Will using shared library in place of static library effect memory usage?

Question

I am linking against 10 static library.

My binary file size is getting reduced when I am using dynamic library.

As I know using dynamic library will not reduce memory usage.

But my senior told me that using shared library will also reduce memory usage ? (when multiple process are running for the same executable code. )

Is that statement is right ?

he told me that as there will no duplicate copy of function used in library , so memory usage will be less. when you create n instance of that process.

When the process start it fork it's 10 children. So will using dynamic library in place of static library reduce total memory usages ?

score 11 · Accepted Answer · answered Mar 05 '11 at 06:44

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In your example, dynamic libraries won't save you much. When you fork your process on a modern OS all the pages are marked copy on write rather than actually copied. So your static library is already shared between your 10 copies of your process.

However, where you can save is when the dynamic library is shared between different processes rather than forks of the same process. So if you're using the same glibc.so as another process, the two processes are sharing the physical pages of glibc.so, even though they are otherwise unrelated processes.

answered Mar 05 '11 at 06:44

Don Neufeld

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Haha. It's amusing how similar our answers are. All the way down to the italicized word "different". – Vinay Pai Mar 05 '11 at 06:45
Well, I guess the answer is really obvious then :) – Don Neufeld Mar 05 '11 at 06:47

score 4 · Answer 2 · answered Mar 05 '11 at 06:44

If you fork given process there shouldn't be much of a difference, because most operating systems use copy-on-write. This means that pages will only be copied if they're updated, so things like the code segments in shared libraries shouldn't be affected.

On the other hand different processes won't be able to share code if they're statically linked. Consider libc, which practically every binary links against... if they were all statically linked you'd end up with dozens of copies of printf in memory.

The bottom line is you shouldn't link your binaries statically unless you have an excellent reason for it.

score 2 · Answer 3 · answered Mar 05 '11 at 06:41

Your senior in this instance is correct. A single copy of the shared library will be loaded into memory and will be used by every program that references it.

There is a post regarding this topic here:

http://www.linuxquestions.org/linux/articles/Technical/Understanding_memory_usage_on_Linux

Will using shared library in place of static library effect memory usage?

3 Answers3