RAM speed vs CAS

  • Thread starter Thread starter rjd
  • Start date Start date

rjd

rjd

rjd

Associate
Joined
4 Mar 2006
Posts
168
Location
London
Hi all

I notice a number of people here (including me) have trouble deciding on the best combination of speed vs CAS.

There are probably other/better ways, but keeping it simple I use CAS divided by speed and go with the one with the lowest number:

e.g:-

CAS 6/800 = 0.0075

CAS 9/1333 = 0.0067516 (faster)

CAS 10/2400 = 0.0041666 (fastest)

EDIT: See post #7 & #8. If the 'lowest numbers' are close, it's best to go for the one with the highest frequency.

I got this from a post on another forum, which I can't find ATM, but will post the details if/when I find it again.

Any other 'simple' ways worth trying?

EDIT http://www.tomshardware.co.uk/forum/299012-30-latency-speed
 
Last edited:
David Bisset said:
That gives you the best for latency, though depending on the task it's not necessarily the best overall. Still a useful rule of thumb :)
Click to expand...

Hi David

Will you please expand on 'depending on the task it's not necessarily the best overall'?

If bandwidth is fixed, what are the pros and cons of aiming for low latency vs high frequency?

I'm not using my PC for gaming but would like to get the best/fastest performance in general use and would be interested in any suggestions on how to achieve this.
 
Why would bandwidth be fixed?

Increasing the frequency increases the bandwidth (good), decreasing the timings decreases the latency (good), the two interact however so e.g. 2400MHz C12 is the same latency as 1600MHz C8 and your way of working it out will have these two setups draw though the 2400MHz one has higher theoretical bandwidth burst rate. Of course, C8 is only one of the timings that we're interested in, so life gets more complicated and there isn't a 'right' answer for most people as 'general use' is a mix of different needs.

Your way of working out which is best will work well enough, stick with it :) I'd not bother testing to see what timings you get at super-slow frequencies or anything, just a few of the fastest options.

I don't really understand it all well enough to explain it very well I'm afraid, maybe someone else can help out who knows more.

Edit: Have a look here and we can see the 2400MHz C12 compared with the 1600MHz C8 - both get their first word at the same time (normally latency gets compared by this) but the 2400MHz gets it's fourth and eighth words faster
 
Last edited:
Hi David

Thank you for your reply.

I thought the bandwidth was fixed as people usually only mention changing the frequency and CAS settings.

My RAM data transfer bandwidth is 19,200MB/Sec (PC3 19200) and current settings are 2400MHz 1.648v 10-12-12-31 CR 2T tRFC 314.

In a review it mentions using 9-11-11-28 but not tried that yet.

Re your Edit, I see it is more complicated so as you say maybe someone else can help out who knows more.
 
The bandwidth figure you're giving is just:
effective frequency * 8 = 2400 * 8 = 19,200

strictly the right way of working it out per-channel is:
actual frequency * bits per line * lines per clock (DDR) / bit per byte
1200 * 64 * 2 / 8 = 19,200MB/s

Obviously most of us run in dual-channel mode so we'd be getting double the above.
 
Hi David

Thanks for the extra info.

Looking at my original post, I will add 'if the 'lowest numbers' are close, it's best to go for the one with the highest frequency'

Thanks for your help.
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom