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Mispredicted branches can multiply your running times

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Re: Mispredicted branches can multiply your running times

#21

Post by pjmlp »

naikrovek wrote: post #15

This is one of those things that is completely lost on someone who has never written in a low level language. I automatically assume JavaScript developers to be completely oblivious to this entire class of software development knowledge. It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software. Software has been…

And when you outsmart the compiler, the CPU gets a microcode update and those clever micro-optimizations are thrown out of the window.

Re: Mispredicted branches can multiply your running times

#22

Post by naikrovek »

meheleventyone wrote: post #20
naikrovek wrote: post #15

This is one of those things that is completely lost on someone who has never written in a low level language. I automatically assume JavaScript developers to be completely oblivious to this entire class of software development knowledge. It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software. Software has been…

> This is one of those things that is completely lost on someone who has never written in a low level language. I automatically assume JavaScript developers to be completely oblivious to this entire class of software development knowledge. This is a shame because not only are there lots of developers who write JS that have low-level backgrounds there are also a lot who haven't and are still interested. It seems rathe…

I realize that it is unfair to categorize ALL JS devs in this way, and it certainly is a tight fit for the JS developers that I have worked with in the past.

Re: Mispredicted branches can multiply your running times

#23

Post by vsareto »

naikrovek wrote: post #15

This is one of those things that is completely lost on someone who has never written in a low level language. I automatically assume JavaScript developers to be completely oblivious to this entire class of software development knowledge. It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software. Software has been…

>It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software.

Let's not drastically increase job requirements for no good reason.

>It's time to learn your platforms, software people.

Many of these platforms have undocumented CPU instructions, so until you get a full accounting of that, what's the point? You can't learn the platform fully if they keep that a secret.

Secondly, we've had CPU-level issues like spectre and meltdown introduced that affected performance in some cases. We can't even trust the platform makers to get it right!

Re: Mispredicted branches can multiply your running times

#24

Post by naikrovek »

gildas wrote: post #17
naikrovek wrote: post #15

This is one of those things that is completely lost on someone who has never written in a low level language. I automatically assume JavaScript developers to be completely oblivious to this entire class of software development knowledge. It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software. Software has been…

> I automatically assume JavaScript developers to be completely oblivious to this entire class of software development knowledge. Surprisingly, In JavaScript the technique described in the article is quite efficient on Firefox whereas the gain is almost negligible on Chrome. https://jsperf.com/mispredicted-branches Edit: Jsperf seeems to be down. Here are the 2 snippets of code I tested: // Unoptimized let howmany =…

Yeah, of course performance gains can be had in JavaScript, and I will add to that fact with the experience that I've had when working with JavaScript developers: only one that I've ever known has ever considered performance and worked to produce performant JS before multiple users complained and an issue was raised. All others simply do not care about Javascript performance until there is an issue created to address it.

This single person is also the only JS developer I know who has any clue how to actually improve performance in existing code.

Re: Mispredicted branches can multiply your running times

#25

Post by naikrovek »

vsareto wrote: post #23
naikrovek wrote: post #15

This is one of those things that is completely lost on someone who has never written in a low level language. I automatically assume JavaScript developers to be completely oblivious to this entire class of software development knowledge. It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software. Software has been…

>It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software. Let's not drastically increase job requirements for no good reason. >It's time to learn your platforms, software people. Many of these platforms have undocumented CPU instructions, so until you get a full accounting of that, what's the point? You can't l…

> Let's not drastically increase job requirements for no good reason.

Well, I would say that it's a very good reason, and that learning about branch prediction and caches is not a "drastic" step by any means.

Is there any software that you write whose users would not be made happier if the software performed better? Any at all?

> Many of these platforms have undocumented CPU instructions

You don't need to know the secrets of a platform to understand branch prediction and caches, or to use that knowledge to produce software that performs far better than software written without that knowledge. You don't need to know the platform on a logic gate level, you need to understand how the platform executes your code, so that you can take advantage of the strengths of the platform. You don't need to know any hidden instructions or secrets to take advantage of the platform.

> we've had CPU-level issues like spectre and meltdown introduced that affected performance in some cases

those things didn't affect performance, the fixes for those things did. The fixes also required no code changes outside of the firmware and the operating system, and knowing the platform is still the best way to write performant software, no matter what is going on to avoid hardware vulnerabilities.

Re: Mispredicted branches can multiply your running times

#26

Post by hamilyon2 »

naikrovek wrote: post #15

This is one of those things that is completely lost on someone who has never written in a low level language. I automatically assume JavaScript developers to be completely oblivious to this entire class of software development knowledge. It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software. Software has been…

What good software modeling techniques could one learn instead of OOP?

Re: Mispredicted branches can multiply your running times

#27

Post by emsy »

vsareto wrote: post #23
naikrovek wrote: post #15

This is one of those things that is completely lost on someone who has never written in a low level language. I automatically assume JavaScript developers to be completely oblivious to this entire class of software development knowledge. It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software. Software has been…

>It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software. Let's not drastically increase job requirements for no good reason. >It's time to learn your platforms, software people. Many of these platforms have undocumented CPU instructions, so until you get a full accounting of that, what's the point? You can't l…

>Let's not drastically increase job requirements for no good reason.

If anything, job requirements for programming are way too low. I wouldn't let a mechanic anywhere near my car if I knew s/he didn't understand the basics of an Otto engine. I'm the first to admit I don't know as much about modern CPUs as I should, but I don't live in a fantasy world where I convince myself that I don't need to know it.

>Many of these platforms have undocumented CPU instructions

So we should get the vendors to publish a proper documentation. This doesn't change anything about software developers responsibilities.

>CPU-level issues

Perfectly answered in a sibling comment so I'll just leave it at that.

Re: Mispredicted branches can multiply your running times

#28

Post by imtringued »

vsareto wrote: post #23
naikrovek wrote: post #15

This is one of those things that is completely lost on someone who has never written in a low level language. I automatically assume JavaScript developers to be completely oblivious to this entire class of software development knowledge. It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software. Software has been…

>It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software. Let's not drastically increase job requirements for no good reason. >It's time to learn your platforms, software people. Many of these platforms have undocumented CPU instructions, so until you get a full accounting of that, what's the point? You can't l…

>what's the point? You can't learn the platform fully if they keep that a secret.

By that logic we should never learn anything.

Re: Mispredicted branches can multiply your running times

#29

Post by kstenerud »

XJ6 wrote: post #19
kstenerud wrote: post #14

Earlier quoted context omitted.

The original code can end with index = 0 as well. The only difference is that in the optimized code, it will write to index 0, whereas in the original it will not.

In the original code, an index = 0 would not be a problem. Here, trying to access out[index-1] would error. Yes, you could then guard against that of course, but then that's even more code to maintain. If this code is a critical hot-path then sure, micro-optimizations can make sense but doing so without over-commenting and a rigorous test suite to catch introduced bugs is a recipe for disaster.

I think you may have misread the code:

    while (howmany != 0) {
        val = random();
        if( val is odd) {
          out[index] =  val;
          index += 1;
        }
        howmany--;
    }
vs

    while (howmany != 0) {
        val = random();
        out[index] = val;
        index += (val bitand 1);
        howmany--;
    }
Both of these store a list of odd numbers in out[], with "index" containing the resulting count of how many numbers are in out[]. Both will have an "index" (count) value of 0 if all inputs were even, and neither attempts to access out[index-1].

Re: Mispredicted branches can multiply your running times

#30

Post by vsareto »

naikrovek wrote: post #25
vsareto wrote: post #23

Earlier quoted context omitted.

>It is important to understand your platform all the way down to the CPU, including things like branch prediction and caches if you want to have performant software. Let's not drastically increase job requirements for no good reason. >It's time to learn your platforms, software people. Many of these platforms have undocumented CPU instructions, so until you get a full accounting of that, what's the point? You can't l…

> Let's not drastically increase job requirements for no good reason. Well, I would say that it's a very good reason, and that learning about branch prediction and caches is not a "drastic" step by any means. Is there any software that you write whose users would not be made happier if the software performed better? Any at all? > Many of these platforms have undocumented CPU instructions You don't need to know the se…

>Is there any software that you write whose users would not be made happier if the software performed better? Any at all?

I'd say security is a bigger issue than performance most of the time.

And most gains are going to happen within the code itself by, e.g., not writing n^2 when there's a log(n) solution or something similar.

Plus we're talking about javascript, and that's likely to be software with network concerns, so your optimizations might be a rounding error compared to performance degradation from slow network connections.

>You don't need to know any hidden instructions or secrets to take advantage of the platform.

You don't know what they do though. Some of those ops could be more advantageous to performance to use in some cases. You can't fully know the platform if there are secret ops.

You can still make optimizations with partial knowledge, but don't pretend to know the platform when the platform manufacturer doesn't tell you everything about it.

>The fixes also required no code changes outside of the firmware and the operating system, and knowing the platform is still the best way to write performant software, no matter what is going on to avoid hardware vulnerabilities.

Good algorithm knowledge and practice is the most cost-effective way of writing performant code and is more than likely going to be the lion's share of issues.

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