On Apr 14, 2013 1:42 AM, "Michael Mol" <mike...@gmail.com> wrote: > > On 04/13/2013 01:45 PM, Pandu Poluan wrote: > > > [snip] > > > Three, AMD has no concept of Hyperthreading. > > Correct. > > > Just match -j to the number of cores your CPU provides, and that's > > it. > > Well, YMMV. You can spend a lot of time adjusting -j on a per-system > basis to account for things like I/O. Right now, I'm in the "-j > $(cores*1.5) -l $(cores)" camp. > > > > > As I wrote, an AMD Quad Core provides actual 4 cores. > > Correct. > > > An "Intel Quad Core with Hyperthreading" actually provides only 2 > > physical cores, but then it performs some internal trickery so the OS > > sees a total of 4 cores. > > Incorrect. Intel Quad Core with Hyperthreading means there are four > physical cores, and there is hyperthreading enabled. This results in the > OS seeing eight logical cores. There is sufficient information available > via ACPI (or is it DMI?) that the kernel knows which virtual cores are > part of which physical cores, which physical cores are part of which CPU > packages, and how everything is connected together. >
Ah yes, thank you for the correction. I misstated there, my bad. What I meant was: given 4 physical AMD cores (but only 2 FPUs, courtesy of AMD's Bulldozer/Piledriver arch) vs 4 virtual Intel cores (2 cores split into 4 by Hyperthreading), I undoubtedly prefer 4 physical ones. (Of course if the Intel CPU has 4 pphysical cores, it should be compared with an 8-core AMD CPU). I had some lively discussion on AMD vs Intel *for virtualization* in the Gentoo Community on Google+, which referenced a thread on ServerFault. The conclusion was: Intel CPUs (provided they support VT-x) can run baremetal virtualization as well as AMD, in the majority of cases. It's the minority of cases -- edge cases -- that I'm concerned with. And, lacking the money to actually buy 2 complete systems to perform comparison, I'll take the safe route anytime. Yes, Intel's top-of-the-line processors might be faster than AMD's, but the latter is cheaper, and exhibited a much more 'stable' performance (i.e., no edge cases to bite me later down the road). That said, I read somewhere about the 'misimplementation' of some hypercalls in Intel CPUs... in which some hypercall exceptions are mistakenly handled by the Ring 0 hypervisor instead of the Ring 1 guest OS, thus enabling someone to 'break out' of the VM's space. This misimplementation is exploitable on KVM and Xen (the latter, my preferred baremetal virtualization). > > > > I much prefer having 4 actual cores than 4 virtual cores (only 2 > > actual cores); less chance of things messing up royally if I hit some > > edge cases where Hyperthreading falls flat on its face. > > Whatever works. I'll note that AMD's piledriver core does something very > complementary to hyperthreading. Where HT uses some circuitry to avoid > context switching when changing whether a core is handling one thread vs > another thread, Piledriver has a small number of physical front-end > cores dispatching to a larger number of backend pipelines. It's a very > curious architecture, and I look forward to seeing how it plays out. HT > and Piledriver are conceptually very similar when you look at them in > the right way...Piledriver might be seen as a more general approach to > what HT does. > True. The main complexity is when an instruction requires access to the FPU, since there's only one FPU per two GP cores. This will somewhat impact applications that uses the FPU heavily... except if they can switch to OpenCL and leverage the embedded Radeon on AMD's so-called "APUs". > Personally, I've enjoyed both Intel and AMD processors. Last I assembled > a system, Intel's midrange offered more bang for the buck than AMD, but > Intel's midrange part was also much more expensive. OTOH, AMD systems > could be upgraded for piece by piece for much, much, much longer, > whereas Intel systems tended to require replacing many more parts at the > same time. > > That was about five years ago, though...I don't know exactly where > things sit today. I'd start with the cpubenchmarking.net CPU value > listing, and find the best-value part that has the performance degree > I'm looking for. > > http://cpubenchmark.net/cpu_value_available.html > > I might also cross-reference that page with this one: > > http://cpubenchmark.net/mid_range_cpus.html > True. My desktop computer died on me about 6 months ago. It was 4.5 years old at the moment of death. It had served me very well. That said, my brother had just purchased an AMD system (store-assembled) with an FX-8350, and he said that it's faster than anything he's ever used before, and he's used many high-end systems in his job (he's a Petroleum Geologist, his line of work involves analyzing a HUGE amount of data to find out the 'oil potential' of an area, to give his company a ballpark figure on how much to bid for the exploitation rights to the area). > If buying an Intel part, I'd be very, very careful to make sure that it > supported all the features I want. I've been bit by that on this > laptop...I had no idea it wouldn't have VT-x. > I almost got bitten by this: My previous employer had wanted to purchase a lot of new workstations. Luckily before the PO came out, I double checked the specs. When I found out that the particular model we were offered does not support VT-x, I immediately halted procurement and ask the supplier to resubmit a quotation, but this time specifying that VT-x support is required. (It's a long story, but we do use VirtualBox extensively. And running VirtualBox without VT-x is an exercise in misery). But that was three years ago; perhaps the latest Intel processors for the desktop today all support VT-x, I don't know... Rgds, --
