memory bandwidth calculations

[mundhra]

I'm hoping some of you can help explain or point me to links about memory bandwidth. I've been trying to research this all week and it's driving me batty.

My situation is: I bought a mobo/proc combo from an AT member. Memtest gives me a memory bandwidth of ~600 MB/sec running 2700 ddr, but that seems a little low to me.

Motherboard: A7V8X-X; Via KT400 chipset
Proc: AMD XP 2400+ (266 fsb)
RAM: Hynix 2.5 2700 ddr (2 x 256)

The BIOS settings for the proc are 15x and 153/31 in the BIOS. I couldn't see anything directly for the RAM except for timing, which I set to use SPD.

My P4 (533 fsb) running dual-channel 2700 on an i865 chipset gives me 2.2 GB/sec in memtest, so I'd expect closer to 1 GB for this system. How far off am I?

 

[Cerb]

Try Sisoft Sandra, and compare against similar chipsets. I don't know what Memtest scores can be compared against.

 

[rise]

interesting. i'd love to find a formula but there are so many variables...

if it helps, i know my systems run ~2700-2900mb/s

 

[dguy6789]

Memtest is not really a benchmark, it is a stability test, so don't worry about the MB/s it tells you.

Sandra is generally pretty close to what it would be.

 

[coomar]

i find that memtest is pretty accurate, its correlates with sandra well

i get 2933 mb/s

couple of things, single channel with an nf2 (pc3200) you'll get a 950-1000 mb/s, with older chipsets you can score anywhere between 500-1000, dual channel pc 2700 you should be getting 700-1000

asus boards tend to put pretty conservative timings if you run them in auto, change them in the bios to whatever they're supposed to be

 

[rise]

Originally posted by: dguy6789
Memtest is not really a benchmark, it is a stability test, so don't worry about the MB/s it tells you.

Sandra is generally pretty close to what it would be.

you've got to be kidding me.

 

[coomar]

sandra/ everest/ memtest will roughly correlate to each other

increasing them doesn't give you that much of a performance boost

 

[Matthias99]

Originally posted by: rise4310

Originally posted by: dguy6789
Memtest is not really a benchmark, it is a stability test, so don't worry about the MB/s it tells you.

Sandra is generally pretty close to what it would be.

you've got to be kidding me.

I've seen memtest (at least an older version of memtest86; maybe the newest memtest86+ doesn't do this) produce some wacky memory bandwidth numbers. Sandra gives, at the very least, pretty consistent numbers that can be compared between systems.

Theoretical bandwidth (in bytes/second) for any kind of memory is (number of transfers per second * number of bits moved per transfer / 8 (the number of bits in a byte)).

For PC3200 (DDR400), the number of effective transfers per second is 400,000,000, and the number of bits moved per transfer is either 64 (in single-channel mode) or 128 (in dual-channel mode).

Single-channel DDR400: 400,000,000 * 64 / 8 = 3,200,000,000 bytes/sec. = 3.2GBps.
Dual-channel DDR400: 400,000,000 * 128 / 8 = 6,400,000,000 bytes/sec. = 6.4GBps.

You should get around 90-95% of those numbers with Sandra (e.g. around 3000MBps for a single-channel system, and around 6000MBps for a dual-channel system). Timings and the exact motherboard will affect this a little bit.

 

[rise]

thanks for that Matthias.

sandra, ime, is handy but not what i'd call accurate. its ok but much more erratic than memtest.