Saturday, July 28, 2018

High Speed VCR 2018


I'm a huge enthusiast of thin desktops. I have no idea why - normally such systems are used for HTPC duties or in very space constrained labs and offices, but my desk is not particularly small and I don't even own a TV.  The low-profile cases are about as small as cases get (they have a smaller interior volume and footprint than the cubes), and fitting everything into <85mm z-height makes for an interesting challenge.

Core Component Thoughts

Most HTPC-type systems are built around the "small" platform - currently, Z370 on the Intel side, X470 on the AMD side. These platforms offer low latencies, high clock speeds, and tons of integrated connectivity, but don't offer many cores compared to the state of the art. In contrast, the "high-end" desktop platforms are derived from server hardware - the boards have loads of PCIe lanes but very little integrated functionality, and the CPU's have many cores lashed together in weird and wonderful ways (rings, grids, clusters, and in the AMD case, multiple dies).

There are currently two possible routes for a USFF high core count system - the current-generation X299e-ITX/ac, or the now-discontinued X99e-ITX/ac. The X299 offers access to the latest platform features and CPU architecture, but as LGA2066 is not shared with any Xeons, the CPU's are quite expensive - the 10c part costs $899 and prices only go up from there. X99, in comparison, is kind of long in the tooth by now, but the CPU's are more accessible; an 18c 2.3/3.6 part used to be about $500 on the used market, and will likely be again once major datacenter upgrades flood eBay with used CPU's. With current pricing, X299 is certainly the correct choice; the 2699 V3 will perform similarly to a 14-core i9, costs about the same right now, and the i9 offers a full generation of platform and core improvements.

There is also no reason to go with anything under 12 cores. Ryzen will get you to 8 cores on a very power efficient platform (trust me, you are not overclocking anything on a computer this dense), and the 10-core i9 costs much more than any of the 8-core processors since Intel charges a "PCIe tax". 

Since I had a 2699 V3 available from the $500 days, I went with a X99 build (I had also hoarded an X99e-ITX from when they were $120 on eBay; prices have since jumped up to $200-300). The final selection was:
  • Motherboard/CPU: ASRock X99e-ITX + E5-2699 V3 - really no other choices here.
  • RAM: Crucial Ballistix Sport LT DDR4-2400: I really like the Ballistix Sport LT series; the gray heatspreaders are inoffensive and functional, and the DIMMS are pretty low profile - there are no useless protrusions on the heatspreaders to run into the CPU cooler.
  • Storage: Inland Professional 256GB NVMe - these are just reference Phison PS5008-E8 + Toshiba BiCS drives. They are incredibly cheap and offer better-than-SATA performance. Being M.2 also means one less cable to route in a case that is incredibly cramped with wires. My usual choice would be a Samsung 970 PRO, but at 3.6 GHz you can't really feel the difference between a fast drive and a slow one, especially when you take into account the Windows scheduler adding extra latency by moving threads between the many cores.
  • Graphics: ...I should really get a real GPU for this thing, but based on previous experiences, anything but the really big cards (Asus STRIX line, I'm looking at you) will fit.

Everything Else

Building these things is really an arts-and-crafts project, especially when you have as many computers as I do.  As such, picking the not-computer parts of the computer is much harder than selecting the parts that do the computing.

Case

My usual case for this type of nonsense is the Silverstone ML08, which is nicely priced and is as thin as possible (the minimum allowable clearance for an ATX case is 58mm). Unfortunately, the extra tight cooler clearance makes fastening a cooler to the board nearly impossible, since 2011/2066 heatsink mounting screws have to go in from the top. I was also interesting in trying the latest crop of Silverstone cases, which add an extra inch or so of clearance in order to fit an ATX power supply. All the 83mm-clearance Silverstones are based on the same chassis, just with different trims. I went with the RAVEN RVZ03, since I am a fan of RGB lighting.

Power Supply

The RVZ03 somewhat misleadingly supports ATX power supplies. While it is true that the mounting holes are for an ATX supply, most supplies flat-out don't fit; the case really requires a 140mm or shallower power supply to leave cable clearance. Furthermore, like the ML08, the RVZ03 uses an internal right angle IEC extender to place the power jack on the case somewhere reasonable. This caused a ton of problems - the CX550M I bought had a power jack to close to the left side of the power supply, which cased the extender to collide with the side of the case, and Seasonic Focus+ 550W had a power switch which collided with the molding on the right-angle connector, cause the switch to get stuck in the "off" position.

I eventually gave up and bought Silverstone's own 500W SFX-L supply. The power supply fit great, but as the X99e-ITX has its power connectors rotated 90 degrees from most ITX boards (the 24-pin is in the upper left corner), the stock 24-pin cable wasn't long enough. Thankfully, Silverstone makes a long cable set for this exact purpose; the kit is amazing for small builds since the 24-pin cable is only 550mm long, which is ~100mm shorter than usual.

Cooler

This whole project was made possible by an obscure-and-discontinued Cooler Master GeminII S heatsink. Low-profile LGA20xx coolers are hard to find - the reference socket backplate uses studs that are tightened from the top, meaning the cooler has to leave sufficient clearance to allow the studs to be tightened. My original plan was to use a Hydro H55 with a slim fan; measurements showed that the clearance would be sufficient. Unfortunately, packing the tubing into the case was pretty much impossible - it could be made to fit, but there was no way to gauge if excessive force was being applied to vertical components on the motherboard. Silverstone claims that a silm fan + slim radiator AIO will fit in this case, but even that seems doubtful...

The stock GeminII S doesn't quite fit - the 25mm fan is about 3mm too tall. I started out by mounting a 15mm fan from a GeminII M4, but that wasn't quite enough, so some more work was required...

Stuffing It All In



This was definitely the hardest computer I've ever assembled. The 58mm Silverstone cases are pretty easy to work on - the top and the bottom both come out, the GPU mounts from the back, and there is an access hole behind the socket to install the CPU cooler. In contrast, the 83mm cases only have one removable side, and the GPU is mounted on a plastic subframe that installs from the top; this makes cable routing far less pleasant. Without the 550mm long 24-pin this would probably have been impossible - I don't think another 100mm of cable would have physically fit in the case.

Performance Tuning

The 2699 v3 has a 80C temperature limit - once it hits 80C, it slowly drops out of turbo to stabilize temperatures. It's a graceful falloff - rather than dithering between 800MHz and 2.8GHz like some processors would, it decreases the multiplier a bin at a time until it achieves thermal equilibrium.

Initial performance was poor; the processor would hit 80C and drop to about 2.2GHz, which is below even the base speed of the 2699 v3. More concerningly, Intel's throttling algorithm seems to favor the core over the uncore - uncore speeds were dropping by as much as 50%, which was sure to affect performance in some applications.

Fortunately, upon further investigation it appeared I had plugged the CPU fan in the 'SYS_FAN' header on the board, which caused the CPU fan to get stuck at its lowest speed (SYS_FAN tracks the chipset temperature, not the CPU temperature). Swapping headers greatly improved performance; the CPU now stabilized at 2.5GHz, and the uncore throttling was gone.

But we can do better! Most 25mm fans have a few mm of superfluous plastic on top - by milling that plastic off I was able to get a 25mm thick Corsair fan to barely fit in the case. Installing the thicker fan bumped clock speeds up another 200 MHz, and and dropping Vcore by 50 mV in XTU allowed the processor to maintain 2.8GHz steady state under full load.

1 comment:

  1. Maybe give the new Noctua NF-A12x15 PWM (or NF-A12x25 PWM if you're feeling adventurous) a try? the 25mm unit doesn't actually have all that much more flow potential than the 15, but handles higher static pressure. Damn good fans.

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