The Virtualization Benchmarking Chaos

There are an incredible number of pitfalls in the world of server application benchmarking, and virtualization just makes the whole situation much worse. In this report, we want to measure how well the CPUs are coping with virtualization. That means we need to choose our applications carefully. If we use a benchmark that spends very little time in the hypervisor, we are mostly testing the integer processing power and not how the CPU copes with virtualization overhead. As we have pointed out before, a benchmark like SPECjbb does not tell you much, as it spends less than one percent of its time in the hypervisor.

How is virtualization different? CPU A that beats CPU B in native situations can still be beaten by the latter in virtualized scenarios. There are various reasons why CPU A can still lose, for example CPU A…

  1. Takes much more time for switching from the VM to hypervisor and vice versa.
  2. Does not support hardware assisted paging: memory management will cause a lot more hypervisor interventions.
  3. Has smaller TLBs; Hardware Assisted Paging (EPT, NPT/RVI) places much more pressure on the TLBs.
  4. Has less bandwidth; an application that needs only 20% of the maximum bandwidth will be bottlenecked if you run six VMs of the same application.
  5. Has smaller caches; the more VMs, the more pressure there will be on the caches.

To fully understand this, it helps a lot if you read our Hardware Virtualization: the nuts and bolts article. Indeed, some applications run with negligible performance impact inside a virtual machine while others are tangibly slower in a virtualized environment. To get a rough idea of whether or not your application belongs to the latter or former group, a relatively easy rule of thumb can be used: how much time does your application spend in user mode, and how much time does it need help from the kernel? The kernel performs three tasks for user applications:

  • System calls (File system, process creation, etc.)
  • Interrupts (Accessing the disks, NICs, etc.)
  • Memory management (i.e. allocating memory for buffers)

The more work your kernel has to perform for your application, the higher the chance that the hypervisor will need to work hard as well. If your application writes a small log after spending hours crunching numbers, it should be clear it's a typical (almost) "user mode only" application. The prime example of a "kernel intensive" application is an intensively used transactional database server that gets lots of requests from the network (interrupts, system calls), has to access the disks often (interrupts, system calls), and has buffers that grow over time (memory management).

However, a "user mode only" application can still lose a lot of performance in a virtualized environment in some situations:

  • Oversubscribing: you assign more CPUs to the virtual machines than physically available. (This is a very normal and common way to get more out of your virtualized server.)
  • Cache Contention: your application demands a lot of cache and the other virtualized applications do as well.

These kinds of performance losses are relatively easy to minimize. You could buy CPUs with larger caches, and assign (set affinity) certain cache/CPU hungry applications some of the physical cores. The other less intensive applications would share the CPU cores. In this article, we will focus on the more sensitive workloads out there that do quite a bit of I/O (and thus interrupts), need large memory buffers, and thus talk to the kernel a lot. This way we can really test the virtualization capabilities of the servers.

Index Independent Real-World Virtualization Benchmarking
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  • JohanAnandtech - Friday, May 22, 2009 - link

    We are definitely interesting in doing this, but of course we like to do this well. I'll update as soon as I can. Reply
  • pc007 - Thursday, May 21, 2009 - link

    I agree, CPU & RAM usage are not really bottlenecks in my experience. Processes hamering slow disk and making everything else slower is the main concern. Reply
  • SeanG - Friday, May 22, 2009 - link

    There are 300 million people in this country and you're surprised that some of them are ignorant/jerks/crazy? We're all supposed to be ashamed because not everyone from this country is mentally stable? It's insulting to people like me who care about this country to hear you talk about being ashamed over something that is a problem with humanity in general and not only in the US.
    Reply
  • lopri - Thursday, May 21, 2009 - link

    It is said to see such a fascist persona in this comment section of such a fascinating article. I feel ashamed as one residing in the U.S. Reply
  • JohanAnandtech - Friday, May 22, 2009 - link

    Don't be. Tshen must be the first US citizen that I have encountered that hates Belgians :-). All other US people I have met so far were very friendly. In fact, I am very much astonished how hospitable US people are. Sometimes we have only spoken over the phone or via e-mail with each other and the minute I arrive in the US, we are having a meal and chatting about IT. When you arrive in Silicon valley, one can only be amazed about the enormous energy and entrepreneurship this valley breathes. Reply
  • tshen83 - Friday, May 22, 2009 - link

    You are a slave, Johan, whether you realize it or not. The people in Silicon Valley are "nice to you" because they are in the process of negotiating a purchased piece of publication from you.

    You don't know anything about the Silicon Valley nor are you qualified to talk about it. If anything is true, Silicon Valley is in the toilet right now with bankruptcies everywhere. The state is broke, with Arnold Schwarzenegger begging for Federal bailouts. The last two big "entrepreneurships" coming out of Silicon Valley: Facebook and Twitter are both advertising scams without a viable business model.

    I don't hate Belgians. I do hate retards like you whether you come from Belgium or not.

    [BANNED]

    [FROM JARRED: We are proponents of free speech, but repeated name calling and insults with little to no factual information to back up claims will not be tolerated. There was worse, and I'm leaving this text so you can see how it started.]
    Reply
  • tshen83 - Thursday, May 21, 2009 - link

    As what I have expected, Johan, your sorry ass came up with a benchmark that invalidates VMmark.

    On page 9 "Nehalem vs Shanghai" http://it.anandtech.com/IT/showdoc.aspx?i=3567&...">http://it.anandtech.com/IT/showdoc.aspx?i=3567&...

    Where is the Nehalem vs Shanghai benchmarks? All I see is a chart pumping Opteron 8389.

    Let me dissect the situation for you [EDITED FOR VULGARITY].

    The 100% performance per watt advantage witnessed by the Nehalem servers was the result of 3 factors: triple channel DDR3 IMC, HyperThreading, and Turbo Boost. The fact that Opterons can no longer compete because they lack the raw bandwidth and the "fake Hyper Threaded" cores that performs like a real core.

    What would AMD do in this situation? Of course, invalidate an industrially accepted benchmark by substituting it with a "paid third party" benchmark that isn't available to the the public. I wonder what kind of "optimizations" were done?

    You know what killed the GPU market? HardOCP.com. That's right, they invalidated the importance of 3DMark by doing game by game FPS analysis. The problem with this approach is that the third party game developers really don't have the energy or resources to make sure that each GPU architecture is properly optimized for. As long as the games run about 30fps on both Nvidia and ATI GPUs, they are happy. What results from this lackluster effort is that there is no Frames per Second differentiation on the GPU vendors, causing prices to free fall and the idiots choosing an architecturally inferior ATI GPU that gave a similar FPS performance.

    Same methodology can be applied here. Since the Opterons lack raw memory bandwidth and core count visible to the OS, why not have a benchmark that isn't threaded well enough, and stress on high CPU utilization situations where memory bandwidth and core count matter less? That is what this new benchmark is doing, hiding Opteron architectural difficiencies.

    The reason why VMmark stresses high number of VMs is to guage the hardware acceleration of VM switching. Having lesser number of VMs doing high CPU workload helps the worse performer(Opteron) by hiding and masquerading the performance difficiency. Nobody runs 100 VMs on one physical machine, but the VMmark does show you a superior hardware implementation. Nobody really prevents AMD from optimizing their CPUs for VMmark.

    Let me be even more brutal with my assessment of your ethics, Johan. Why do you feel you are qualified to do what you do? The people who actually know about hardware are doing the CPU designs themselves in the United States, so the Americans would be the first to know about hardware. When the CPU samples are sent to Taiwan for motherboard design, the Asians would be the second batch of people dealing with hardware. By the time hardware news got to freaking Europe(Fudzilla, The Inq), the information usually was fudged up to the wazoos by Wall Street analysts. Consider yourself lucky that the SEC isn't probing you [EDITED FOR VULGARITY] because you reside in Belgium.

    So Johan, my suggestion for you personally, is that you should consider the morality of your publications. In today's day and age, every word you ever say is recorded for eternity. Thirty years from now, do you want people to call you a [EDITED FOR VULGARITY] for pumping an inferior architecture by fudging benchmark results? Of course, I personally run the same risks. What I can guarantee you is that by June of next year, Johan, you [EDITED FOR VULGARITY] would be pumping Via instead.
    Reply
  • JohanAnandtech - Friday, May 22, 2009 - link

    As long as you are not able to discuss technical matters without personal attacks, I won't waste much time on you. Leave the personal attacks out of your comments, and I'll address every concern you have.

    But for all other readers, I'll show how shallow your attacks are (but they probably figured that one out a long time ago).

    "Why not have a benchmark that isn't threaded well enough"
    Yes, Tshen. In your world, Oracle and MS SQL server have few threads. In the realworld however...

    "http://it.anandtech.com/IT/showdoc.aspx?i=3567&...">http://it.anandtech.com/IT/showdoc.aspx?i=3567&...
    Where is the Nehalem vs Shanghai benchmarks? All I see is a chart pumping Opteron 8389. "

    All other readers have seen a chart that tries to show how the benchmark reacts to cache size and memory bandwidth. All other readers understand that we only have one Nehalem Xeon, and that is a little hard to show empirically how for example different cache sizes influence the benchmark results.

    Lastly, as long as I publish AMD vs Intel comparisons, some people will call me and Anandtech biased. This article shows that Nehalem is between 50 to 80% faster in typical server apps.
    http://it.anandtech.com/IT/showdoc.aspx?i=3536&...">http://it.anandtech.com/IT/showdoc.aspx?i=3536&...
    For some people that meant that we were biased towards Intel. In your case, we are biased towards AMD if Intel does not win by a huge percentage. For the rest of the world, it just means that we like to make our benches as realworld as possible and we report what we find.

    Reply
  • whatthehey - Friday, May 22, 2009 - link

    At least we can be grateful he's kind enough to include his IQ in his user name. You know what's interesting? tshen83 isn't exactly a common user name, and he happens to troll elsewhere:
    http://www.google.com/search?hl=en&q=tshen83">http://www.google.com/search?hl=en&q=tshen83

    "Thirty years from now, do you want people to call you a fucking asshole...?" No need for you to wait 30 years, tshen; we'll be happy to call you a fucking asshole right now. As the saying goes: if the shoe fits....
    Reply
  • JarredWalton - Thursday, May 21, 2009 - link

    Frankly, you make me sad to be an American - as though just because someone is located in Belgium they are not qualified to do anything with hardware? Let's see, Belgium has higher average salaries than the US, so they surely have to be less qualified. And with you as a shining example we can certainly tell EVERYONE in the US is more qualified than in Europe.

    To whit, your assertion that HardOCP - or any other site - "killed the GPU market" is absurd in the extreme. The GPU market has seen declining prices because of competition between ATI and NVIDIA, and because the consumer isn't interested in spending $500 every 6-12 months on a new GPU. However, ATI and NVIDIA are hardly dying... though ATI as part of AMD is in a serious bind right now if things don't improve. Thankfully, AMD has helped the CPU market reach a similar point, but with Core i7 we're going back to the old way of things.

    Your linking to page nine of this article as though that's somehow proof of bias is even better. Johan shows that Nehalem isn't properly optimized for in ESX 3.5, while Shanghai doesn't have that problem. That's a potential 22% boost for AMD in that test, which we outright admit! Of course, as Johan then points out, there are a LOT of companies that aren't moving to ESX 4.0 for a while yet, so ESX 3.5 scores are more of a look at the current industry.

    Again, we know that VMmark does provide one measurement of virtualization performance. Is it a "catch-all"? No more so than the vApus Mark I tests. They both show different aspects of how a server/CPU can perform in a virtualized environment. We haven't even looked at stuff like Linux yet, and you can rest assured that the performance of various CPUs with that environment are all over the place (due to optimizations or lack of optimizations). Anyhow, I expect Nehalem will stretch its legs more in 2-tile and 3-tile testing, even with our supposedly biased test suite.

    Since you're so wise, let me ask you something: what would happen if a large benchmark became highly used as a standard measurement of performance in an industry where companies spend billions of dollars? Do you think, just maybe, that places like Intel, Dell, HP, Sun, etc. might do a bunch of extra optimizations targeted solely at improving those scores? No, that could never happen, especially not in the great USA where we alone are qualified to know how hardware works. Certainly NVIDIA and ATI never played any optimization games with 3DMark.

    In short, the responses to your comments should give you a good idea of how reasoned your postings are. Cool your jets and learn to show respect and thoughtful posting. I don't know why you're so worried about people showing Intel in the best light possible, but you post on (practically) every Intel or AMD article pumping the joys of Intel, and lambasting AMD.

    The fact is, many reviews of Nehalem show inflated benefits for the architecture relative to the real world. VMmark with ESX 4.0 definitely falls into that category - or do you think a range of 14.22@10 tiles with ESX 3.5 Update 4 to 24.24@17 tiles with ESX 4.0 is perfectly normal? I'm not sure anyone actually runs a real workload that mimics VMmark to the point where simply an update to ESX 4.0 would boost performance and virtualization potential by 70%.

    Does Intel make the currently better CPU? Of course they do. Does that mean AMD isn't worth a look? Hardly. There are numerous reasons an architecture might perform better/worse. VMmark - or any benchmark - will at best show one facet of performance, and thus what we really need are numerous tests showing how systems truly perform.
    Reply

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