DragonFly kernel List (threaded) for 2007-07
Re: 4mb PAGES for mbuf clusters
:To avoid memory fragmentation what I am thinking is that at system
:startup reserve two memory pools for mbufs and use just one of them.
:When it becomes fragmented (upto 80% or user configurable), switch to
:the next one and recover the fragmented memory. However, this may not
:be a general solution but at least it may work for specific workloads.
This only works if the memory allocations are temporary. Unfortunately,
many memory allocations are persistent. Without a way to reallocate
the persistent store there is no way to undo the fragmentation.
Reallocation is often integrated into high level interpreted
languages through the means of a double indirect. That is,
instead of holding a pointer to something you hold a pointer to a
pointer to something. That way the 'something' can be reallocated
by just changing the underlying pointer and not all the active
references. But doing this imposes fairly severe overheads unsuitable
for kernel code as well as locking issues.
In absence of that you either have to pre-reserve the memory for
use ONLY as 4MB pages, which is easy to do but almost impossible to
balance, or there needs to be a mechanism whereby individual subsystems
can reallocate memory for active structures - something very difficult
to do properly.