Date: Sun, 31 Jul 2016 16:51:29 +0300 From: Slawa Olhovchenkov <slw@zxy.spb.ru> To: Bruce Evans <brde@optusnet.com.au> Cc: Mateusz Guzik <mjg@freebsd.org>, svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r303583 - head/sys/amd64/amd64 Message-ID: <20160731135129.GA22212@zxy.spb.ru> In-Reply-To: <20160731220407.Q3033@besplex.bde.org> References: <201607311134.u6VBY81j031059@repo.freebsd.org> <20160731220407.Q3033@besplex.bde.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sun, Jul 31, 2016 at 11:11:25PM +1000, Bruce Evans wrote: > Misalignment of this loop made it almost twice as slow on old Turion2 with > slow DDR2 memory. It made no difference on Haswell. I added an extra > movnti, but that makes little or no differences. 2 more movnti's wouldn't > fit in a 16-byte cache line so are slower unless even more care is taken > with alignment (or with less care, 4 with misalignment are not less than > twice as slow as 1 with alignment). > > I thought that alignment and unrolling didn't matter here, because movnti > has to wait for memory and almost any loop runs fast enough to keep up. > The timing on my old system is something like: CPUs at 2 GHz; main memory > at 4 GB/sec; movnti is only 4 bytes wide on i386 (so this problem > only affects i386, at least with slow memory). So sustaining 4 GB/sec > requires 1 G movnti's/sec, so the loop needs to run at 2 cycles/iteration > to keep up. But when it is misaligned, it runs at 3-4 cycles/iteration. > Alignment makes it take about 2, and the extra movnti is for safety and > to work with faster memory. > > On Haswell with CPUs at 4 GHz, 2 cycles/iteration gives 8 GB/sec on > i386 and 16 GB/sec on amd64 with wider movnti. IIRC, 16 GB/sec is about > the main memory speed so nothing better is possible but just 1 extra > movnti gives more with faster memory. This is just worse than bzero() What about modern system with 120 GB/sec main memory speed?
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20160731135129.GA22212>