[9fans] [Fwd: [olpc-software] Multi-protection VMAs and memory usage]

[Ronald G Minnich <rminnich@lanl.gov>]

Just got this from the OLPC list. Another reason to love shared 
libaries. Are things getting kludgy here or what ...


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LWN lwn.net had an article this week discussing a patch that can
result is some nice memory savings for 'multi-protection VMAs'

Here is a link the relevant kernel mailing list thread

http://thread.gmane.org/gmane.linux.kernel/402675

and here is an excerpt from the lwn article,

"Ordinary Linux users could also benefit from this patch, however.
Ulrich Drepper explained how the C library uses VMAs currently; it
turns out that linking to a single shared library can create up to
five separate VMAs. An application which brings in a large number of
libraries - as many desktop applications do - can end up creating
hundreds of VMAs for shared library mappings. That leads to many VMAs
being created on the system; just how many can be seen by looking at
the vm_area_struct line in /proc/slabinfo. Your editor's system
currently has over 13,000 VMAs active, using about 2.5MB of memory.

Of the five VMAs potentially created by glibc for each shared library
mapping, four are mappings into the same file with different
permissions. The ability to have multiple permissions settings within
a single VMA has the potential to collapse those four VMAs into one,
leaving a single file mapping and an anonymous memory segment for each
library. The result would be significantly reduced memory usage and
faster kernel performance. Those benefits are likely to motivate the
inclusion of this patch, sooner or later. "