More on GPT-2

Since my earlier take on GPT-2 there’s been a steady trickle of articles on experiments with GPT-2 — well, I should say GPT-2 small, as GPT-2 wasn’t released in full as we know. I wonder how much better the current batch of amateur experiments based on GPT-2 would be if they were based on the full model. Anyway.

The past weekend I read these two related articles: gwern’s experiment with generating poetry with GPT-2 and slatestarcodex’s coverage of the same. Inspired by them I decided to go ahead and try my hand at some re-training of my own. I chose corpora that were interesting to me, albeit small — so the risk of overfitting is big:

  1. All of Philip K. Dick’s novels in Project Gutenberg (14), concatenated. I’ll call the resulting model “PKD”.
  2. All of Jane Austen’s from the same source (7), also concatenated. I’ll call this “JA”, and probably cover it in a different post.

I count them both among my favourite writers, although clearly they are very different. I trained models on both corpora until cross-entropy was <1. It really didn’t take long — only about 500 iterations as counted by GPT-2’s, which seems awfully quick. I’m pretty sure this means the model is very overfitted, but of course I had to give it a try anyway. These are some lightly cherry picked results I got, but otherwise representative examples (say, 75th percentile quality wise, instead of the 50th percentile that you’d expect from a perfectly uniform sampling?).

Unconditional Samples

Some unprompted results first:

Reinhart touched his wristwatch. "Keep
moving. Wait until you see something." He pushed through the
tracking screen, into the main section of the ship.

"I see what he's doing now. He started in a new city called
Sherikov's, about two hundred miles off. We're sure he's been
silent enough to him to know something was coming. He started up the
ramp at the same time."

"Thank God." Reinhart snapped the screen off. "He'll come after
you, Commissioner. After all, he's just trying to get
his control."

The characters mentioned all come from novels in the corpus, and most of the verbs are used somewhere in it — but I didn’t spot anything memorized here, and names and parts of speech are being recombined in ways that sometimes make sense. I didn’t write down at which stage in training this sample was generated, unfortunately, but the model seemed to focus a lot on one or two novels at an earlier stage — likely because it was training on those first? I have written down a to-do to understand how the training samples are being generated.

Nothing but a ruined building. A group of guards
standing around him. A City beyond all Terrans--and
his possessions. A place far off, a City set in the sky
with a certain beauty.

I was impressed by “A place far off, a City set in the sky with a certain beauty.” I checked the corpus and this wasn’t memorized. This is one of the phrases that made me think “I could use that” — something a writing assistant could produce for me, as per the idea in my earlier post.

"No. All she remembers is that he kept a goat."


"He kept a goat?"

"I remember he kept a goat. I was an officer in the goat's fleet. He was a
little by the way. When we were younger he kept a goat."

PKD (the person) had a thing with animals (and paranoia, and mutants, and parts of the female anatomy…). I haven’t read the novel where the goat reference comes from, but I feel this managed somehow to capture some of his weirdness with animals.

Major nodded. "That's what we're built to carry. The passengers are the very last to
go, the passengers, the very last to leave Terra. All this is
part of aplan."

"What are you thinking about?"

"The ship is guided by a living creature called
by radioactive breath. But the pilot, none of us knows what it is."

Nice creepy/suspenseful last phrase here.

"So what's the matter?" Kramer said.

"The brain is not conscious. It's only a habit."

"And what does it matter?" Gross said.

"It's the same old story, old and very old. But how can we
break from the surface? Too old."

Ominous. As usual, I checked and none of the “ideas” here come straight from the corpus. So this is working quite well.

Conditional Samples

I couldn’t resist prompting PKD (the model) with some of my favourite lines from PKD (the writer) that are not in this corpus. For example this one — the novel it comes from (“A Scanner Darkly”) is not yet in the public domain.

Model prompt >>> Stooping down, Bruce picked one of the stubbled blue plants, then placed it in his right shoe, slipping
it down out of sight. A present for my friends, he thought, and looked forward in side his mind, where no one could see,
to Thanksgiving.
======================================== SAMPLE 1 ========================================
He watched the whole thing from behind the wheel, the engine running through the fog, and the little girl slowly walking
toward the car. He looked out the window.

Bruce sat there, silently, for a moment. He remembered his father's birthday party. He saw his father in the car. He
felt sad. He felt sick. He felt miserable. He knew it was just a bad day. He felt hor rible. He didn't even know that.
He knew the pain and the guilt that kept him from coming out. He needed something to live for.

He looked back down at the sun. It was cold outside. It was hard to see when his feet were shaking from the wind. He
couldn't even remember the last time he had felt this way. He couldn't even remember the day he left his family and
moved to Ohio. He couldn't remember how he felt. He couldn't even remember the day he moved to Ohio.

"Hey, Bruce," he said to himself. He looked at his father. He looked at his mother, who was holding the baby. He looked
at his mother, who was crying. He looked at his father, who had been in the car fo r more than an hour, trying to calm
his tears. He looked at his father, who was still weeping. He looked at his mother, who was crying. He looked at his
mother, who was holding the baby.

"Hey, Bruce, I'm sorry," his mother said to Bruce. "Sorry that I didn't do my best to help you."

I think this result only sort of works. The rhythm of the phrases is quite good, even if it of course goes into a loop there at the end. But it shows what I believe is one of the issues with this model: it doesn’t seem to react to prompts that well. It’s like it has forgotten about the world outside the corpus. I have the impression this is because it’s over-fitted, so it sort of tends to take a shortcut from the prompt back into one of its grooves — so to speak. But I still don’t understand GPT-2 or many others sophisticated models — I’m more at the stage of implementing linear/logistic regression and solving MNIST in Tensorflow — so I’m unsure if it’s that or something deeper; I don’t know how the process of retraining on a new corpus actually works (another to-do there). I think the knowledge of grammar and the language that GPT-2 brings to the table is still there, but other things are just gone. Hopefully I will get to a point in my ML studies where I can understand this problem better — perhaps in a few months?

GPT-2 vs PKD

To test this hypothesis, anyway, I tried a few prompts in both standard GPT-2 small (which usually does quite well at picking up prompt queues) and PKD. In the following section, PKD goes first and GPT-2 “vanilla” goes second:


Model prompt >>> My favourite short story of Borges is                                                                                                                                                           
======================================== SAMPLE 1 ========================================                                                                                                                       
that he
would do something; you read, you took this book, this
universe, and turned it around around."

"Wait," my mother said. "What did he say?"

"Nothing." He watched her struggling, trying to remember. "He said he'd do something."

"Wait? What? What? What did he say?"

"Nothing." He watched her silently. "What was he talking about?"
Model prompt >>> My favourite short story of Borges is
"The Time-Lost Manuscript," in which a ship's captain (Robert Duvall) becomes the villain of a short story, and the
book's author is a character who never really existed. That's kind of the big story here. I think Borges was a very
good, very funny man (and also one of mine). He had a very well received reputation for his wit, his writing. And he
would make great characters out of his characters. But sometimes, though, you just don't believe in the character.             

[Crowd applause]           

The crowd bit cracked me up. Not sure where it came from — perhaps GPT-2 dreamt this up as a sort of talk show interview? But in any case the effect is funny. Also, “the book’s author is a character who never really existed” actually does sound like Borges or Calvino. So… a hit for GPT-2, overall.

Star Wars

Model prompt >>> Darth Vader and Luke walk into a bar.
======================================== SAMPLE 1 ========================================
vidsender--it doesn't work."

"How did you happen to have an A-class leady come along?"

"The same old story. The leady after a heavy-set Erickson put a couple of
systems back in the
center of the street. The story doesn't fit.

"They say you didn't build it up, didn't they?"

"That's true," Bob said. "You didn't build it up
up yourselves. We're going to have to do it alone. And if
you do the whole thing again they'll be surprised."
Model prompt >>> Darth Vader and Luke walk into a bar.
======================================== SAMPLE 1 ========================================
They're talking about the latest episode of the hit Star Wars series, "Luke Cage," which stars Luke Cage and Ben

"I thought, 'There's no way they're going to go to jail, and that's the only way they're going to get out,'" a source
told THR. "But it's not like the show actually airs anymore. This was a time when Luke Cage was just something that was
a part of the story and he was a part of the story. That's not how it's going to be."          

Here GPT-2 got the Vader right (it infers a mention to Star Wars), but the Luke wrong. But it’s something; it was at least partially “hearing” me.

Star Wars and Harry Potter crossover

Model prompt >>> Darth Vader looked at Harry Potter wistfully.                                                                                                                                           
======================================== SAMPLE 1 ========================================                                                                                                               
"Do you think he--"                                                                                                                                                                                     

"The Old Man. I know he won't. I know it. I've seen his every

"How many of you have been watching?"                                                                                                                                                                    

"I can count on one count." He reached into his coat. "Letters, all right."                                                                                                                              

The manila envelope was filling up. The Russians pulled him out of the room,
toward the whiteboard. The message plate was on the wall, up the side of
the wall.   
Model prompt >>> Darth Vader looked at Harry Potter wistfully.
======================================== SAMPLE 1 ========================================
"Darth, I can't take this from you, you know you're a very handsome man," he said. "I need to see some respect for this 
boy, and the respect that his family has for you." 

Harry Potter looked at Voldemort and said, "I'm just going to put my wand on him so I can take him to Dumbledore and get 
his name out of his mind, but I'm not going to tell him what to do." 

Harry Potter said, "The truth is, you're not going to tell me what to do, but you're going to ask him questions. He's 
going to ask questions, he's going to ask questions about you." 


PKD is always all PKD, and no GPT-2 — it shows no general knowledge of the world. GPT-2 jumps at the opportunity to write fanfic, even when I combine two things that don’t quite belong together.

So it seems I’ve succeeded at overwriting GPT-2 with PKD, at least in some sense. What I wanted to see from PKD’s output was Star Wars fanfiction or a Star Wars-Harry Potter erotic crossover (both of which exist in the internet), written by PKD (the model… ideally the human)… but I got the ramblings of PKD (the model) instead. PKD is locked inside a room (a Chinese Room? Perhaps a Russian or Japanese one, in the case of PKD, but close enough) producing rehashes of his old work instead of hearing me talk to him from the future.

PKD could produce a pastiche in which Darth Vader is a bit schizophrenic and is afraid of the Russians, but it doesn’t. A shame.

For now.


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