Shared library in containers

Question

For two processes A and B, the both use the library libc.so, libc.so is loaded into memory only once. This is a normal situation when A and B both run on the same host and the same rootfs.

When it comes to container, if A and B are running in different containers, are A and B sharing same memory area?

for example

imageA

--libc.so

--programA

imageB

--libc.so

--programB

we use chroot to run A and B in different rootfs. The two libc.so are same. Will libc.so be loaded into memory twice?

Answer 1

Actually, processes A & B that use a shared library libc.so can share the same memory. Somewhat un-intuitively it depends on which docker storage driver you're using. If you use a storage driver that can expose the shared library files as originating from the same device/inode when they reside in the same docker layer then they will share the same virtual memory cache pages. When using the aufs, overlay or overlay2 storage drivers then your shared libraries will share memory but when using any of the other storage drivers they will not.

I'm not sure why this detail isn't made more explicitly obvious in the Docker documentation. Or maybe it is but I've just missed it. It seems like a key differentiater if you're trying to run dense containers.