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The 321 Sieve project has come to an end, and no more 321 Sieve tasks will be sent out. If you currently have only 321-Sieve tasks selected, I suggest switching to the PPS-Sieve project. It's a similar task, albeit longer in duration.

Source: http://www.primegrid.com/forum_thread.php?id=9403

Wasn't this the high paying Primegrid sub project?

You might be thinking of PPS Sieve the GPU application.

Among the CPU-only projects, 321-Sieve was the highest paying, in the same vein as GCW-Sieve before it. The sieving projects get a hefty bonus because you can't find primes with them = can't acquire glamor and fame.

The GPU-supported PPS-Sieve is the only active sieving project now; this one too pays better than the prime-searching GPU projects. (I idly wonder how the CPU application of PPS-Sieve compares with prime-searching CPU projects.)

I was indeed thinking of the GPU application.