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 _______________________________________________ freebsd-toolchain@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-toolchain To unsubscribe, send any mail to "freebsd-toolchain-unsubscr...@freebsd.org"
