On Thu, 23 Feb 2012 15:00:34 -0500
Rik van Riel [off-list ref] wrote:

Some programs have a large number of VMAs, and make frequent calls
to mmap and munmap. Having munmap constantly cause the search
pointer for get_unmapped_area to get reset can cause a significant
slowdown for such programs. 

Likewise, starting all the way from the top any time we mmap a small 
VMA can greatly increase the amount of time spent in 
arch_get_unmapped_area_topdown.

For programs with many VMAs, a next-fit algorithm would be fastest,
however that could waste a lot of virtual address space, and potentially
page table memory.

A compromise is to reset the search pointer for get_unmapped_area
after we have unmapped 1/8th of the normal memory in a process.

ick!

For
a process with 1000 similar sized VMAs, that means the search pointer
will only be reset once every 125 or so munmaps.  The cost is that
the program may use about 1/8th more virtual space for these VMAs,
and up to 1/8th more page tables.

We do not count special mappings, since there are programs that
use a large fraction of their address space mapping device memory,
etc.

The benefit is that things scale a lot better, and we remove about
200 lines of code.

We've been playing whack-a-mole with this search for many years.  What
about developing a proper data structure with which to locate a
suitable-sized hole in O(log(N)) time?

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