Date: Sun, 13 Jan 2013 07:50:14 -0700 From: Ian Lepore <freebsd@damnhippie.dyndns.org> To: John-Mark Gurney <jmg@funkthat.com> Cc: arch@freebsd.org, toolchain@freebsd.org Subject: Re: Fast sigblock (AKA rtld speedup) Message-ID: <1358088614.32417.23.camel@revolution.hippie.lan> In-Reply-To: <20130112230435.GJ1410@funkthat.com> References: <20130107182235.GA65279@kib.kiev.ua> <50EBAA1F.9070303@freebsd.org> <20130112230435.GJ1410@funkthat.com>
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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
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