real.world.technologies

The first generation of virtualization has really focused on decreasing the hardware footprint required by IT staff.  However, the lasting value from virtualization will not be savings from hardware, but the savings from manageability and other higher level features.  Virtualization is already pretty good for a single user – both VMware and Hyper-V are very easy to use for an individual and can probably address most real needs there.  However, there are tremendous opportunities to save real time and money by making it easier to manage groups of VMs across an entire enterprise.

This has certainly been the talk of the industry for a while, focusing on management tools above and beyond the VMM itself.  To a large part, this a response to open source VMMs (like Xen or KVM) and free VMMs (like Hyper-V). If the VMM is free, then vendors must somehow make money elsewhere in the
stack – for Microsoft and Linux vendors, it’s obvious how this works, but VMWare is clearly driving to differentiate in other ways.  Products from VMware are looking at high-availability, disaster recovery and backup as ways to add value by bringing formerly high-end server features to standard x86.

To some extent this creates a bit of tension though.  One of the reasons why virtualization is necessary is that modern operating systems have grown incredibly large and perform a lot of complex and non-essential work inside the kernel.  This additional complexity can result in instability and security vulnerabilities.  For VMMs to really add value, they need to stay lean (like VMware ESXi – which can be as small as 32MB) and avoid the temptation to throw in every feature and the kitchen sink.  That just will result in another bloated software layer – and then someone will come along and try to put yet another layer of indirection beneath the VMM, or perhaps remove the VMM entirely! Read More »

The old phrase, ‘what cannot be measured cannot be improved’ is a favorite amongst those in the computer industry – and it contains more than a kernel of truth.  That logic has been behind the establishment of a variety of industry organizations, such as SPEC and TPC, which seek to establish standard benchmarks for various workloads.

Virtualization is certainly one of the trendiest technologies and ripe for measurement.  The latest announcements of CPUs from Intel and AMD have all explicitly mentioned and showcased improvements in virtualization performance, along with a flurry of feature names like VPID, Pacifica, Nested Page Tables and Extended Page Tables.  Most of these I’ve described in prior article, but to summarize, a lot of these features are about shifting the burden of virtualization from software (that is, the VMM) onto the hardware by making some hardware operations more complete.  Take for example, THE VPID in Nehalem (incidentally AMD has had an equivalent feature for a while), which reduces transition times between VMs by about a third (compared to the prior generation Penryn).  While it’s great that VM transitions are faster, it’s really hard for an average user to understand what that really means in terms of virtualization performance. Read More »

In our last post, we discussed some of the ways that virtualization is used both by individual consumers, and by enterprises. Recapping, the most prominent use is server consolidation, but there are plenty of other ways that virtualization can prove beneficial.

One particularly interesting topic is how the various uses of virtualization have influenced and impacted the design of modern servers. For the most part, the goal of virtualization and server consolidation is to minimize the power, heat and space foot print consumed by servers. As it turns out, this generally means that workloads are consolidated onto larger four socket servers which tend to maximize the capacity for guest operating systems, rather than the smaller single or dual socket servers. Read More »

Virtualization emerged as an area of interest in IT earlier this decade and is now unarguably one of the hottest trends in IT at the moment.  Prior posts explored the technical details of how processor virtualization and I/O virtualization function.  But what are the uses of virtualization that are driving all this attention and interest from IT, vendors and the press? 

The most important use today for virtualization is server consolidation.  During the late 1990’s and early part of this decade, small rack-mounted x86 servers proliferated rapidly.  In part, this was due to the low cost of x86 servers using Linux or Windows, compared to their UNIX predecessors.  But this was also driven by the flakiness of the software stacks on those same servers.  The software and operating systems were not nearly as robust as they are today and it was problematic enough that a lot of IT staff considered isolating applications from one another a best practice – even if it lead to low utilization.  There were plenty of peculiar bugs that resulted from a combination of corner cases in the OS and different software packages and using a server dedicated to a specific application avoids that problem entirely.

The proliferation of small, underutilized servers was slowed down by the dot-com bust in 2001, but also by constraints on data center space, cooling and power.  Cooling and power were particularly sensitive, because while Moore’s Law means that you can get more computing power in a chip over time – it also means that the power density (whether it is W/cm² or W/cm³) increases substantially.  So a data center designed for 1995 servers is wholly inadequate for those in 2003.  Worse yet, data centers are incredibly expensive to build or redesign – for many companies there is a bit of pain as your data center approaches 80% capacity and the cooling or power system gets stretched.  But typically, building another data center is far too painful and expensive to be an acceptable solution. Read More »

In my earlier post, I discussed some of the main concepts, challenges and techniques that are essential for virtualizing a CPU or instruction set.  To a large degree, these concepts and techniques are not new – they are really just variations on a theme that has been around for a long time in computer science.  Virtualization is fundamentally about preserving the appearance of isolating resources, while actually sharing the resources harmoniously and efficiently.  Perhaps the most obvious example was virtual memory – I can still remember being excited about virtual memory when I upgraded to System 7 on an Apple Quadra!  Tricks with virtual memory would let you run applications without buying new RAM (although the loading performance was often horrible as a result…at least it ran).

 

One of the most difficult and complicated areas of a modern PC is the I/O architecture.  The I/O architecture governs how the processor and memory talk to the devices that make a computer interesting – keyboards, mice, network cards, GPUs and hard drives.  It’s essential to remember that I/O is really a first class priority of a computer, because as a CPU architect, I/O moves at a glacial speed. 

 

Modern CPUs operate at around 3GHz, so a single cycle is only 0.33ns.  In comparison, reading data from a disk takes around 5ms – or 15 million cycles!  The engineering maxim of ‘make the common case fast, and the uncommon case correct’ is sometimes erroneously simplified into ‘ignore the uncommon case’ – and when I/O only happens every 15 million cycles, it’s pretty uncommon. Read More »

Virtualization is one of the oldest and most fundamental concepts in computer science and IT; if you look carefully, it is almost everywhere – desktops, notebooks, servers, SANs, cell phones, even GPUs!

Virtualization entails replacing a dedicated computing or storage resource (e.g. CPUs) in a system with an abstraction of that resource (e.g. a virtual CPU) that appears to be dedicated, but is actually shared between several uses and managed to achieve higher efficiency. The appearance of being dedicated (i.e. isolation) is key because dedicated resources are very easy and convenient to use; there’s no concern that someone else might be hogging that resource.

This sounds deceptively simple, but the reality is quite complex. Many times, different elements of a system will conflict with one another – those conflicts must be seamlessly arbitrated. Even if this is done correctly, sometimes the performance overhead is so bad that the result is almost unworkable.

Virtual memory is a particularly relevant example. To virtualize memory, there are two key issues – resources and how to access them. Typically virtual memory is broken down into two sections – kernel space, which is a private region of memory for ring 0 (i.e. the most privileged) software – that is the OS kernel, IO devices, drivers and a few other odds and ends. Kernel space cannot be paged out to disk, so there are no page faults in the kernel. This restriction is one reason why most kernels are small. User space is made available to ring 3 software (aka applications, the least privileged) and can be paged out. The OS provides and a separate user space for each application, which is how they are protected from each other – so that bad data in one application will not cause problems elsewhere, and that applications cannot maliciously or accidentally access each other’s data. Read More »