Date: Sat, 4 Oct 1997 20:05:25 +0600 (ESD) From: "Serge A. Babkin" <babkin@hq.icb.chel.su> To: hackers@freebsd.org Subject: PCI slowness ? Message-ID: <199710041405.UAA09967@hq.icb.chel.su>
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Hi! I've made a simple driver to test the SCSI throughput. It takes 2 NCR53c810A SCSI cards and starts to transfer data between them at (theoretically) 10MBps synchronous rate. But in fact I get at most 8.5MBps ! I was able to rase it from 7.5MBps to 8.5MBps by changing the memory access options in NCRs from simplest to maximal optimization so probably the PCI or memory bus limits the throughput. Can anyone suggest me what's the problem ? The chipset is Intel Triton on some chineese motherboard with 75MHz Pentium, memory is 60ns EDO. Theoretical PCI throughput is 33M of 4-byte transfers per second (the card claims to work in burst mode). Theoretical memory throughput is at least 10M of 4-byte transfers per second if we suppose that the memory cycle with all overhead is 100ns and the card reads by 4 bytes at a time. But the experiment shows throughput of only 17MBps or 4.25M of 4 byte transfers. Does the processor eats all the remaining throughput (although I think it must load most of the code it runs at idling into the cache) ? And another thing. I know that expensive machines like DEC Prioris have possibilities to change some PCI timing parameters for PCI cards. My cheap box does not have anything like. May be that's the problem ? Thanks for any clue! -SB
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