The Bulldozer Aftermath: Delving Even Deeper
by Johan De Gelas on May 30, 2012 1:15 AM ESTIt has been months since AMD's Bulldozer architecture surprised the hardware enthusiast community with performance all over the place. The opinions vary wildly from “server benchmarks are here, and they're a catastrophe” to “Best Server Processor of 2011”. The least you can say is that the idiosyncrasies of AMD's latest CPU architecture have stirred up a lot of dust.
Now that the dust has settled, the Bulldozer chips now account for more than half of Opteron shipments and revenues. Since AMD's Financial Analyst Day (February 2, 2012), we have new code names: the improved Bulldozer architecture "Piledriver" will power the "Abu Dhabi" chip, a replacement for the current top server chip "Interlagos". AMD is clearly committed to the new "Bulldozer" direction: fitting as many cores as possible into a certain power envelope to improve thread throughput, while trying to "hold the line" on single-threaded performance.
In theory, the new 16-core Interlagos should have offered somewhere around a 33% boost in most highly-threaded applications. The reality is unfortunately not that rosy: in many highly-threaded server applications such as OLAP databases and virtualization, the new Opteron 6200 fails to impress and is only a few percent faster than it's older brother the 12-core Magny-Cours. There are even times where the older Opteron is faster.
Some, including sources inside AMD, have blamed Global Foundries for not delivering higher clocked SKUs. Sure, the clock speed targets for Interlagos were probably closer to 3GHz instead of 2.3GHz. But that does not explain why the extra integer cores do not deliver. We were promised up to 50% higher performance thanks to the 33% extra cores, but we got 20% at the most.
The combination of low single-threaded performance, the failure to really outperform the previous generation in highly-threaded applications, the relatively high power consumption at full load, and the fact that the CPU is designed for high clock speeds gives a lot of people a certain sense of Déjà vu: is this AMD's version of the Pentum 4 ?
One of our readers, "Iketh", spoke up and voiced the opinion of many of our readers:
" Unfortunately, the thought still in the back of my mind while reading was why did AMD reinvent the Pentium 4? I just don't get it."
Another reader nicknamed "Clagmaster" commented:
"A core this complex in my opinion has not been optimized to its fullest potential. Expect better performance when AMD introduces later steppings of this core with regard to power consumption and higher clock frequencies."
Although there have already been quite a few attempts to understand what Bulldozer is all about, we cannot help but not feel that many questions are still unanswered. Since this architecture is the foundation of AMD's server, workstation, and notebook future (Trinity is based on the improved Bulldozer core with the codename "Piledriver"), it is interesting enough to dig a little deeper. Did AMD take a wrong turn with this architecture? And if not, can the first implementation "Bulldozer" be fixed relatively easily?
We decided to delve deeper into the SAP and SPEC CPU2006 results, as well as profiling our own benchmarks. Using the profiling data and correlating it with what we know about AMD's Bulldozer and Intel's Sandy Bridge, we attempt to solve the puzzle.
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Schmide - Wednesday, May 30, 2012 - link
I do remember from some analysis that the L2 cache reads were as slow as main memory. That's great if you hit a L2 cache, but it's not going to buy you anything if it's that slow.SocketF - Wednesday, May 30, 2012 - link
Impossible, you probably mix some things up, maybe latency and bandwidth?Schmide - Wednesday, May 30, 2012 - link
Yup. It was late at night, I was thinking writes. the L1 write through basically makes L1 writes the same as L2 writes.Homeles - Wednesday, May 30, 2012 - link
Not even close. L2 is about 10 times faster than main memory.http://www.anandtech.com/show/4955/the-bulldozer-r...
jcollake - Wednesday, May 30, 2012 - link
Through research here at Bitsum on the AMD Bulldozer platform (specifically the 9150), I found a couple things of interest.First, disabling CPU core parking seems to make a big difference in performance. I believe that by default the CPU core parking is just too aggressive. I wrote a tool to let you enable or disable CPU parking in *real time* without a reboot, so you can test this yourself. It is called ParkControl, http://bitsum.com/about_cpu_core_parking.php . For *me*, it seemed to make a night and day difference.
Second, I am working on a neat little benchmarking tool called ThreadRacer, currently only in alpha prototype. It allows you to really see the effects of these paired cores, and how much it matters that the scheduler is properly aware of them. Take this 1 second or so sample, as seen in the screenshot here (downloads available, but it is an early prototype that I'll quickly be finishing up): http://bitsum.com/forum/index.php/topic,1434.0.htm...
The scheduler update that Microsoft issued of course treats these paired cores as it would a hyper-threaded core. Indeed, the concept is very similar, except perhaps to avoid patents, AMD took the 'share a little' instead of 'share a lot' approach when it comes to shared computational resources. This was the proper way to *quickly* address the issue, but I believe the scheduler is still suboptimal on these processors (likely to be resolved in Windows 8 or a later update to Windows 7/Vista).
For Bulldozer, as you know, they are two real processors, but because they have shared dependencies, the performance can really be drained if the other processor in the 'pair' is busy. You can see the effects from ThreadRacer, the core without its pair busy quickly out-paced the paired cores that were both busy.
jcollake - Wednesday, May 30, 2012 - link
I should have also mentioned that ThreadRacer also allows you to see how a single CPU consuming thread gets swapped around to different cores (the multi-core thread in the utility). This is its other use. The less the thread gets swapped from core to core, the greater the performance will be. It is interesting to compare and contrast the behavior of the scheduler. I fully believe that most the problems with Bulldozer are due to the Windows scheduler, something that could be tested by using linux and replacing the scheduler with a custom one, or an off the shelf alternative that may behave substantially differently than the Windows scheduler.SocketF - Wednesday, May 30, 2012 - link
Some people running BOINC programs have reported that Windows-applications run faster when they use a Linux and WINE or a VM.The Win-scheduler especially hurts AMD chips, because of the huge exclusive caches. If a thread on an intel CPU is switched to another core, it can load the warmed up L2 portion from the L2 inclusive L3.
I did some google-search and it seems that under Linux, each core has its own run-queue, whereas on Windows, there is only one run queue for all cores.
But i didn't delve into it deeply, there are so many different schedulers for Linux, seems to be a complex issue ;-)
Btw. your link to download is off limits for non-members of your discussion board:
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Maybe you can upload it somewhere else?
jcollake - Saturday, September 1, 2012 - link
Sorry for the late reply. First, the forum permissions were fixed. Second, the utility (still in early stages) is included in Process Lasso *and* available here: http://bitsum.com/threadracer.phpeoerl - Wednesday, May 30, 2012 - link
Very interesting article, together with the hardware.fr report there's a lot of information. One question though, if you read commentaries : you didn't speak much about the influence of compilers. This proved to change a lot of things on Linux (see phoronix extensive tests on both ivy bridge and bulldozer depending on compiler used and compiler options, for examplehttp://www.phoronix.com/scan.php?page=article&...
http://www.phoronix.com/scan.php?page=article&...
Benchmark results really change a lot with bulldozer, much more than with ivy or sandy bridge. Do you think AMD lost being oversensitive to compiler optimisations, due to a very original architecture ?
JohanAnandtech - Thursday, May 31, 2012 - link
I deliberately avoided the compiler issues as this would make the article too convoluted. But notice that what we found is not influenced by compiler choice: we find the same indications in SAP and SQL server (compiled by "conservative" compilers and compiler settings) as in CPU CPU 2006, which uses the best optimized settings and compiler as possible.