Notices by Graham Sutherland / Polynomial (gsuberland@chaos.social), page 3

TIL you can closely estimate the quality factor a JPEG was encoded with by looking at the AC table and doing some fairly simple maths.

as part of my blog generator I'm optimising images before publishing, so if I look at a JPEG and can see that it was already compressed below Q=85, I probably won't see enough of a size saving by re-encoding it down to Q=80 to justify the extra perceptual losses of a repeat encoding.

for PNG images I attempt an indexed colour encoding, subtractively combine it with the original image data, threshold the luma and chroma deltas to get a per-pixel "different/same" value, sum those, and if less than 1% of the image is affected then I keep it. if not I fall back to max compression RGB888. (I don't use transparency in blog images)

on average this is saving me about 50-60% over whatever random PNGs and JPEGs I'm passing in. most of the savings are coming from PNG optimisation on screenshots.

anyone got a copy of that "I am the keyboard I have an important message... E" processor interrupt meme with the pissed off looking bird laying around? I just remembered it and I can't find a copy.

UPDATE: Success, thanks to @SnoopJ

https://hachyderm.io/@SnoopJ/113999524381604960

and this is a 100% known problem. and it's solved using active current balancing on the GPU side. you put shunt resistors in series with the lines, measure the current, and actively shift the current draw in realtime to keep everything balanced. (you can do this with an ideal diode-OR controller, or separating the high-side feeds to separate sets of VRM phases)

nVidia *has* done this on some prior cards. but they reduced the shunt count here, running stuff in parallel, and this is the result.

source of the photo and lots more details here:

https://www.youtube.com/watch?v=Ndmoi1s0ZaY

and more technical details of the 5090 shunt issue here:

https://www.youtube.com/watch?v=kb5YzMoVQyw

one note on the der8auer video: he mentions that the power headroom on the 5090 is just 15%, due to the high power draw and only a single connector being used, and states his opinion that they should've gone for two connectors to offer more headroom.

I agree with this in general (15% cuts it too fine), but it's important to contextualise the problem here: assuming perfect sharing, two connectors would give you a headroom of 130%.

the magnitude of current imbalance within the cable is 350%.

@phenidone it's right in the der8auer video, you can go watch him take the clamp measurements. 20A down two cables, <2A down the rest.

@ignaloidas @phenidone I have my suspicions that there is a sneaky runaway effect from thermal expansion increasing contact pressure, in part due to the ridiculously high current density at the connector, and cards with proper current balancing are keeping it in check. but I can't test that theory without hands on the gear and I don't have the cash right now (not even for buying the same connectors and some cables, unfortunately)

@ignaloidas @phenidone ultimately though I'm firmly of the belief that when burning people and their expensive equipment is the impact of the risk, you need secondary safety controls (active current balancing, lockout when sufficiently bad connections occur) to account for bad connectors or user error. 12VHPWR has problems but the cards should be protecting against these dangerous failure cases regardless, and especially given that it's a known problem.

@ignaloidas @phenidone the 3090 and prior manage this fine by balancing across split high side power domains - if one leg is compromised it won't boot, and you don't get massive imbalances. the Asus ROG 4-series cards added per line shunts to detect bad connections (they were required to use nvidia's single combined high side design so this was the next best thing) and warn the user / refuse to power on to protect against the issue. we have the capability to be safe. nvidia just didn't do it.

so to be clear the problem here is 100% the GPU and nvidia being fucking idiots.

you've got 500W going through this cable at 12V. so that's 40A.

the cable has a bunch of 12V lines and a bunch of GND lines. if the current is shared across those lines, it's all good. they're well within spec.

but what happens is if you put them all in parallel, whichever cable has a slightly lower resistance will carry more current, resulting in most of the current going down one or two wires and getting hot.

don't worry, it doesn't stay at 128 degrees!

it goes up. like... 150 plus.

12VHPWR is going great

inductance is a scam. I've looked at tons of inductors and there are no ducks in sight.

got my meds

@ryanc @manawyrm hot air and flux would be how I'd remove it if you were taking the chip off.

@ryanc @manawyrm although if you mean the main CPU, I don't think that'll get you anywhere

@ryanc @manawyrm the flux or the solder paste?