"brakes make my car slower, so I'd expect my average velocity to decrease if you fixed them"
Just to be pedantic... in some contexts better brakes actually allow you to go faster. Consider racing on an oval track... a car with better braking ability can maintain speed longer as it approaches a corner, then scrub off speed more quickly to navigate the corner. A car with inferior brakes has to start slowing down sooner or risk crashing by taking the corner too fast. So better brakes can lead directly to faster lap times.
Yes, and in other contexts, better brakes make you go slower. Better brakes means bigger brakes: larger rotors (discs), larger and heavier calipers, etc. Larger brake rotors take more energy to accelerate (they're basically a bigger flywheel), so they decrease the car's acceleration. So if your goal is to have a car that accelerates as fast as possible, better brakes are actually a big detriment.
Some people argue that the Earth is flat. That doesn't mean they have a good point.
The morons you're referencing probably think cars should all go back to drum brakes and bias-ply tires so people think more about brake fade and tire grip. It's an idiotic argument. Disc brakes on bikes are better in every way, except weight (they add about 1 pound, maybe). Reducing performance available to a cyclist doesn't make any sense at all; you never know when you're going to need to stop suddenly in the real world.
Of course both things could be true...
No, you have it the other way around. Rubber brake pads on wheel rims fade with heat; disc brakes are able to dissipate far more heat, and are basically essential for long descents.
The only valid argument against disc brakes on bikes is that they weigh a little more (maybe 1 pound). I suppose you could also argue that brake fluid is more trouble to deal with than a cable, and certainly not as easy to jerry-rig, but hundreds of millions of cars use hydraulic brakes without any trouble these days, and I wouldn't want to have jerry-rigged brakes anyway.
Some drivers have complained about rain because the tears have been literally pulled out of them under the deceleration
Also, I drift corners instead of braking, it seems to be a lot faster if you nail the tire orientation on the exit.
Fair point. As the old saying goes "looser is faster". The risk, of course, is that you wind up in the wall with your velocity = 0. :-)
> In the UK, in 2009, it was revealed that the Department for Transport had previously discouraged green waves as they reduced fuel usage, and thus less revenue was raised from fuel taxes.
Ugh...
The degree by which governments routinely violate the accounting principle of matching revenues to expenses is awful.
A carbon tax on consumers (incl. businesses) is effective at producing good market behavior regardless of how the tax revenue is spent.
In the example I reacted to, a government agency actively worked to inhibit that behavior in order to preserve its revenue. Your point would hold if governments didn't have the power to do things like this.
> Earmarked funds are stuck.
If the revenue is being raised to pay for damage caused by society, those funds should be stuck to that purpose and should rise and fall based on the level of damage being done.
A governmental bad actor is basically orthogonal to the idea of a carbon tax. The government could equally try to increase sales of liquor and cigarettes for sin taxes, or go around murdering people for the estate taxes. The UK example is shameful but reflects on that government body rather than the specific tax on petrol.
The problem is earmarking or allocating a variable set of revenue as funding for something unrelated, and in particular something unrelated that's otherwise valuable or important.
And NOT earmarking or allocating 'sin taxes' to pay for the supposed costs of those sins, borne by the public in the form of the government, seems pretty stupid (to me) if it's actually necessary or desirable for the government to 'manage' those sins or those sin's consequences.
There is this thing called the "waiting time paradox" [0] (or more generally the "inspection paradox") that suggests a surprising thing, the average time between discrete events observed by many other randomly interspersed observation is the same as the average time between events. This should be surprising, because it suggests that many more people experience times LONGER than the average wait to balance out people who experience a shorter wait because they were randomly closer to the event.
This happens because longer intervals, when they do exist, get a larger proportion of the random observations than the shorter intervals between events. More precisely and generically, when the quantity being observed affects the observer, the observations will be distorted by the quantity in question and have to be normalized.
In the case of a green wave, those rare times when someone waits too long on a green because they were looking at their phone or a naked person in a nearby window and create slowdowns and stalls in the pipeline? Those moments are longer, so there is a longer window where you could experience them.
This actually comes up a lot in observations of things in nature. Imagine if instead of light flips or bus arrivals we were observing clicks on a geiger counter and you'll realize how fundamental this is to experiencing the world.
tl;dr: The Green Wave only works in averages and it can be correct even as your experience of it never working is also correct. :)
[0]: https://jakevdp.github.io/blog/2018/09/13/waiting-time-parad...
Of course traffic was heavy enough that you'd be stuck behind someone doing the speed limit most days, but sometimes you got to ride the wave.
Timed lights work, I've seen it in action. Top of my head, Indianapolis 20 some years ago, Capitol Avenue going north I'd bet you could go from (what is effectively) Zero Street to 38th and not hit a red light.
If two cars both say they drive at 50 km/h, and one is actually 45 km/h and the other 55 km/h, that is quite a bit relative margin of error. And we only need two cars like that in the whole group to start causing problems.
If you can't design a green wave to be tolerant of varying speeds, then it isn't useful.
Great Highway in SF, if you drive 35MPH you will just get greens on the evenly spaced lights. The worst is when you get stuck behind a person who drives 40, then brakes heavily, then drives 40, then brakes heavily, etc.
Here it's street view of the road, it's not perfectly visible, because those are LED displays: https://goo.gl/maps/T7RzQCUS4nk3QVZN9
Edit: (or go double the speed limit if you must but please stop creating stops)
It was one my little pleasures to drive along that at 35 (or 30, I forget what the speed limit was) and see cars zip ahead at each light at 45, only to watch them ride the brakes as they hit the next red.
Imagine a translucent checker board of blue and black squares overlaid on top of a map of roads. You can see the color of the squares but you can also see the roads underneath them.
If a light falls on a blue square, make its principal north/south direction Red at t=0 and it’s principal east/west direction green. For black squares do the opposite.
Calculate how long it would take cars to travel through each square unimpeded. It helps if this time is roughly the same regardless of direction. It also helps if the squares are roughly large enough that it takes roughly one light phase worth of time to travel through it. Call that time N.
At t=N, switch the lights to the opposite color, so now north/southbound traffic gets green lights on blue squares, and east/west gets red. Black squares get the opposite.
What you’ll see if you imagine yourself as a car driving through this, is that as you enter a black square as it turns green, the green lights will stay green until you enter the nearest blue square, at which point the phase shifts and now the blue square gets green lights as you drive through it.
This works in both directions, with both north and south bound traffic.
Now, this falls apart when roads are diagonal, although it’s mitigated if diagonal roads have higher speed limits. It also means if you make a turn, you’re in the wrong light cycle, but it will correct itself after the next red light.
It also doesn’t work well at all if you have left turn arrows complicating your light cycles, so you’ll need to have something like a Michigan Left and the requisite wide medians to make this work.
It’s not as ideal as I described in real life, but the basic principle works.
I guess the real issue here is that 'fast' is ambiguous. In terms of racing it can be used for both lap-times or velocity. lap-times and maximum velocity are not necessarily correlated.
</nerdsnipe>
[1] these sort of assumptions of course tend not to age well.
This just isn’t true.
Here’s a concrete example - many JIT compilers use counters to work out when to compile a method. They allow data races in updates to the counters because performance is more important than correctness for them and the impact of a lost write is basically zero. It’s only a bug if you decide it’s a bug.
> A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is dependent on the sequence or timing of other uncontrollable events. It becomes a bug when one or more of the possible behaviors is undesirable.
Or read the canonical Encyclopaedia of Parallel Computing page 1693
> [A race condition is when there is] some order among events [where the] order is not determined by the program
Data races are one of those cases where "anything may go" undefined behavior is actually quite necessary. Actual hardware memory models (particularly on "weak" architectures like ARM or PowerPC, or especially the infamously weak Alpha) result in cases where the order of memory traffic is not a consistent view between different threads. Formally specifying the hardware memory model is actually surprisingly difficult, made all the more difficult when you want to give a hardware-agnostic definition [1]. And should you want to introduce hardware transactional memory, break down in despair at the complexity [2].
The great breakthrough in memory models is the definition of the "data-race-free" model. This model says that, if memory accesses are properly protected with locks (i.e., there are no "data races" [3]), you cannot observe a violation of sequential consistency, which makes reasoning much easier. Java used this model for its corrected memory model in Java 5, and then C++11 adapted Java's memory model, and C11 took C++11's model verbatim. The upshot of this approach is that, if you make data races undefined behavior, you don't have to try to figure out how typical memory optimizations, both by the compiler and by the physical hardware, affect visible memory. Trying to work out the effects of these optimizations on the memory system is extraordinarily difficult, and the benefits of doing so are manifestly unclear, since most programmers will need to properly synchronize their code anyways.
[1] The C++ memory model attempts to do this with atomic memory references. The resulting specification (in C++11) is generally considered to be wrong, and the question of how to fix it is still up in the air as of right now.
[2] PLDI 2018 had the first formal semantics that combined transactional memory and weak memory semantics, and went on to prove that ARM's hardware lock elision was broken. This should be showing just how difficult this is to grasp even for theoreticians pushing the boundary, let alone a language trying to make it accessible to typical programmers.
[3] There are two definitions of "data race" going on here. Vernacular definitions usually define it as accesses not protected by synchronization, which permit "benign" data races for regular loads/stores. C/C++ tweaks the definition so as to use it to proscribe undefined behavior, so "benign" data race is oxymoronic in those languages. However, the use of memory_order_relaxed is intended to indicate a permitted data race in the first sense but not the second sense.
Could you or someone else please give a minimal example of an undefined access in C, just so that I can be sure of whether we're talking about the same phenomena here?
#include <threads.h>
int x;
int do_thread(void *) {
for (int i = 0; i < 10000; i++)
x = i;
return 0;
}
int main() {
thrd_t t;
thrd_create(&t, do_thread, NULL);
do_thread(NULL);
thrd_join(&t, NULL);
return 0;
}
This is a very dangerous way to understand semantics of programming languages. Programming languages are usually defined via some sort of operational semantics on an abstract machine which occasionally bears some resemblance to how processor semantics are defined. But sometimes there is a massive disconnect between processor semantics and language semantics--and perhaps nowhere is that disconnect greater than memory.
In hardware, there is a firm distinction between memory and registers. Any language which approximates hardware semantics (which includes any mid-level or low-level compiler IR) is going to mirror this distinction with the notion of a potentially infinite register set (if pre-register allocation) and use of explicit instructions to move values to and from memory. But in every high-level language, there is no concept of a memory/register distinction: just values floating around in a giant bag of them, often no notion of loads or stores. You can approximate this by annotating values (as C does, with the volatile specifier), but the resulting semantics end up being muddy because you're relying on an imperfect mapping to get to the intended semantics (don't delete this load/store). Optimization that screws up the mapping of the language memory model to the hardware memory model is anticipated, even 30 years ago--that's why C has a volatile keyword.
It is possible, if you are very careful, to write C code that exposes the underlying hardware model that allows you to reason about your program using that model. But this is not typical C code, and instead requires modifying your code in such a way that you make the intended loads and stores explicit. And if you write your code in the typical way and expect it to map cleanly to hardware semantics, you're going to have a bad time.
I use the term 'race condition' about once a week to point out something may happen before or after something else, because the things are happening in parallel.
Simple example:
me: I just uploaded the file to our shared dropbox
you, refreshing: I don't see it yet
me: race condition
The latter is the colloquial definition.
You can use it to refer to any race, but in reality the race you are referring to is only subjective to the user. The server is generally processing all requests in the order they are received.
A race condition usually is referring to a multithreaded system where the threads interfere with each other, causing undesired effects.
Relaxing "race condition" to refer to all causality... is such a loose definition as to give it no meaning at all.
I did a PhD partially on irregular parallelism where understanding race conditions are essential for understanding the topic.
That's literally how 'race condition' is defined in the industry.
You’re not wrong that not all race conditions are harmful, but i feel you’re doing a pretty bad job of explaining what a race condition is.