Copying this from the BOINC forums:

I've been having some discussions on several sites regarding GPUs and bandwidth usage for distributed-computing projects, and I wanted to broaden things by hopefully getting some BOINC experts in on the matter.

Now most of us are probably familiar with GPU mining and how hardware is generally deployed in these farms, but for anyone who isn't too familiar with it, a mining farm is typically comprised of a cheap motherboard with as many PCI-e slots of any size, some basic RAM, a cheap CPU, and of course, the GPUs. Due to space limitations, these GPUs are normally connected to the motherboard via a flexible riser which is an x1 adapter at the motherboard end, an x16 adapter on the GPU, and a USB 3.0 cable connecting the two to each other. Essentially, these are PCI-e x1 extension cables. They do not actually use a USB interface. a 3.0 cable is used simply because it has the right number of wires inside to map to an x1 interface.

Now, given that these risers are bottlenecked at x1 bandwidths, this would limit performance for high-bandwidth applications such as gaming, and significant performance reductions would be observed. Since cryptocurrency mining does not require high bandwidth, no performance loss occurs here, as x1 bandwidth on PCI-e 2.0 or 3.0 is never maxed out.

I had assumed that since mining does not require such high levels of bandwidth, perhaps distributed-computing projects might be the same. In the past few weeks, I've been discussing this over on the Folding@Home forums, and to my disappointment, anything less than PCI-e 3.0 x4 or PCI-e 2.0 x8 would result in bandwidth saturation, and thus a performance loss occurs due to the GPUs never reaching full load. This was rather disappointing for me, as I had wanted to build a system specced similarly to a mining rig, for the purposes of distributed-computing.

After a bit of thinking, I started to wonder if every project would require the same levels of bandwidth as F@H, so here I am. With the lengthy backstory out of the way, my question to you guys is simply this: Are there any GPU projects on the BOINC platform that do not saturate the PCI-e x1 interface?

I would love to get some data from anyone working on GPU projects. MSI Afterburner shows bus usage, so if a few people are willing to spend two or three minutes to take a few measurements, I would really appreciate it. Please let me know what size and version the PCI-e slot of your GPU is as well.

I've been discussing this over on the Folding@Home forums, and to my disappointment, anything less than PCI-e 3.0 x4 or PCI-e 2.0 x8 would result in bandwidth saturation, and thus a performance loss occurs due to the GPUs never reaching full load.

I was afraid of that. Really wanted to drop a good $10K on a dedicated F@H/BOINC rig and use PCI-e splitters to multiply the GPU capacity. Would have saved me a lot of money buying extra mobos and CPUs.

You think every other GPU project on BOINC would be the same?

You think every other GPU project on BOINC would be the same?

I was afraid of that. Really wanted to drop a good $10K on a dedicated F@H/BOINC rig and use PCI-e splitters to multiply the GPU capacity. Would have saved me a lot of money buying extra mobos and CPUs.

You think every other GPU project on BOINC would be the same?

Thanks for the info. Guess my ultimate system vision might not be exactly how I'd have wanted it. I should be able to get away with multiple triple or quad card setups though, given the right chipset.

So if each DC project is to some extent similar to each other when it comes to secondary hardware requirements, what kind of CPU would be recommended if each separate system were to have three or four cards each?

Some of the work units require the use of a CPU core in addition to a process running on the GPU.

With SETI for example, some of the work units require only very light CPU usage - and a single CPU core can quite easily keep many parallel processes running across multiple GPUs. Some of the work units however, require a dedicated CPU core or thread. A quick look in task manager or CPUID HWMonitor (very useful little app if you haven't already got it) will show what is going on.

As far as I am aware, it is not currently possible to automatically select a certain type of work unit or refuse another. This has also resulted in a reduction in RAC as the work units that are CPU dependent dont necessarily yield higher credits, they just take several times longer to run.

With SETI, I have found peak performance on my system (3770K and 980 Ti) occurs when crunching 4 work units at the same time on the GPU and no more than 3 at a time of the CPU (leaving 5 CPU threads free - 4 of which are used as required by the work units that require GPU and CPU).

The older work units that do not require much of the CPU complete in about 15 minutes each. So I crunch about 16 per hour. The CPU intensive units take an hour each, so I only crunch 4 per hour. The credit received for either is broadly the same.

The situation with GPUGRID is the opposite as this almost exlusively is designed to run just within the GPU. I am sure I could happily fit 4 x 980s in my PC case and work away at GPUGRID with the added bonus of not having to turn the heating on in my house during winter providing I am sitting in the same room as the computer!

The situation with GPUGRID is the opposite as this almost exlusively is designed to run just within the GPU.

I am sure I could happily fit 4 x 980s in my PC case and work away at GPUGRID with the added bonus of not having to turn the heating on in my house during winter providing I am sitting in the same room as the computer!