Intel's Haswell Architecture Analyzed: Building a New PC and a New Intel
by Anand Lal Shimpi on October 5, 2012 2:45 AM ESTTSX
Johan did a great job explaining Haswell's Transactional Synchronization eXtensions (TSX), so I won't go into as much depth here. The basic premise is simple, although the implementation is quite complex.
It's easy to demand well threaded applications from software vendors, but actually implementing code that scales well across unlimited threads isn't easy. Parallelizing truly independent tasks is the low hanging fruit, but it's the tasks that all access the same data structure that can create problems. With multiple cores accessing the same data structure, running independent of one another, there's the risk of two different cores writing to the same part of the same structure. Only one set of data can be right, but dealing with this concurrent access problem can get hairy.
The simplest way to deal with it is simply to lock the entire data structure as soon as one core starts accessing it and only allow that one core write access until it's done. Other cores are given access to the data structure, but serially, not in parallel to avoid any data integrity issues.
This is by far the easiest way to deal with the problem of multiple threads accessing the same data structure, however it also prevents any performance scaling across multiple threads/cores. As focused as Intel is on increasing single threaded performance, a lot of die area goes wasted if applications don't scale well with more cores.
Software developers can instead choose to implement more fine grained locking of data structures, however doing so obviously increases the complexity of their code.
Haswell's TSX instructions allow the developer to shift much of the complexity of managing locks to the CPU. Using the new Hardware Lock Elision and its XAQUIRE/XRELEASE instructions, Haswell developers can mark a section of code for transactional execution. Haswell will then execute the code as if no hardware locks were in place and if it completes without issues the CPU will commit all writes to memory and enjoy the performance benefits. If two or more threads attempt to write to the same area in memory, the process is aborted and code re-executed traditionally with locks. The XAQUIRE/XRELEASE instructions decode to no-ops on earlier architectures so backwards compatibility isn't a problem.
Like most new instructions, it's going to take a while for Haswell's TSX to take off as we'll need to see significant adoption of Haswell platforms as well as developers embracing the new instructions. TSX does stand to show improvements in performance anywhere from client to server performance if implemented however, this is definitely one to watch for and be excited about.
Haswell also continues improvements in virtualization performance, including big decreases to guest/host transition times.
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jigglywiggly - Friday, October 5, 2012 - link
wish the onboard gpu was better =/woula been nice for a laptop
tipoo - Friday, October 5, 2012 - link
2x the HD4000 is pretty decent for integrated. I wonder if that's 2x with or without the eDRAM cache though.ElvenLemming - Friday, October 5, 2012 - link
It's been known for a while that Haswell was only going to have a moderate improvement in the iGPU and the next big overhaul would be coming with Broadwell.csroc - Friday, October 5, 2012 - link
This is impressive, it might convince me it's time for a new laptop. On the other hand I also need to build a new desktop workstation and Haswell so far hasn't impressed me in that space.mayankleoboy1 - Friday, October 5, 2012 - link
Is Intel sacrificing Desktop CPU performance to make an architecture that is geared to the mobile space ?csroc - Friday, October 5, 2012 - link
It feels that way to me. Mobile performance seems to be their big concern now, that and improving the GPU. Two things I generally can't be bothered to care about when I'm looking to build a new workstation. I suspect I'll build an Ivy Bridge system because I could use it now and see nothing worth getting excited about.dishayu - Friday, October 5, 2012 - link
I fully share your sentiment. TO be very crude, i don't mind at all, paying for power imporvements, because it will pay back for itself in the long term (by consuming less power AND needing lesser cooling). But i DO mind very much, paying for 40 EUs of GPU on my desktop build which i will not use even for a second. Me, you and many others do not care about on-die graphics and Intel should realize that.I don't know why intel can't offer us both GPU and GPU-less options, the way they did with motherboards back in the days? P965 had no graphics, G965 did. Pretty sure it's technologically not an issue.
DanNeely - Friday, October 5, 2012 - link
If it makes you feel any better; reports elsewhere are that GT3 will be mobile only, because desktops don't have the power/size constraints driving the need for premium IGPs.Intel's not IGP CPUs are the E series parts; unfortunately they've failed to execute on the enthusiast side in terms of price/launch date leaving them as mostly server parts.
There just aren't enough of us to justify Intel adding another die design for their mass market socket that doesn't have an IGP at all instead of just letting us turn it off and use the extra TDP headroom for more time at boost speeds.
Omoronovo - Friday, October 5, 2012 - link
I'm somewhat in disagreement with you both.Whilst I share a concern that Intel is no longer focusing on raw performance improvements in the purely desktop space, they are still delivering incremental updates to the architecture that will benefit all current software (even if only marginally). However, processor performance has been reaching more and more diminishing returns in recent years, namely that software is simply not able to take advantage of multiple cores and improved performance because of (primarily) locks and complexity in creating multi-threaded applications.
As such, Intel has been focusing on that area - to make it easier for software and software developers to take advantage of the performance that exists *now* rather than brute forcing the issue by simply delivering more raw performance (much of which will be wasted/remain idle due to current software constraints).
With this, Intel has been able to focus on keeping performance high whilst subsequently dropping power usage substantially - the fact the iGPU is oftentimes not being used in a desktop environment does not invalidate it's utility - QuickSync is a prime example of where the gpu can accelerate certain types of processing, and if more software takes advantage of this we should see even more gains in future.
For the last 6 years or so, Intel has shown that it knows what demands will be placed on future computing hardware, and they seem convinced that this is the way to go. We might not be there yet, but technologies like C++AMP, OpenCL and such make me hopeful that this will change in a few years.
cmrx64 - Friday, October 5, 2012 - link
I solved this problem by buying an Ivy Bridge Xeon (specifically, an E3-1230v2). No GPU, lower power consumption than the equivalent i5/i7, has hyperthreading, performs really good, and a lot cheaper than an i7.If you don't care about the GPU, look to the Xeon line.