Messages in this thread | | | Date | Fri, 4 Dec 1998 11:17:26 +0100 | From | Kurt Garloff <> | Subject | Re: IDE-DMA strangeness (another one) |
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On Thu, Dec 03, 1998 at 07:39:36PM -0500, John Kodis wrote: > On Wed, Dec 02, 1998 at 11:43:34PM +0100, Kurt Garloff wrote: > > > So 33MB/s is quite realistic [for reads from buffer cache], but in > > no way characteristic for your HD. > > I get a similar speed, but don't understand why. I would expect that > given a 4-byte wide bus clocked at 33MHz performing a read from cache > and a write into the destination userspace buffer, the buffer cache > read speed should be more like 66MB/s. Is there another read/write > pair that I've overlooked?
Basically you measure the main memory bandwidth for copying.
So, you should expect: ~50MB/s with Pentium and ~100MB/s with P-II (100MHZ FSB), as reported by the ctcm tool. (c't cache monitor, www.heise.de/ct)
P-II does what it's expected to: 100MB/s.
I don't know exactly, why Pentium is slower than expected. It may be Linux buffer cache handling overhead (not very likely). I think it's some of the strangenesses of Pentium architecture. Memory copying speed on a Pentium depends on whether you * use movb or movl or use fpu * implement software write allocation (loading target L1 cache lines) * how many bytes you copy within the loop body (branch prediction issue?)
I don't know what mechanism the linux buffer cache uses. I've seen 36MB/s on my old Cx6x86-P200+ (2x75MHz), IIRC.
Regards, -- Kurt Garloff <K.Garloff@ping.de> (Dortmund, FRG) PGP key on http://student.physik.uni-dortmund.de/homepages/garloff
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