Date: Sun, 18 Aug 2002 12:33:34 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Julian Elischer <julian@elischer.org> Cc: Ian Dowse <iedowse@maths.tcd.ie>, Bruce Evans <bde@zeta.org.au>, arch@FreeBSD.ORG Subject: Re: Solving the stack gap issue Message-ID: <200208181933.g7IJXYC5072982@apollo.backplane.com> References: <Pine.BSF.4.21.0208181024530.35342-100000@InterJet.elischer.org>
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:On Sun, 18 Aug 2002, Ian Dowse wrote: :> :> If there is agreement on the td vs. curthread issue, then that would :> obviously be easy to change. : :A few days ago, Peter gave some comments as to the expense of using :curthread. I must admit this is something where some architectureal :guidance would be a good thing.... maybe something like :"Use a local if you need to access a Per-cpu variable more than twice :in a function", and "passing a thread pointer as an argument (is/is not) :preferable to calling curtread explicitly in the child function". : :Julian I would consider this to be more expensive: proc1() { struct thread *td = curthread; ... proc2(td) } proc2(td) { ... } And this to be less expensive: proc1() { proc2(); } proc2() { struct thread *td = curthread; ... use td several times ... } At least for I386. Ultimately I think this will be generally true on any architecture. If a procedure uses 'curthread' multiple times loading it into a local at the top of the procedure should be a sufficient optimization. Passing td around to dozens or hundreds of procedures just for the sake of avoiding accessing 'curthread' is bad design. -Matt Matthew Dillon <dillon@backplane.com> To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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