Re: DRAM to CPU Frequency Ratio (Athlon)

From: Marc Mutz (Marc@Mutz.com)
Date: Tue Aug 29 2000 - 13:00:13 EST

Ookhoi wrote:
>
> Hi Marc,
>
> > > I have two exactly the same Athlon systems at 750MHz and 512 meg memory
> > > (@ 133MHz). In the BIOS there is a option: "DRAM to CPU Frequency
> > > Ratio", which can be 3:3 or 4:3. The Help says: "Using this item to set
> > > the operating frequency of DRAM".
> > >
> > > I just installed Debian on the machines, rebooted, changed one machine
> > > to 3:3, the other was 4:3, and compiled a kernel on both of them. The
> > > time it took was almost exactly the same (about 5 min 32 sec). The 3:3
> > > one was 0.24 seconds faster.
> > >
> > > Does Linux ignore the DRAM to CPU Frequency Ratio (if at all possible),
> > > is it just a useless BIOS option (because of hardware limits or
> > > something), or is it not supposed to make the machine faster (if, what
> > > _does_ it do then ;-) ? (the book that came with the machine doesn't
> > > tell me much about the option).
> > >
> > <snip>
> >
> > You have PC133-333 RAM installed (PC133-222 SDRAM is still hard to find)
> > which may also be run as PC100-222. So, when you select a ratio of 3:3,
> > then the RAM runs in PC100-222 mode, which is not too much slower than
> > PC133-333:
> >
> > 2*1/100e6 = 1/50e6 is actually smaller than 3*1/133e6 = 1/44e6.
> >
> > There are _many_ factors to SDRAM timing, but the bottomline is that you
> > will only see a difference (of then ca. 5%, which sounds tiny, but is
> > approximately the same gain as when going from a 600 to a 650 MHz CPU)
> > if you use PC133-222 SDRAM.
>
> Thank you for your reaction. Others on the list also said that with a
> kernel compile, disk speed is a bottleneck, not memory.
>
> What is the difference between PC133-333 and PC133-222, and is that also
> the difference between RAM and SDRAM? Do you have an url to more info
> about this (and other factors to SDRAM timing)? TIA!
>

The full spec:
http://developer.intel.com/technology/memory/pcsdram/spec/index.html

The most important parameters (from approx. 30?)

PARAMETER SYMBOL
cycle period t_CK
CAS Latency CL
RAS-to-CAS Delay t_RCD
RAS Precharge Time t_RP
Output Valid from Clock t_AC
RAS Cycle Time t_RC
RAS Active Time t_RAS

The PC-blah-blah convention is also spec'ed in the above document, but
here comes the short story:

PC<1/t_CK>-<CL><t_RCD><t_RP>

where 1/t_CK is in MHz, CL, t_RCD and t_RP are in units of t_CK.

If you speak German, you can read all that in c't, issue 17/00, pp.166.

Marc 

-- 
Marc Mutz <Marc@Mutz.com>        http://marc.mutz.com/Encryption-HOWTO/
University of Bielefeld, Dep. of Mathematics / Dep. of Physics

PGP-keyID's:   0xd46ce9ab (RSA), 0x7ae55b9e (DSS/DH)

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