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I noticed a short thread on this today, but I deleted the messages 
before commenting (just too busy this morning).

Anyway, I think it's a pretty serious regression to disable kernel map 
entry merging.  A highly fragmented map is going to have a negative 
impact on performance.  (FWIW, I do think it would be cool if we had a 
Solaris-like vmem, but I don't think it's necessary to solve this 
problem.)

So, in reference to this commit to uvm_map.c:

----------------------------
revision 1.153
date: 2004/01/29 12:06:02;  author: yamt;  state: Exp;  lines: +382 -98
- split uvm_map() into two functions for the followings.
- for in-kernel maps, disable map entry merging so that
   unmap operations won't block. (workaround for PR/24039)
- for in-kernel maps, allocate kva for vm_map_entry from
   the map itsself and eliminate MAX_KMAPENT and
   uvm_map_entry_kmem_pool.
----------------------------

Instead of disabling entry merging for kernel maps, how about the 
following:

	* For every kernel map entry that would be merged and freed,
	  merge it, but instead of freeing the map entry, hang it off
	  a free list for that map.

	* If, when unmapping, you need to allocate a new map entry because
	  of a split, pull an entry off that free list.

	* Don't use those map entries on the free list for anything other
	  than the unmap-split case.

I guess the only issue then is to determine when free list entries can 
be freed back when a split is not required on unmap.

Anyway, I think this, or some variation of this, could solve the 
problem and still let us have map entry merging for kernel maps.

         -- Jason R. Thorpe <thorpej@wasabisystems.com>

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