Can someone tell me why yac gpu miner works so well with my 7870LE Tahiti?

[6strings]

Basically the 7870 LE EZ is a tahiti based gpu like the 7950/70. It is not the same as the standard 7870. Anyway, its terrible at scrypt however it is just as good as a 7950 when mining BTC and now I find out it performs just as well with the Yac GPU miner, which is based off cgminer to the best of my knowledge. For the life of me, and from what I see from many many others its damn near impossible to get this above 400khash mining scrypt coins like LTC.

Can someone out there please tell me why the modfied cgminer for Yac works so well but regular cgminer won't mine scrypt worth a damn for me? The super technical bits of software/hardware are not something I am skilled with but I would love to know if its some how possible to remodify that yacminer to mine regular scrypt just as good on the 7870 tahitis. Its a cheap and powerful gpu with much lower power usage than the 7950 and nearly equal performance where it counts.

[6strings]

bumping,
for some reason this was moved from the mining software forum to here. which sucks cause i wanted an answer from a technical minded person rather than just someone who mines this specific coin. silly mod.

hopefully someone brainy can answer the above for me? much thanks.

[forsetifox]

I believe it's because of the faster memory bandwidth.

[6strings]

is it at all possible for someone in the know to make cgminer work better for this card by tweaking the yac version? I ask because I have no clue.

[mikaelh]

Well, the scrypt-chacha kernel is a curious thing.

I think performance is mostly linked to memory. Ideally you want to access the main memory as little as possible even though the scrypt algorithm is forcing you to do so. Large register files and caches help with that. I haven't looked at the exact specs of all the chips but there may be some differences there.

Wikipedia has a nice table with the specs for the HD 7000 series:
http://en.wikipedia.org/wiki/Radeon_HD_7000_Series

The chip on the 7870 XT is code named Tahiti LE. It has a 256-bit memory bus which puts it behind the 7900 series in memory bandwidth. And it also has less cores than the 7900 series. I would assume they didn't change the Tahiti design but simply disabled some cores and a few memory controllers. Compared to the 7970, they probably have disabled 33% of memory controllers and 25% of cores. So the memory bandwidth is the biggest issue with your card.

Hanzac did a pretty good job with the OpenCL kernel in his initial release. I only improved it by taking some ideas from the normal scrypt kernel. So it has some new ideas and it has some old ideas. I think it may be possible to apply some of Hanzac's tricks to the normal scrypt kernel.

The scrypt-chacha kernel certainly seems to perform differently. For example on my 7790 there doesn't seem to be a "sweet spot" for the combination of engine and memory clock. Admitted, I didn't test this thoroughly, but the kernel seemed to always simply run faster at higher clocks.

[6strings]

thanks mikaelh.
good info there.
The thing with this card is its so damn cheap and it comes with 4 games like the 7950/70, when sold make it even cheaper. It also consumes much less power. If someone can make it run scrypt worth a damn than its probably the next best thing to an fpga. $200 is nothing if it can get up to 500khash. Theres some good potential here if someone can make it work right. Heck i'd buy 8 more of these cards next week if so.

[6strings]

a bit of a bump to keep this alive. if i knew what to change in that cl file myself I would.

[notaek]

thanks mikaelh.
good info there.
The thing with this card is its so damn cheap and it comes with 4 games like the 7950/70, when sold make it even cheaper. It also consumes much less power. If someone can make it run scrypt worth a damn than its probably the next best thing to an fpga. $200 is nothing if it can get up to 500khash. Theres some good potential here if someone can make it work right. Heck i'd buy 8 more of these cards next week if so.

Can't help you with your main question, but be aware that you can pick up 6950's that will hit 500khash for under 200 bucks.

[6strings]

good point. used vs new can make a big difference when mining however....if you bake the things you have a chance at RMA at least if you go with new. Also 7000 series is more energy efficient, especially the 7870 LE in comparison to the 6950.