On 2/21/2013 11:41 AM, David Brown wrote: ... > HT may not be of help in a pure file server setup, but in many other > server applications such as web servers (and IMAP, as you mentioned), HT > is a huge benefit. HT benefits workloads with more heavy active processes than cores, which causes functional unit/pipeline contention. If your workload has no resource contention, then HT is of no benefit. In these cases having it enabled can actually decrease performance, yes, even with Intel's recent implementation, and it can cause other issues as I described earlier, such as simple system administration headaches. > It is not coincidence that the big server cpu > architectures (MIPS, Power, SPARC) all use 2 or even 4 time SMT. Quoting Wikipedia again...but making incorrect assumptions about what it says. MIPS CPUs haven't been used in a "big server" for a decade, the last machine being the Origin 3900, the CPU being the single core R16000A, which had no SMT. HPC workloads don't benefit from it. The short lived SiCortex machines were obviously "big" with up to 5832 cores using 6-way SMP SOCs. These cores did not have SMT either. And yes, the SiCortex section of the MIPS page is my edit, as well as some of the SGI related edits. (Hated to see SiCortex fold as their machines not only offered performance and unique features, but had a cool aesthetic missing in the supercomputer space since the glory days of Cray) Imagination Technologies today offers two MIPS IP cores with SMT, of over hundreds of cores/designs in their portfolio. Both are used in embedded applications only. Both are 32 bit CPUs. And both hit the market within the past 2 years. I.e. SMT is very new for MIPS chips. WRT Power and SPARC these are targeted at consolidation workloads which can benefit from SMT. Note that on the Power CPUs destined for HPC platforms SMT is typically disabled. So again, whether HT/SMT is of benefit depends entirely on the workload. In Adam's iSCSI server case, it decidedly does not. > I have no numbers of my own to back this up - but I would certainly not > consider disabling HT on a server without very concrete reasoning. I've demonstrated the reasoning, twice now. People fear what they don't understand. You fear shutting off HT because you don't yet have a complete understanding of how it actually works, and when it actually helps. > (I too was a great fan of AMD, and used them almost exclusively until > the Core 2 architecture from Intel. And while I am glad that Intel have > made very nice chips in recent years, I think it is a shame that they > did so using ideas copied directly from AMD - and AMD can no longer > seriously compete.) Absolute performance was no longer an issue long before Intel introduced the Core architecture. The vast majority of cycles on all systems were executing the idle instruction, still are. And there were/are few server applications deployed that performed significantly better with Xeon than Opteron. Thus I continued with AMD for a few reasons. The IO infrastructure was superior, and it wasn't until QuickPath that Intel caught up here. And AMD still offers a better price/performance ratio. -- Stan -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
