Memory usage across multiple jails

[Tom Haapanen]

I have been using FreeBSD (and other forms of *BSD) a long time, but I'm new
to the world of jails.  I have been doing reading on them, but there is one
question I have not been able to find an answer to, and that's the
efficiency of memory usage when using multiple jails on a single system.

 

With "conventional" virtual machines (VMware, Virtual Server et al),
essentially each VM is opaque to the host OS, and thus has to be allocated X
MB of memory, which that VM then manages internally.

 

However, since jails are based on the FreeBSD kernel, and both host and
guest OSs are identical, I am wondering whether there are any comparative
efficiencies in memory utilization.  Will the jails share the disk cache,
for example, or does each jail allocate its own?  Will other kernel
structures (and code!) be shared across jails, or allocated multiple times?
And what about userland applications, like httpd, for example?  (I suspect
userland would not be able to benefit, but that's just a guess.)

 

Thanks for any insight into this ...

 

Tom