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Message boards : General discussion : Advice on buying used computers

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Profile CoffeemanProject donor
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Message 147332 - Posted: 3 Jan 2021 | 19:03:52 UTC

I was thinking to get a couple of used computers for use with Primegrid; I have a list of cpus that support AVX-512. But on ebay and craigslist, its hard to find them.

Any advice for what the "sweet spot" is, for used computers?

Laptop or desktop?
i3, i5, or i7?
What video cards to seek or avoid (desktop)

I am looking for the most computing power per dollar, how to find that? The CPU in my laptop is an i7-1065G7, and I am also searching on ebay for used laptops that work but have broken screens; can use an external monitor.

Intel has so many cpus, I dont see why there are so many, its ridiculous!

Any advice is appreciated.

Kevin

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Message 147333 - Posted: 3 Jan 2021 | 20:06:00 UTC - in response to Message 147332.

I was thinking to get a couple of used computers for use with Primegrid; I have a list of cpus that support AVX-512. But on ebay and craigslist, its hard to find them.

Any advice for what the "sweet spot" is, for used computers?

Laptop or desktop?
i3, i5, or i7?
What video cards to seek or avoid (desktop)

I am looking for the most computing power per dollar, how to find that? The CPU in my laptop is an i7-1065G7, and I am also searching on ebay for used laptops that work but have broken screens; can use an external monitor.

Intel has so many cpus, I dont see why there are so many, its ridiculous!

Any advice is appreciated.

Kevin


Laptops aren't a very good choice; they are expensive for the hardware due to size constraints, and the performance ain't good either because of cooling. Unless they are particularly cheap due to defects like a broken screen or wifi or whatnot, I wouldn't go for them.

For the hardware, getting AVX512 machines is probably a trap. The CPUs are expensive, the motherboards even more so, and the cooling is a nightmare. You'd be better off of the money going with FMA3 capable machines, aka Haswell and older or 3rd gen Ryzen and up (1st and 2nd have a bad implementation). If I was to bet, sweetspot would be 8th/9th gen i5 due to 6 cores per machine, or possibly 9th gen i7 for 8c. Xeons could offer more cores per machine, but the motherboards are probably a lot more expensive than some cheapo H310s.


For GPUs, the more modern, the better efficiency. Nvidia Gtx 16xx are a nice balance of cost (due to not having RTX) and power eficiency until the new cheap 3xxx drop.

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Message 147334 - Posted: 3 Jan 2021 | 20:37:14 UTC - in response to Message 147332.

AVX512 is new enough and only useful on the Core-X HEDT/expensive Xeon Gold 6000+ Server CPUs that it's going to cost a lot to acquire, even used, for now.

Look for CPUs that fully support AVX2/FMA3. Since you're going used, that would be Intel Haswell (4000/Xeon E3v3) or newer, and AMD Ryzen 3000 and newer. For price/performance, I'd say Haswell is a pretty good buy, and the 5800/5900/6800/6900 HEDT are also very nice and coming down in price. Quad core i5 or better, i7 if you can swing it. Skylake(6000)/Kaby Lake (7000) are also rapidly dropping in price thanks to AMD forcing Intel to step up its core count game. I'm finding complete Optiplex i7 4770 systems around the $200 mark on ebay (no GPU of course). Not bad.

If you are adventurous, special mention goes to building your own from used parts. Haswell CPUs and motherboards are readily available. Ultimately you will spend more than with a prebuilt system, but you are getting higher quality parts and faster memory. Xeon CPUs are quite affordable and for the massive selloff of Haswell/Broadwell-EP (Xeon 26xx v3/v4) parts, there are inexpensive Chinese-made compatible motherboards you can put them into, or for a bit more, used single and dual CPU server motherboards from good manufacturers, but you'll need to also spring for cooling, power, memory, storage, case if you want one, etc. But, IMHO, these represent some of the best bang for buck you can get in used parts. Many cores, high memory bandwidth (ECC is not required), but will certainly use more power than a used consumer/office machine for more work done.

Desktop is going to be your best bet if you want it to last; laptop cooling is rarely adequate for PG loads. Would I turn down a Haswell laptop for $50? No, and I'd run it bottoms up (I have one that I got for free I do that with. It is very loud.). Be wary of "SFF" desktops. These are the tiny little boxes that are smaller than keyboards and feature low power CPUs and poor cooling.

I'm going to go stoke some flames and say that any GPU that comes with a used system is probably garbage, and was garbage back when it was new, too. This is the place where newer is always going to be leaps and bounds better than older. We don't see +50% gains in CPUs (AVX512 notwithstanding) every year or two, but we do with GPUs. Prebuilts will likely have big limitations in what can be installed, whether it's power usage or simply fitting inside the case (and it's a crapshoot if a prebuilt system can be reinstalled into a larger case or use an off the shelf power supply). Buying used PC parts separately (or as combos) can overcome this limitation.

Intel really does have a long list of CPUs, don't they? They want something in just about every $10 price bracket, and OEMs like that option, too. Searching ebay by cpu model is a great way to find hidden gems.
____________
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Message 147377 - Posted: 5 Jan 2021 | 11:06:48 UTC

Depends on your expectations, but following are some general suggestions:


For CPU work focusing on LLR projects but can also apply to GFN:

Fastest per-core are AVX-512 enabled CPUs with two unit implementations. Practically speaking that is Skylake-X and possibly Cascade Lake-X. If you can find a low core count model with a cheap mobo to go with it, they could be interesting. 6 cores aren't that interesting to many these days so try 7800X in particular for a possible bargain.

Other HEDT CPUs can be interesting due to their higher ram bandwidth, which is often limiting for the bigger task projects. 5820k is particularly cheap, but again finding a cheap mobo to use it is key here.

Into the consumer realm, I'd suggest looking at Haswell or newer i5 CPUs (i5 4xxx or higher). LLR doesn't benefit from hyper-threading, and they also perform relatively worse for gaming, so they're often sold much cheaper than their i7 equivalents.

CPUs older than Haswell lack AVX2 support so perform notably worse, and thus not recommended.

On the AMD side, I wouldn't recommend anything older than Zen 2 which is their first CPU to finally match AVX performance with Intel. This is desktop Ryzen 3000 excluding APUs. Down side is these are still relatively new and their replacement generation is mostly out of stock, so there might not be great bargains there yet.

For the Intel CPUs, ram speed is particularly important. Prioritise filling the ram channels first. That's an easy win for bandwidth. Second, get the fastest affordable ram. Target 3200 or so. If dual rank, that's a bonus but unlikely to be found outside of very old 8GB modules, or higher capacity modules. Equivalently, can be implemented by putting two modules per channel. Timings are much less important.

For AMD Zen 2 and newer CPUs, ram matters much less since most work with correct settings can be done on CPU cache.


On GPU side, I have less experience here, but as a rough guide newer is better. nvidia Maxwell (9 series) isn't very efficient now so I wouldn't get that. Pascal (10 series) bought with it a good bump up in performance and efficiency, and again with Turning (20 series). I don't have personal data on Ampere (30 series). I don't have any recommendations for AMD side. Polaris and Vega were pretty meh. No experience of navi. Radeon VII is an odd ball that could be interesting in niche cases where FP64 is able to be used.

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Message boards : General discussion : Advice on buying used computers

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