEWITHSUNDEEP
javamemory-managementmmap
millimoose
millimoose

Reputation: 39950

Memory usage of FileChannel#map

Does FileChannel#map allocate all the memory needed for the resulting ByteBuffer immediately, or is it only allocated on-demand during reads from the buffer?

I just tried mapping all of a 500+ MB file in a trivial test program, and looked at the memory usage of the process. (Using both Runtime#totalMemory and eyeballing it in the OS X Activity Monitor for a groovysh process.) The memory usage never passed 30-ish MB.

So, could a Java implementation "hide" some of its memory usage in native calls? And if so, is there a way to find out how much that is on OS X?

Upvotes: 5

Views: 2084

Answers (3)

yetanothercoder
yetanothercoder

Reputation: 1709

java.nio.MappedByteBuffer#load will trigger loading of all file contents into memory (otherwise it's on demand):

This method makes a best effort to ensure that, when it returns, * this buffer's content is resident in physical memory. Invoking this * method may cause some number of page faults and I/O operations to * occur.

Upvotes: 1

kohlerm
kohlerm

Reputation: 2624

Yes. It's not part of the heap. Still, depending on the OS there will still be memory reserved. On OS X, because it's UNIX-like you should be able to use top or ps to check how much memory it needs.

Upvotes: 1

Tom Hawtin - tackline
Tom Hawtin - tackline

Reputation: 147164

Memory usage is never straightforward. The actual buffer used by FileChannel.map is not part of the Java heap. Indeed the memory might be shared with other processes. The file may not even be read from the disc until the pages are touched.

Upvotes: 5

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