Date: Sun, 29 May 2005 23:52:03 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: scottl@samsco.org Cc: gibbs@scsiguy.com, arch@FreeBSD.org, nyan@jp.FreeBSD.org Subject: Re: [RFC] remove bus_memio.h and bus_pio.h Message-ID: <20050529.235203.74669295.imp@bsdimp.com> In-Reply-To: <4299FD87.1000505@samsco.org> References: <20050525.212009.71136852.nyan@jp.FreeBSD.org> <20050525.111945.41668351.imp@bsdimp.com> <4299FD87.1000505@samsco.org>
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In message: <4299FD87.1000505@samsco.org>
Scott Long <scottl@samsco.org> writes:
: This kind of makes me sad. I don't see how this was harming anything,
: it just wasn't documented so people didn't know how to use it. If it
: didn't apply to non-i386 and amd64, fine, just don't implement it for
: those platform. This optimization might have seemed trivial, but it's
: all of the little trivial optimizations that add up to make a nice
: system. I'm guessing that Justin only put effort into this originally
: because he did see a benefit; discounting it without doing any testing
: of your own is a bit disingenuous.
I've been unable to measure any difference in any of timing solution's
drivers between having the bus_pio.h include and not having it at all
(which disables the optimization). This is on a 266MHz Pentium. I'm
guessing that the drivers did inb/outb/etc so infrequently that any
benefit was swamped by the actual I/O. Even at the maximum data rates
that we could see (which did about 20k inb/outb a second) I couldn't
measure any CPU difference, nor could I measure any performance
difference. I did this in the 4.3 time frame in our tree when looking
to optimize a driver that was giving us trouble. A back of the
envelope calculation suggests that I should have seen about 100us/s of
extra CPU time. Since I ran the tests for about 1000 seconds, I
should have seen .1s extra in CPU. Maybe I did, but iirc the standard
deviation of the measurements as much more than that. I'll see if I
can find the data I gathered at the time.
I've not measured anything with memio to see if that matters, or if
there is anything different about newer pentiums and the branching
effects. However, when Justin introduced them in the 3.0 time frame,
which is 1998. According to Intel's web site, the Pentium II had just
been introduced, which puts the CPU speeds at just a little faster
than the embedded systems we run at work. I also recall discussions
with Justin at the time that said the biggest win was for 386 and 486
machines, but I might be misremembering those discussions, since they
were over lunch about 7 years ago.
Warner
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