Fast sigblock (AKA rtld speedup)

[Ian Lepore]

On Sat, 2013-01-12 at 15:04 -0800, John-Mark Gurney wrote:
> David Xu wrote this message on Tue, Jan 08, 2013 at 13:09 +0800:
> > and can not be freed until process is exited, the page is doubly
> > mapped into in kernel and userland, accessing the shared data
> > in kernel has zero overhead though.
> 
> Don't forget that there are arches out there w/ VIVT caches which will
> probably eliminate most of the performance benifits if we have the same
> page mapped writable in two different virtual addresses..
> 

Even worse than eliminate the benefits, since multiple mappings with one
writable disables caching on the whole page, there can be a big penalty
depending on what other data is nearby that suddenly becomes
uncacheable.  I was initially very interested in the work to read system
clocks without a syscall until I realized it was going to suffer from
the same problem.

-- Ian