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Ubuntu 24.04 LTS will enable frame pointers by default

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Re: Ubuntu 24.04 LTS will enable frame pointers by default

#71

Post by peterfirefly »

mathiasgredal wrote: post #58
mdpye wrote: post #48

Earlier quoted context omitted.

Because then it would need more registers for the other purpose? But actually the decision about which general purpose registers to use for what is made at compile time (hence we're discussion a compiler flag here, the frame pointer is not a hardware dictated feature), so the question is actually kind of moot. If the compiler is out of registers to allocate and instead uses the stack, the CPU isn't reasonably going t…

Sure, but wouldn’t it make sense to extend the instruction set to allow the compiler to use these registers instead of reserving them for speculative / out-of-order execution? It was just a thought i had after watching a talk by a compiler guy: https://youtu.be/2EWejmkKlxs?feature=shared&t=2409

jcranmer got it right. Read that reply (and mine). And then maybe rewatch watch Chandler Carruth says.

The current practice allows for CPUs to transparently increase their physical register count (to gain performance) and still run old code -- and older CPUs can still run new code. That's usually quite practical...

Adding more register names takes more bits for the register numbers -- which leads to larger instructions. It also leads to more complicated encodings if we want backwards compatibility. AMD64 does that by adding an optional prefix byte that carries a payload of 4 more instruction bits. That's one bit each for the three possible register names encoded in a traditional IA32 instruction + a bit to indicate whether to operate on 32-bit or 64-bit data (the actual rules are a bit more complex). Intel published a whitepaper recently suggesting a future encoding with a different (optional) prefix that encodes 8 more bits -- so each of the three register names can be extended to 5 bits (32 register names). It all ends up being quite complicated + new code won't run on older CPUs, which is not great.

I think you are suggesting not just bigger register names but also doing away with register renaming -- that would be... less than entirely useful because you would lose almost all your out-of-order capability and thereby almost all your ability to hide cache misses. Cache misses are very, very hard to predict statically (before actually running the code on a real CPU with real data) so good luck trying to do magic ahead-of-time allocation of those registers...

Re: Ubuntu 24.04 LTS will enable frame pointers by default

#72

Post by alfalfasprout »

londons_explore wrote: post #66

So the whole world should take a 1-2% performance penalty on everything so some users can maybe run a profiler? Wouldn't it make more sense to just have an 'apt reinstall all --with-frame-pointers' command that power users could run before they wanted to profile something?

This is very hyperbolic and inaccurate.

In 2023 it's not a 1-2% performance penalty anymore and certainly not for most use cases. Only if the 15th register is critical for performance on an x86_64 CPU.

Certain workloads might suffer more, but most will certainly suffer less than a 1-2% hit.

Re: Ubuntu 24.04 LTS will enable frame pointers by default

#73

Post by m463 »

londons_explore wrote: post #66

So the whole world should take a 1-2% performance penalty on everything so some users can maybe run a profiler? Wouldn't it make more sense to just have an 'apt reinstall all --with-frame-pointers' command that power users could run before they wanted to profile something?

You are prematurely optimizing.

"can make use of this improved debugging information to diagnose and target performance issues that are orders of magnitude more impactful than the 1-2% upfront cost."

Also, can't you get reliable stack dumps when something goes wrong too?

Re: Ubuntu 24.04 LTS will enable frame pointers by default

#74

Post by kmeisthax »

duskwuff wrote: post #6
brancz wrote: post #2

Frame pointers are such a destructive micro-optimization to omit by default, I am beyond excited about this collaboration with the folks at Canonical to make Ubuntu debuggable by default!

This option was almost certainly a holdover from the bad old 32-bit x86 days, when disabling frame pointers gave you a seventh valuable general-purpose register. It's no longer beneficial on x86_64 -- even with rbp locked down, you still have fourteen registers there.

Also, apparently Intel is planning to extend x86_64 to 32 GPRs, with an extension called... sigh[0]... Intel APX[1]. So the overhead of frame pointers will be even lower in the future.

[0] Intel APX is extremely confusable with the iAPX 432, a failed non-x86 architecture Intel made that's completely unrelated to doubling the size of the x64 register file.

[1] https://www.intel.com/content/www/us/en/developer/articles/t...

Re: Ubuntu 24.04 LTS will enable frame pointers by default

#75

Post by mratsim »

PaulDavisThe1st wrote: post #24
duskwuff wrote: post #6

Earlier quoted context omitted.

This option was almost certainly a holdover from the bad old 32-bit x86 days, when disabling frame pointers gave you a seventh valuable general-purpose register. It's no longer beneficial on x86_64 -- even with rbp locked down, you still have fourteen registers there.

and 14 registers should be enough for everyone!

It's not. 14 is still low compared to other ISAs.

Re: Ubuntu 24.04 LTS will enable frame pointers by default

#77

Post by mratsim »

brancz wrote: post #21
herodoturtle wrote: post #11

For the layman, what does this mean?

We wrote pretty extensively about what was needed to be able to profile things without frame pointers [1]. It's still possible at less than 1% overhead with the right set of technologies, but frame pointers unwinding is virtually free. [1] https://www.polarsignals.com/blog/posts/2022/11/29/dwarf-bas...

Intel VTune, Apple Instruments and perf have been able to profile without frame pointers.

Re: Ubuntu 24.04 LTS will enable frame pointers by default

#78

Post by brenns10 »

brancz wrote: post #67
brenns10 wrote: post #62

This is a good idea for the short-term. As of now, frame pointers are the most reliable way to ensure that software can be profiled by tools like perf*. The core issue is that the kernel must be the one to unwind the userspace stack, and it only knows how to unwind stacks with frame pointers**. The .eh_frame data will never be supported by the kernel, because it involves a turing-complete program that must be execute…

We’ve also figured out an alternative format to use from within eBPF to unwind stacks (we happen to only support dwarf at the moment but theoretically any source information could work): https://www.polarsignals.com/blog/posts/2022/11/29/dwarf-bas...

Yeah I saw Vaishali & Javier's presentation [1] at LPC last year! Great stuff, & certainly available to use now rather than when SFrame becomes available and supported.

In the same spirit, it seems that the .eh_frame -> BPF unwind table process could be (relatively) easily modified to produce SFrame, which you could attach to the binaries if you have a trustworthy way of doing that (which is... a big if). So that once SFrame support becomes available in the kernel, you could apply it to applications without rebuilding them.

[1]: https://lpc.events/event/16/contributions/1361/

Re: Ubuntu 24.04 LTS will enable frame pointers by default

#79

Post by PeterisP »

jcranmer wrote: post #26
herodoturtle wrote: post #11

For the layman, what does this mean?

There are two key effects of this decision. The first effect is that it makes one additional general-purpose integer register unavailable for use for code. x86-64 has 16 general-purpose registers, but one of these is the stack pointer and basically can't be used for any other purpose; this would add a second reserved register for the frame pointer. This effect may cause slowdowns if the 15th register was critical for…

Does this provide there any benefit at all to the majority of machines which aren't intended to be used for development of binaries (i.e. user workstations, developers working with interpreted languages, and servers), what would be the use cases where frame pointers would help on those machines?

Like, even for developers, I assume a random web development shop using Ubuntu and hosting stuff on Ubuntu would likely not ever attempt debugging a binary executable, and likely don't have any employees who could do it if they wanted. Of course there are companies who can and do debugging and profiling of binaries running on their servers, but IMHO those who are capable and willing to do that a relatively small minority of users of Ubuntu systems.

Re: Ubuntu 24.04 LTS will enable frame pointers by default

#80

Post by brancz »

mratsim wrote: post #77
brancz wrote: post #21

Earlier quoted context omitted.

We wrote pretty extensively about what was needed to be able to profile things without frame pointers [1]. It's still possible at less than 1% overhead with the right set of technologies, but frame pointers unwinding is virtually free. [1] https://www.polarsignals.com/blog/posts/2022/11/29/dwarf-bas...

Intel VTune, Apple Instruments and perf have been able to profile without frame pointers.

A lot of profilers have, it’s literally what unwind information is for, but we did it in kernel therefore not the entire stack needs to be copied to user space so it’s way less overhead.
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