Making Sense of the Intel Haswell Transactional Synchronization eXtensions
by Johan De Gelas on September 20, 2012 12:15 AM ESTHaswell's TSX
The new Transactional Synchronization eXtensions (TSX) extend the x86 ISA with two new interfaces: HLE and RTM.
Restricted Transactional Memory (RTM) uses Xbegin and Xend, allowing developers to mark the start and end of a critical section. The CPU will thread this piece of code as an atomic transaction. Xbegin also specifies a fall back path in case the transaction fails. Either everything goes well and the code runs without any lock, or the shared variable(s) that the thread is working on is overwritten. In that case, the code is aborted and the transaction has failed. The CPU will now execute the fall back path, which is most likely a piece of code that does coarse grained locking. RTM enabled software will only run on Haswell and is thus not backwards compatible, so it might take a while before this form of Hardware Transactional Memory is adopted.
The most interesting interface in the short term is Hardware Lock Elision or HLE. It first appeared in a paper by Ravi Rajwar and James Goodman in 2001. Ravi is now a CPU architect at Intel and presented TSX together with his colleague Martin Dixon TSX at IDF2012.
The idea is to remove the locks and let the CPU worry about consistency. Instead of assuming that a thread should always protect the shared data from other threads, you optimistically assume that the other threads will not overwrite the variables that the thread is working on (in the critical section). If another thread overwrites one of those shared variables anyway, the whole process will be aborted by the CPU, and the transaction will be re-executed but with a traditional lock.
If the lock removing or elision is successful, all threads can work in parallel. If not, you fall back to traditional locking. So the developer can use coarse grained locking (for example locking the entire shared structure) as a "fall back" solution, while Lock Elision can give the performance that software with a well tuned fine grained locking library would get.
According to Ravi and Martin, the beauty is that the developer of your locking libraries simply has to add a few HLE instructions without breaking backwards compatibility. The developer uses the new TSX enabled library and gets the benefits of TSX if his application is run on Haswell or a later Intel CPU.
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Paul Tarnowski - Thursday, September 20, 2012 - link
So it's good that the locking is finally being addressed at the CPU level, but that just means that even fewer developers will bother using fine-grain locking.Which isn't necessarily a bad thing, because they will be able to either spend the time and money on something else, or save, but it does mean that their software will be less efficient on older CPUs. Which in turn means that unless AMD comes up with a similar system that achieves the same effect, they will be even more behind. In the short term, of course, this just means that Haswell might improve any multi-threaded program that has been recompiled using the updated libraries.
The one thing that does make me hesitant is that it works on only one chacheline at a time, as well as all the abort conditions. That makes me think that the graph shown is a best-case scenario, and actual improvements in real world scenarios would be far less.
anubis44 - Thursday, September 20, 2012 - link
AMD and Intel have cross-licensing agreements for instructions each of them come up with. That's why Intel can build AMDx64 compatible CPUs (AMD designed the 64 bit extensions we're all using in both AMD and Intel CPUs) and AMD has SSE instructions. This is an automatic thing, so no further agreements need to be made. You can bet these instructions will show up in the next generation of AMD CPUs after Intel releases them.Brutalizer - Saturday, September 22, 2012 - link
Sun built Transactional Memory years ago with their SPARC ROCK cpu, to be used in the new SuperNova Solaris servers. But for various reasons ROCK was killed (I heard that it used too much wattage, among other problems).The good news is that ROCK research is not wasted, most of it is used in newer products. The new coming SPARC T5 to be released this year, has Transactional Memory. It will probably be the first sold cpu that offers TM.
HibyPrime1 - Thursday, September 20, 2012 - link
I don't really understand why this has to be implemented in hardware.Can't a developer just write the programming assuming the threads wont interfere with each other? After doing that, the program does a simple check to see if something went wrong, and if it did, it falls back to coarse grained locking.
I'm not sure how this is supposed to make it significantly easier for the developers. I'm sure that I'm missing something, but doesn't TSX just mean the developer doesn't have to write code that checks to see if something got broken? seems to me that part would be the easiest part of all this locking coding...
twotwotwo - Thursday, September 20, 2012 - link
The short answer is, an approach like that kind of *does* exist (search for optimistic concurrency control), but it still takes work to detect when things went wrong and be able to clean up.In Intel's bank example, you might need some kind of indexed concurrency-proof transaction history so the bank can know that when I gave $50 to Alice and $60 to Bob, both transactions used a $100 starting balance. And the code needs to know how to undo a transaction that collided with another. To complicate things, many live systems deal with larger transactions than just two-person money transfers like Intel's example. Optimistic control can still be a step up from spinlocks (or people would never use it) but it doesn't come for free.
Tuna-Fish - Thursday, September 20, 2012 - link
Transactional memory can, and has been, implemented in software. The typical examples are Clojure and Haskell. However, doing the tracking in software generally takes a lot of resources, especially because you have to deal with all kinds of race conditions. Remember that without some kind of hardware support for concurrency, every single write and read operation is independent, and something could go wrong at any point, including during the verify/restore phase.name99 - Friday, September 21, 2012 - link
This is not a technology to make parallel programming automatic or even easier. It is a technology to make ONE PART of parallel programming, namely the locking MORE EFFICIENT. That is all.This has the consequence that one can write an app using fewer, coarser grained locks, and have it perform as well as if you'd used finer grained locked, but again, that is all. In particular
- it doesn't do the locking for you, it doesn't tell you what needs to be protected by locks, it doesn't catch stupid usage of locks, etc etc
- it doesn't help with everything else related to parallel programming, from choosing appropriate data structures to choosing appropriate algorithms.
As for doing it in SW, well, yes, at the end of the day you can do EVERYTHING in software. But Intel is in the business of moving as much as possible of what is slow in software into faster hardware. That's why we have everything from branch prediction to AES instructions to QuickSync in modern CPUs.
Finally, it's foolish to obsess too much about implementation details, like how the L1 cache is used. EVERYTHING in HW is a tradeoff and, just like you can invent some pathological code that runs slower under branch prediction, you can invent pathological code that runs slower under this implementation of the locking mechanism. As always, Intel will look at how these extensions are used in practice, and how they fail, and will modify how they are implemented as a result. This is just common sense.
softdrinkviking - Monday, September 24, 2012 - link
hmm. so would you say that recent generation Intel CPUs are well utilized? All of the instructions and features not only make sense, but are well utilized by a majority of developers?epobirs - Thursday, September 27, 2012 - link
That depends on what you consider an acceptable time scale for widespread usage of a hardware feature. Everything has to start somewhere. Nobody today would bother to mention that their software takes advantage of MMX and its successors but at one time it carried some novelty value. The real benefits came when it was so common in the hardware and compiler support that it no longer merited mention.epobirs - Thursday, September 27, 2012 - link
For the traditional reason you create any dedicated function in hardware: performance. Intel is betting this is going to matter hugely to scaling up the value of multi-core processors.Such things have a long history. MMX was the result of examing many, many pieces of of software and looking for functions then done entirely in software on most systems that could be accelerated for a minimal investment in transistor real estate.
There was a period of a few years when a 3D accelerator board was a separate item from the video board. The 3Dfx Voodoo series worked this way for several generations until the company faltered in trying to transition to complete video solutions on a single board. In that time 3D had gone from something exclusively of interest to gamers and some other specialty apps to a thing expected of every system to some extent. It wasn't long before integrated graphics adapters had the kind of 3D performance that formerly lead people to make a costly separate purchase to obtain.
If something is worth putting in the hardware, it will reveal itself through what is done in the software. From there it is only a question of how many transistors does it take to embody and at what cost? If the numbers are right, into the hardware it goes.