Date: Wed, 3 Mar 2004 09:16:30 -0500 (EST) From: Daniel Eischen <eischen@vigrid.com> To: Peter Grehan <grehan@freebsd.org> Cc: freebsd-threads@freebsd.org Subject: Re: User-space context switch and floating-point Message-ID: <Pine.GSO.4.10.10403030909510.17237-100000@pcnet5.pcnet.com> In-Reply-To: <40459478.9060702@freebsd.org>
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On Wed, 3 Mar 2004, Peter Grehan wrote: > Here's the comment I currently have in libpthread/.../powerpc/context.S: > > * XXX XXX > * Floating-point is a big issue. Since there's no way to determine > * if the caller has used FP, all volatile register need to be saved. > * If FP hasn't been used, this results in a lazy FP exception in > * the kernel and from that point on FP is always switched in/out > * for the thread, which may be a big performance drag for the system. > * An alternative is to use the the getcontext system call, which > * will do the right thing for floating point but will save all > * registers rather than the caller-saved subset, and has the overhead > * of a syscall. > * Maybe another option would be to give a light-weight way for a > * thread to determine if FP is in used: perhaps a syscall that > * returns this info immediately in the asm traphandler, or an > * OSX-style read-only per-thread page with a flag to indicate FP state. > > Any opinions ? I noticed the alpha/sparc64 routines don't save FP. You don't need to save floating point registers when saving the context from userland. The userland _thr_getcontext() function is called as a result of an application calling pthread_yield(), sleep(), nanosleep(), etc (yielding/sleeping calls), or by something that blocks in userland like pthread_mutex_lock(), pthread_cond_wait(), low-level libc/libpthread locks, etc. Look at the i386 thr_getcontext(); it only needs to save the FP control word. -- Dan Eischen
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