Date: Sun, 26 Oct 2003 00:45:31 -0700 (PDT) From: Don Lewis <truckman@FreeBSD.org> To: alc@cs.rice.edu Cc: cvs-src@FreeBSD.org Subject: Re: cvs commit: src/sys/i386/i386 pmap.c Message-ID: <200310260745.h9Q7jVeF020616@gw.catspoiler.org> In-Reply-To: <20031026041909.GY20658@cs.rice.edu>
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On 25 Oct, Alan Cox wrote: > On Sat, Oct 25, 2003 at 11:14:48PM -0400, Jeff Roberson wrote: >> >> Uhm, dumb question, why don't we just allocate one page of kva per >> processor and avoid the mutex, the switchin/out, etc? To save KVA? At 3 >> pages per processor and a max of 8 processors on intel, that's 192k. We >> can probably spare it. I just saved that much with some UMA tuning. >> > > The mutex and the switchin mechanism are addressing two distinct needs. > > The mutex exists because there is a single KVA for some low-level pmap > functions on i386. (This could be changed, eliminating the need for > the mutex.) > > If, however, you eliminate the need for the mutex, you still require > the switchin mechanism as a means of handling preemption, > specifically, migration of the preempted thread between CPUs. The > alternative is to make pmap_zero_page() and the like critical > sections. Why not allow preemption, but just mark the thread as being ineligable for migration while it is executing pmap_zero_page() and similar bits of code? The thread might have to wait longer to get the CPU again after it was preempted, but so what.
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