Date: Fri, 15 May 1998 06:42:54 -0700 From: Mike Smith <mike@smith.net.au> To: Greg Lehey <grog@lemis.com> Cc: dyson@FreeBSD.ORG, sos@FreeBSD.ORG, current@FreeBSD.ORG Subject: Re: People having problems with X windows? Message-ID: <199805151342.GAA00557@antipodes.cdrom.com> In-Reply-To: Your message of "Fri, 15 May 1998 18:09:16 %2B0930." <19980515180916.J1953@freebie.lemis.com>
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> > Excuse my ignorance, but what's WC in this context? Just to elaborate on the previous explanations: A PCI bus transaction is relatively expensive to set up and tear down. Uncached memory accesses via PCI (and accesses to PCI targets that do not support cache operations) are performed in a word-at-a-time fashion. In the case of, eg. a PIO blit from X server backing store to the framebuffer, a large portion of the time is taken setting up and tearing down PCI transactions, each of which transfers at most 32 bits of data. Some PCI chipsets provide a facility whereby writes to successive locations within a single PCI target are cached, and the resulting set of transactions are then performed as a burst cycle, where a single set up/tear down overhead is associated with a transfer of more than 32 bits of data. This is "write combining". -- \\ Sometimes you're ahead, \\ Mike Smith \\ sometimes you're behind. \\ mike@smith.net.au \\ The race is long, and in the \\ msmith@freebsd.org \\ end it's only with yourself. \\ msmith@cdrom.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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