Skybuck Flying wrote:


"Paul" wrote in message ...

wrote:
So these cards have 3.5 GiB and 0.5 GiB of memory.
We went throught this about 10 years ago with Intel northbridge,
where a 32-bit Windows saw typically 3.25 GiB with 4 GiB installed.
If you had PAE linux, you got almost 4 GiB, but not continuous.
Didn't nvidia learn from history? Repeat!


"
That's not exactly true. The card in question has
one (or more) functional units turned off (for yield reasons),
preventing the memory interface unit for it from
working. There really is only 3.5GiB of memory.
"

This would be news to me ?!

I think you might be miss-understanding.

There is 4 GB of RAM on it.

However only one section can be used at a time for reading.

So either 3.5 GB or 500 MB.

It is claimed to be possible to read from 3.5 GB and write to 500 MB at
the same time or vice versa.

Bye,
Skybuck.

You are correct.

http://www.anandtech.com/show/8935/g...ory-allocation

But you don't really want to do that.

"In the case of pure reads for example, GTX 970 can read the 3.5GB segment
at 196GB/sec (7GHz * 7 ports * 32-bits), or it can read the 512MB segment
at 28GB/sec, but it cannot read from both at once.

The same is also true for writes, as only one segment can be written to
at a time.

Unfortunately what this means is that accessing the weaker 512MB segment
blocks access to the stronger 3.5GB segment if both memory operations are
identical; or put another way, using the 512MB segment can harm the
performance of the 3.5GB segment. For example, if we want to issue reads
to both segments at once, reading the 512MB segment blocks any other reads
to the 3.5GB segment for that cycle. If the 3.5GB segment is blocked in
this fashion and doesn't have a non-blocking write to work on instead,
it would have to go idle for that cycle, which would reduce the effective
memory bandwidth of the 3.5GB segment. This means that taken over time
in our example, the larger the percentage of the time the crossbar is
reading the 512MB segment, the lower the effective read memory bandwidth
would be from the 3.5GB segment."

So it's best if the 512MB section is not used.

One of the tools listed in that article, shows this. And is
probably why I concluded 3.5GB was present when I glanced at
the article the first time.

http://images.anandtech.com/doci/8931/970_35GB.png

I wasn't really all that interested, as the chances of me
owning one of those is just about zero :-) I don't have
money for 4GB (or 3.5GB) video cards :-) And I still haven't
read the article to the very end. My eyes are glazing over.

Paul