HPX - High Performance ParalleX


Governing Principles applied while Developing HPX

As it turn out, we do not have to start from scratch. Not everything has to be invented and designed anew. Many of the ideas needed to combat the 4 horsemen have already been had, often more than 30 years ago. All it takes is to gather them into a coherent approach. So please let me highlight some of the derived principles we think to be crucial for defeating SLOW. Some of those are focused on high-performance computing, others are more general.

Focus on Latency Hiding instead of Latency Avoidance

It is impossible to design a system exposing zero latencies. In an effort to come as close as possible to this goal many optimizations are mainly targeted towards minimizing latencies. Examples for this can be seen everywhere, for instance low latency network technologies like InfiniBand, caching memory hierarchies in all modern processors, the constant optimization of existing MPI implementations to reduce related latencies, or the data transfer latencies intrinsic to the way we use GPGPUs today. It is important to note, that existing latencies are often tightly related to some resource having to wait for the operation to be completed. At the same time it would be perfectly fine to do some other, unrelated work in the meantime, allowing to hide the latencies by filling the idle-time with useful work. Modern system already employ similar techniques (pipelined instruction execution in the processor cores, asynchronous input/output operations, and many more). What we propose is to go beyond anything we know today and to make latency hiding an intrinsic concept of the operation of the whole system stack.

Embrace Fine-grained Parallelism instead of Heavyweight Threads

If we plan to hide latencies even for very short operations, such as fetching the contents of a memory cell from main memory (if it is not already cached), we need to have very lightweight threads with extremely short context switching times, optimally executable within one cycle. Granted, for mainstream architectures this is not possible today (even if we already have special machines supporting this mode of operation, such as the Cray XMT). For conventional systems however, the smaller the overhead of a context switch and the finer the granularity of the threading system, the better will be the overall system utilization and its efficiency. For today's architectures we already see a flurry of libraries providing exactly this type of functionality: non-preemptive, task-queue based parallelization solutions, such as Intel Threading Building Blocks (TBB), Microsoft Parallel Patterns Library (PPL), Cilk++, and many others. The possibility to suspend a current task if some preconditions for its execution are not met (such as waiting for I/O or the result of a different task), seamlessly switching to any other task which can continue, and to reschedule the initial task after the required result has been calculated, which makes the implementation of latency hiding almost trivial.

Rediscover Constrained Based Synchronization to replace Global Barriers

The code we write today is riddled with implicit (and explicit) global barriers. When I say global barrier I mean the synchronization of the control flow between several (very often all) threads (when using OpenMP) or processes (MPI). For instance, an implicit global barrier is inserted after each loop parallelized using OpenMP as the system synchronizes the threads used to execute the different iterations in parallel. In MPI each of the communication steps imposes an explicit barrier onto the execution flow as (often all) nodes have be synchronized. Each of those barriers acts as an eye of the needle the overall execution is forced to be squeezed through. Even minimal fluctuations in the execution times of the parallel threads (jobs) causes them to wait. Additionally it is often only one of the threads executing doing the actual reduce operation, which further impedes parallelism. A closer analysis of a couple of key algorithms used in science applications reveals that these global barriers are not always necessary. In many cases it is sufficient to synchronize a small subset of the threads. Any operation should proceed whenever the preconditions for its execution are met, and only those. Usually there is no need to wait for iterations of a loop to finish before you could continue calculating other things, all you need is to have those iterations done which were producing the required results for a particular next operation. Good bye global barriers, hello constraint based synchronization! People have been trying to build this type of computing (and even computers) already back in the 1970's. The theory behind what they did is based on ideas around static and dynamic dataflow. There are certain attempts today to get back to those ideas and to incorporate them with modern architectures. For instance, a lot of work is being done in the area of constructing dataflow oriented execution trees. Our results show that employing dataflow techniques in combination with the other ideas, as outlined herein, considerabley improves scalability for many problems.

Adaptive Locality Control instead of Static Data Distribution

While this principle seems to be a given for single desktop or laptop computers (the operating system is your friend), it is everything but ubiquitous on modern supercomputers, which are usually built from a large number of separate nodes (i.e. Beowulf clusters), tightly interconnected by a high bandwidth, low latency network. Today's prevalent programming model for those is MPI which does not directly help with proper data distribution, leaving it to the programmer to decompose the data to all of the nodes the application is running on. There are a couple of specialized languages and programming environments based on PGAS (Partitioned Global Address Space) designed to overcome this limitation, such as Chapel, X10, UPC, or Fortress. However all systems based on PGAS rely on static data distribution. This works fine as long as such a static data distribution does not result in inhomogeneous workload distributions or other resource utilization imbalances. In a distributed system these imbalances can be mitigated by migrating part of the application data to different localities (nodes). The only framework supporting (limited) migration today is Charm++. The first attempts towards solving related problem go back decades as well, a good example is the Linda coordination language. Nevertheless, none of the other mentioned systems support data migration today, which forces the users to either rely on static data distribution and live with the related performance hits or to implement everything themselves, which is very tedious and difficult. We believe that the only viable way to flexibly support dynamic and adaptive locality control is to provide a global, uniform address space to the applications, even on distributed systems.

Prefer Moving Work to the Data over Moving Data to the Work

For best performance it seems obvious to minimize the amount of bytes transferred from one part of the system to another. This is true on all levels. At the lowest level we try to take advantage of processor memory caches, thus minimizing memory latencies. Similarly, we try to amortize the data transfer time to and from GPGPUs as much as possible. At high levels we try to minimize data transfer between different nodes of a cluster or between different virtual machines on the cloud. Our experience (well, it's almost common wisdom) show that the amount of bytes necessary to encode a certain operation is very often much smaller than the amount of bytes encoding the data the operation is performed upon. Nevertheless we still often transfer the data to a particular place where we execute the operation just to bring the data back to where it came from afterwards. As an example let me look at the way we usually write our applications for clusters using MPI. This programming model is all about data transfer between nodes. MPI is the prevalent programming model for clusters, it is fairly straightforward to understand and to use. Therefore, we often write the applications in a way accommodating this model, centered around data transfer. These applications usually work well for smaller problem sizes and for regular data structures. The larger the amount of data we have to churn and the more irregular the problem domain becomes, the worse are the overall machine utilization and the (strong) scaling characteristics. While it is not impossible to implement more dynamic, data driven, and asynchronous applications using MPI, it is overly difficult to so. At the same time, if we look at applications preferring to execute the code close the locality where the data was placed, i.e. utilizing active messages (for instance based on Charm++), we see better asynchrony, simpler application codes, and improved scaling.

Favor Message Driven Computation over Message Passing

Today's prevalently used programming model on parallel (multi-node) systems is MPI. It is based on message passing (as the name implies), which means that the receiver has to be aware of a message about to come in. Both codes, the sender and the receiver, have to synchronize in order to perform the communication step. Even the newer, asynchronous interfaces require to explicitly code the algorithms around the required communication scheme. As a result, any more than trivial MPI application spends a considerable amount of time waiting for incoming messages, thus causing starvation and latencies to impede full resource utilization. The more complex and more dynamic the data structures and algorithms become, the larger are the adverse effects. The community has discovered message-driven and (data-driven) methods of implementing algorithms a long time ago, and systems such as Charm++ already have integrated active messages demonstrating the validity of the concept. Message driven computation allows to send messages without that the receiver has to actively wait for them. Any incoming message is handled asynchronously and triggers the encoded action by passing along arguments and - possibly - continuations. HPX combines this scheme with work queue based scheduling as described above, which allows to almost completely overlap any communication with useful work, reducing latencies to a minimum.