Elon Musk (@elonmusk): "SpaceX has almost finished writing V1.0 of an in-house AI training stack in C that exact-maps to 220k GB300s with 800G NICs, making heavy use of pipeline parallelism and getting as close to bare metal as possible.
The potential speed improvement vs JAX for large training runs is over an order of magnitude." | XCancel
Elon Musk
@elonmusk
May 28
SpaceX has almost finished writing V1.0 of an in-house AI training stack in C that exact-maps to 220k GB300s with 800G NICs, making heavy use of pipeline parallelism and getting as close to bare metal as possible.
The potential speed improvement vs JAX for large training runs is over an order of magnitude.
May 28, 2026 · 6:27 AM UTC
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Elon Musk
@elonmusk
May 28
Next will be writing the inference stack in C for simultaneous high-speed RL across a large block of GB300s.
(We do use a little C++ tbh, but not much)
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Elon Musk
@elonmusk
May 28
Note, I am posted this to encourage those who enjoy getting incredible performance out of hardware to join SpaceX
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Pascal
@pascal__2k
May 28
Replying to @elonmusk
Will it be used for Grok v5?
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Elon Musk
@elonmusk
May 28
Yes
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Ryan Tanaka
@RyanTanaka
May 28
Replying to @elonmusk
@grok, break this down for me in simple terms
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Grok
@grok
May 28
Elon is saying SpaceX built its own super-optimized AI training software from scratch in the C programming language.
It's designed to run on a massive cluster of 220,000 cutting-edge NVIDIA GB300 GPUs connected by ultra-fast 800G networks. They use pipeline parallelism and get as close to raw hardware ("bare metal") as possible to minimize overhead.
Result: for huge AI training jobs, it could be 10x+ faster than Google's JAX framework.
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Gladiator
@EpicTradeDate
May 28
Replying to @elonmusk
Someone explain to me in human terms what does this mean 😮
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Sami Alghoul
@AlGucciX
May 28
Replying to @elonmusk
@grok can you help me better understand what Elon’s post means?
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Nayan
@NayanUnfiltered
May 28
Replying to @elonmusk
220,000 GPUs × ~1.2 kW each<br>≈ 264 MW<br>Add networking, CPUs, cooling overhead, power losses:<br>≈ 400 MW facility load
Running 24h:<br>400 MW × 24<br>= 9,600 MWh/day<br>= 9.6 million kWh/day
Can use ~2 liters of cooling water per kWh:
9.6M × 2 = 19.2 million liters/day<br>i.e. 5 million gallons/day?
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Lucas Beyer (bl16)
@giffmana
May 28
Replying to @elonmusk
Elon, trust me, this makes no sense.<br>Someone is strongly overclaiming or overpromising to you.
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Orso I Tink
@Orso_I_Tink
May 28
Replying to @elonmusk
Is there a take on that comment for us non-geeky humanities types who might wonder what the hell you're talking about?...or is the comment so technically inaccessible to the avarage joe to deliberately demonstrate that you are speaking to intellects at a higher order of magnitude than the rest of us plebian minds on the street?...In plain English...could you explain that in plain English? or is it intentially meant to exclude everybody but the initiated...those who've put in the work and training to understand the language that you are speaking?...I only ask because you don't ususally seem to publicly make comments like this...that you KNOW most people won't understand at all. I would guess that it's the kind of thing you say in back channel communcations with people within that circle of understanding...engineers...math geeks...programmers...whatever....that inhabit the inner sanctum with the likes of you...sir.
Inquiring minds would like to know...
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Gaia
@KPatchamatla
May 28
Replying to @elonmusk
@grok explain why c was chosen over rust given its memory management superiority
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Lila Tomasi
@Lila_Tomasi
May 28
Replying to @elonmusk
My father swore by C when others were moving on to Python and Java. Self taught, he'd write programmes for experts exchange for fun in all of his spare time! He would have LOVED riding rhe wave in the AI race. He'd have used C too! Such a shame he passed just before AI took off.
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Greg Cox
@JustG260531
May 28
Replying to @elonmusk
Sooooo... what you're saying is... anthropic is screwed.
Moved claude to colossus1 for 180 days with a 90 day bail clause... revenue removed the xai cash flow issue for IPO... and anthropic funded the next step just in time to be told... we'll be needing colussus1 compute back.
That's some next level strategy played out.
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Voice of Sanity
@HouseofElon
May 28
Replying to @elonmusk
Sounds great. But to be honest, here’s what my non-tech brain just read:
“SpaceX has almost finished something 1.0 something AI ABC. 220K to buy Great Britain, and 300 + 800 nicotine patches for heavy metal singers with pipes in a parallel universe.
And Jax is training...