Concept# FIFO (computing and electronics)

Résumé

In computing and in systems theory, first in, first out (the first in is the first out), acronymized as FIFO, is a method for organizing the manipulation of a data structure (often, specifically a data buffer) where the oldest (first) entry, or "head" of the queue, is processed first.
Such processing is analogous to servicing people in a queue area on a first-come, first-served (FCFS) basis, i.e. in the same sequence in which they arrive at the queue's tail.
FCFS is also the jargon term for the FIFO operating system scheduling algorithm, which gives every process central processing unit (CPU) time in the order in which it is demanded. FIFO's opposite is LIFO, last-in-first-out, where the youngest entry or "top of the stack" is processed first. A priority queue is neither FIFO or LIFO but may adopt similar behaviour temporarily or by default. Queueing theory encompasses these methods for processing data structures, as well as interactions between strict-FIFO queues.
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vignette|Ici Agner Krarup Erlang, ingénieur et mathématicien Danois ayant travaillé sur la théorie des files d'attente.
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For time-sensitive networks, as in the context of IEEE TSN and IETF Detnet, cyclic dependencies are associated with certain fundamental properties such as improving availability and decreasing reconfiguration effort. Nevertheless, the existence of cyclic dependencies can cause very large latency bounds or even global instability, thus making the proof of the timing predictability of such networks a much more challenging issue. Cyclic dependencies can be removed by reshaping flows inside the network, by means of regulators. We consider FIFO-per-class networks with two types of regulators: per-flow regulators and interleaved regulators (the latter reshape entire flow aggregates). Such regulators come with a hardware cost that is less for an interleaved regulator than for a per-flow regulator; both can affect the latency bounds in different ways. We analyze the benefits of both types of regulators in partial and full deployments in terms of latency. First, we propose Low-Cost Acyclic Network (LCAN), a new algorithm for finding the optimum number of regulators for breaking all cyclic dependencies. Then, we provide another algorithm, Fixed-Point Total Flow Analysis (FP-TFA), for computing end-to-end delay bounds for general topologies, i.e., with and without cyclic dependencies. An extensive analysis of these proposed algorithms was conducted on generic grid topologies. For these test networks, we find that FP-TFA computes small latency bounds; but, at a medium to high utilization, the benefit of regulators becomes apparent. At high utilization or for high line transmission-rates, a small number of per-flow regulators has an effect on the latency bound larger than a small number of interleaved regulators. Moreover, interleaved regulators need to be placed everywhere in the network to provide noticeable improvements. We validate the applicability of our approaches on a realistic industrial time-sensitive network.

Jean-Yves Le Boudec, Ludovic Bernard Gérard Thomas

For time-sensitive networks, as in the context of IEEE TSN and IETF Detnet, cyclic dependencies are associated with certain fundamental properties such as improving availability and decreasing reconfiguration effort. Nevertheless, the existence of cyclic dependencies can cause very large latency bounds or even global instability, thus making the proof of the timing predictability of such networks a much more challenging issue. Cyclic dependencies can be removed by reshaping flows inside the network, by means of regulators. We consider FIFO-per-class networks with two types of regulators: per-flow regulators and interleaved regulators (the latter reshape entire flow aggregates). Such regulators come with a hardware cost that is less for an interleaved regulator than for a per-flow regulator; both can affect the latency bounds in different ways. We analyze the benefits of both types of regulators in partial and full deployments in terms of latency. First, we propose Low-Cost Acyclic Network (LCAN), a new algorithm for finding the optimum number of regulators for breaking all cyclic dependencies. Then, we provide another algorithm, Fixed-Point Total Flow Analysis (FP-TFA), for computing end-to-end delay bounds for general topologies, i.e., with and without cyclic dependencies. An extensive analysis of these proposed algorithms was conducted on generic grid topologies. For these test networks, we find that FP-TFA computes small latency bounds; but, at a medium to high utilization, the benefit of regulators becomes apparent. At high utilization or for high line transmission-rates, a small number of per-flow regulators has an effect on the latency bound larger than a small number of interleaved regulators. Moreover, interleaved regulators need to be placed everywhere in the network to provide noticeable improvements. We validate the applicability of our approaches on a realistic industrial time-sensitive network.

We study networks of FIFO nodes, where flows are constrained by arrival curves. A crucial question with these networks is: Can we derive a bound to the maximum delay that a packet can experience when traversing the network, and to the maximum queue size at each node? For a generic FIFO network these are still open issues: Some examples show that, contrary to common sense, no matter how low the maximum node utilization is in the network, it is possible to build an example of an unstable FIFO network. The importance of this issue lies in the necessity of hard bounds on packet delay and queue size, in order to enable QoS guarantees in these networks. For this reason we choose to tackle this problem through a deterministic approach, based on worst-case behavior. Our first result is the determination of a general method to derive sufficient conditions for the stability of a network: We show how, with a proper choice of the observed variables in the network and with the use of network calculus results, it is possible to derive the expression of an operator whose properties are associated to the stability of the network. Exploiting this method on a simple example, we first derive a generalization of the RIN result to heterogeneous settings and to leaky bucket constrained flows. Through some realistic examples, we show how this method allows networks to achieve a level of utilization which is more than three times larger than the best existing result. By applying the general method to three different variable classes, we derive some new sufficient conditions for stability, that perform largely better than all the main existing results, and we show how they can all be derived from the new sufficient conditions. Finally, we present a new formula for the computation of end-to-end delay bounds in a network of GR nodes.