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Comment by saagarjha | Hacker News Reader

QEMU TCG is not particularly optimized for performance. It's not all that hard to do better than it, especially if you target only one architecture.

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It's also not that easy though, and blink's code generation is comparable to QEMU in 2007 or so. Even if TCG remains relatively basic, 2x better code generation then QEMU is still a strong claim.

I suspect that the reason why blink is faster in this experiment is not related to code generation, as I mentioned in another comment. Looking again at the screenshot, the 28 context switches (vs 0 for blink) might be a clue as well.

Qemu generates pretty good code. I've skimmed through your codebase. Qemu knows all the ISA-specific operation constraints, that lets it work very similar to a compiler. All I'm saying is that Qemu generates that quality code slowly. That's what I mean by a locomotive. A train can go hundreds of miles per hour faster than a Tesla sports car. But which is quicker off the mark? Blink basically just glues together functions that were created by the compiler. So when it runs a program like GCC, which has an enormously long initialization ritual, the JIT path generator is able to plow through it at a speed comparable to memcpy.

While I have your attention, may I ask if you've considered saving your JIT's output to a file, that can be rapidly loaded if GCC is launched a second time? Ephemeral commands tend to be executed independently many times, which would seem to favor an AOT approach. But I don't see any reason why a JIT can't persist its output to gain the advantages of AOT. I'd call it EOT.

> may I ask if you've considered saving your JIT's output to a file, that can be rapidly loaded if GCC is launched a second time?

I don't work that much on QEMU TCG actually, but it would be possible to do so. In fact recent (unrelated!) changes to QEMU might even make it possible to preserve ASLR with such a first-execution JIT compilation.

That said I have rarely seen code generation in QEMU's profiles. In the end what matters is real world performance and I need to redo the test myself to understand what's going on and whether your benchmark is representative of e.g. building a small but nontrivial program (let's say blink itself) with both QEMU and blink. In that case there would be repeated cold-start recompilation, but also the compiled code would run at least once per function so QEMU would have an edge.

To be honest blink is probably more like a bicycle than a sports car. It will start faster than a locomotive, but the cruise speed is definitely lower.

> A train can go hundreds of miles per hour faster than a Tesla sports car. But which is quicker off the mark?

Top speed of most trains is below that of a Tesla let alone super cars. The trains that can go significantly faster (but not really hundreds of miles per hour faster - Maglevs - are capable of similar acceleration). This analogy is not the greatest for the point being conveyed.

I think this comes down to a difference is perspective of trains. Coming from a European perspective when I first read the analogy I was imagining the incredible fast passenger trains over here (~320km/h).

I suspect if you are coming from the perspective of a US rail user then yes, they aren't exactly known for high speed train travel.

320 km/h is not hundreds of miles per hour faster than the top speed of a Tesla, especially one like the Plaid marketed for speed. I don’t give a fuck if you’re from Europe or the US, high speed trains aren’t faster than high speed sports cars. This is from the perspective of someone who’s not an idiot.

I think the point here is that trains go for hundreds of miles faster than a Tesla. If you're going 800 miles, a train might well get there first, though a Tesla's top speed is 4x the trains.

OP: “A train can go hundreds of miles per hour faster” Anyway I think this proves the limitations of this analogy.

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