What thread concurrency were you running? It's absolutely possible to fire up a miner at any value of N with 1-2GB per thread, but if your cards have 4GB of ram on each of them and you are firing them up using only 1GB on each, then it's really not a great test. I'd be happy to create a command line for you that replicates how I run my 4x4GB card system. I couldn't even fire up a single thread allocating 3.5GB memory per card when I had 4 GB system ram. Let me know if you want to test it out.
Just to clarify: how exactly does the buffer size relate to the memory size on the card and the N? If you have a link to something that explains this for noobs I'd appreciate it. Thanks.
@suchmoon
https://github.com/Thirtybird/YACMiner/blob/master/SCRYPT-READMEstarting around line 56. buffer-size replaces thread-concurrency because all that is really doing behind the scenes is setting the scrypt buffer-size. Simplification IMHO.
@phzi
my exact configuration command line is as follows:
yacminer --scrypt-chacha --worksize 128 -g 1 --lookup-gap 2 -R 1280 --buffer-size 3040 --auto-gpu --temp-target 70 --temp-overheat 76 --gpu-engine 900-1100 --gpu-memclock 1000 --gpu-powertune 20 -o stratum+tcp://yac.m-s-t.org:3333 -u Thirtybird.R7240 -p x
Now, this doesn't even try to push the memory boundary on each card because with the number of shaders those R7 240's have, it's not necessary at N=14. For N=15 I will need to increase the buffer-size. I haven't found the top of buffer-size, but to push the buffer-size a bit, I can use --buffer-size 3664.
To interpret this for cgminer, you'll need to change a few things - probably something like this...
cgminer --scrypt --worksize 128 -g 1 --lookup-gap 2 -I 12 --thread-concurrency 58624
it will probably be about the same for vertminer. The parameters that affect the scrypt buffer are thread-concurrency and lookup-gap. You can change anything else you want, but you'll need to use the two values I specified to allocate 3,664 MB per thread (don't use -g 2 [or any number higher than 1]!!!!). Honestly though, these parameters are independent of what coin you're mining. Why it doesn't show up at lower n factors is because scrypt mining, and vertcoin mining both require much less RAM by comparison, that setting up to mine those coins is much simpler.