What?! Linux provides me memory-barrier primitives: smp_mb(), smp_rmb(), smp_wmb() etc. Even an everyday Linux hacker does not need to understand the peculiarities of each architecture, because the locking peculiarities are all abstracted out in commonly used spinlock_*() interfaces. Any sane compiler-writer would use these primitives too: so, unless you're writing raw architecture-dependent assembly, or are a very hardcore Linux hacker, you do NOT need to understand this.
Linux supports Alpha, which provides the weakest ordering guarantees anyway, so no: any architecture stronger than Alpha puts absolutely no burden on anybody. In practice, weaker ordering constraint means more freedom for Linux.
Sure, if you use a portable set of primitives you only need to understand how and when to use those primitives and only the relaxed memory model they are based on. But even that is a lot to ask for, and too much for many programmers.
And this particular solution only works inside the Linux kernel and only using GCC. It doesn't matter what we think a sane compiler-writer or a sane computer-architect would do. Many times we just have to live with the choices other people made.
I think you got something wrong here. Architectures with strong memory ordering put more burden on the hardware and less on the programmer. That was the point I was trying to make: weak memory ordering is not worth it, because it is better to let the hardware do the hard work and not to burden the programmer with it.
Linux may work well with relaxed memory ordering, but that does not come for free, a lot of people had to understand first, what a relaxed memory model is, etc.
weak memory ordering is not worth it, because it is better to let the hardware do the hard work and not to burden the programmer with it
My question is very simple: why do we have architectures that reorder aggressively if the performance gain is not worth it? CPU manufacturers do understand that weaker guarantees leads to complexity that must be handled at the software level: so, are they mad? The fact of the matter is that we have been moving away from Lisp machines, CISCs and "human" instruction sets to RISCs, because the kernel/ compiler is in a much better position to make optimization decisions than the hardware (it can see the bigger picture).
These architectures exist, and the infrastructure to handle all this complexity has already been written (yes, all major compilers too [1]). Yes, a lot of people had to study a _lot_ to get us where we are today, and the success of our little iPhone apps stands on the shoulders of those giants.
I seriously don't get what software complexity you're whining about. Complexity is not the exception in Linux; it's the bloody rule! Have you seen the fs/ tree implementing various filesystems? Perhaps the kernel/ tree with the state-of-the-art scheduler? Or even the net/ tree implementing TCP/IP?
I'm not interested in discussing some hypothetical idealized textbook world where everything is simple and elegant: I'm interested in contributing whatever little I can to tomorrow's concurrency infrastructure.
The mammoth question in the room has still not been answered: how does ARM manage significantly lower power consumption? Aggressive reordering (=> weaker guarantees) seems to be part of the answer.
No they are not. But in the early 90ties when most of these decisions were mande, the situation was very different. Processors got twice as fast every two process generations. Shared memory multiprocessors were an exotic niche. But then we hit the power wall, and all of a sudden shared memory multiprocessors became the norm. Today, you can not just wait for a new processor to make your program magically faster, you have to rewrite it. If you asked the same people who took these decisions in the 90ies today, many would (and many actually did so publically) say that they regret this decision.
On, complexity. Yes, software is incredibly complex. That's why you don't want to add even more complexity to it!
How does ARM manage significantly lower power consumption?
Easy, ARM is targetting the low power market. So they don't need to build the fastest chip. When you try to build the fastest chip, energy efficiency suffers a lot. Once ARM starts to compete with Intel on performance, their power usage will skyrocket.
Comments
What?! Linux provides me memory-barrier primitives: smp_mb(), smp_rmb(), smp_wmb() etc. Even an everyday Linux hacker does not need to understand the peculiarities of each architecture, because the locking peculiarities are all abstracted out in commonly used spinlock_*() interfaces. Any sane compiler-writer would use these primitives too: so, unless you're writing raw architecture-dependent assembly, or are a very hardcore Linux hacker, you do NOT need to understand this.
Linux supports Alpha, which provides the weakest ordering guarantees anyway, so no: any architecture stronger than Alpha puts absolutely no burden on anybody. In practice, weaker ordering constraint means more freedom for Linux.
Sure, if you use a portable set of primitives you only need to understand how and when to use those primitives and only the relaxed memory model they are based on. But even that is a lot to ask for, and too much for many programmers.
And this particular solution only works inside the Linux kernel and only using GCC. It doesn't matter what we think a sane compiler-writer or a sane computer-architect would do. Many times we just have to live with the choices other people made.
I think you got something wrong here. Architectures with strong memory ordering put more burden on the hardware and less on the programmer. That was the point I was trying to make: weak memory ordering is not worth it, because it is better to let the hardware do the hard work and not to burden the programmer with it.
Linux may work well with relaxed memory ordering, but that does not come for free, a lot of people had to understand first, what a relaxed memory model is, etc.
My question is very simple: why do we have architectures that reorder aggressively if the performance gain is not worth it? CPU manufacturers do understand that weaker guarantees leads to complexity that must be handled at the software level: so, are they mad? The fact of the matter is that we have been moving away from Lisp machines, CISCs and "human" instruction sets to RISCs, because the kernel/ compiler is in a much better position to make optimization decisions than the hardware (it can see the bigger picture).
These architectures exist, and the infrastructure to handle all this complexity has already been written (yes, all major compilers too [1]). Yes, a lot of people had to study a _lot_ to get us where we are today, and the success of our little iPhone apps stands on the shoulders of those giants.
I seriously don't get what software complexity you're whining about. Complexity is not the exception in Linux; it's the bloody rule! Have you seen the fs/ tree implementing various filesystems? Perhaps the kernel/ tree with the state-of-the-art scheduler? Or even the net/ tree implementing TCP/IP?
I'm not interested in discussing some hypothetical idealized textbook world where everything is simple and elegant: I'm interested in contributing whatever little I can to tomorrow's concurrency infrastructure.
The mammoth question in the room has still not been answered: how does ARM manage significantly lower power consumption? Aggressive reordering (=> weaker guarantees) seems to be part of the answer.
[1]: http://en.wikipedia.org/wiki/Memory_ordering#Compiler_suppor...
No they are not. But in the early 90ties when most of these decisions were mande, the situation was very different. Processors got twice as fast every two process generations. Shared memory multiprocessors were an exotic niche. But then we hit the power wall, and all of a sudden shared memory multiprocessors became the norm. Today, you can not just wait for a new processor to make your program magically faster, you have to rewrite it. If you asked the same people who took these decisions in the 90ies today, many would (and many actually did so publically) say that they regret this decision.
On, complexity. Yes, software is incredibly complex. That's why you don't want to add even more complexity to it!
Easy, ARM is targetting the low power market. So they don't need to build the fastest chip. When you try to build the fastest chip, energy efficiency suffers a lot. Once ARM starts to compete with Intel on performance, their power usage will skyrocket.