GPU Concurrency: Weak Behaviours and Programming Assumptions

Concurrency is pervasive and perplexing, particularly on graphics processing units (GPUs). Current specifications of languages and hardware are inconclusive; thus programmers often rely on folklore assumptions when writing software. To remedy this state of affairs, we conducted a large empirical study of the concurrent behaviour of deployed GPUs. Armed with litmus tests (i.e. short concurrent programs), we questioned the assumptions in programming guides and vendor documentation about the guarantees provided by hardware. We developed a tool to generate thousands of litmus tests and run them under stressful workloads. We observed a litany of previously elusive weak behaviours, and exposed folklore beliefs about GPU programming---often supported by official tutorials---as false. As a way forward, we propose a model of Nvidia GPU hardware, which correctly models every behaviour witnessed in our experiments. The model is a variant of SPARC Relaxed Memory Order (RMO), structured following the GPU concurrency hierarchy

O Sorriso da Avó em ‘Guarde Segredo’ de Esmeralda Ribeiro

No conto “Guarde Segredo” (1991), Esmeralda Ribeiro (1958-) substitui os pais “verdadeiros” ou biológicos de uma jovem afrodescendente por dois pais “falsos” (vovó Olívia e Lima Barreto). Por qual razão Ribeiro nos apresenta esta família alternativa? Neste artigo, defendo que a imaginação de uma nova família permite a Ribeiro combinar uma abordagem da invisibilidade literária, histórica e política das mulheres afrodescendentes com uma visão crítica e ambivalente da maternidade, quando esta se torna excessivamente visível na esfera privada. Apropriando-se dos princípios utilizados por sociedades disciplinares para regular relações raciais e sexuais, “Guarde Segredo” introduz estruturas panópticas de disciplina (Foucault, 1977) no cotidiano familiar dos inquilinos da casa situada à rua Major Mascarenhas, de modo a inverter o processo de invisibilidade crescente da jovem narradora, enquanto mãe afrodescendente em potência. A solução panóptica encontrada por Ribeiro cria uma plataforma privilegiada de acesso aos seus pais enquanto observadores/educadores, um dos quais é, como veremos, mais invisível do que o outro. A imaginação de uma nova família em “Guarde Segredo” permite a Ribeiro oferecer pistas teóricas necessárias para uma discussão sustentável e não essencialista da maternidade no contexto do projeto político-literário da escritura afro-brasileira

A Derived Transformation of Valence Functions Across Two 8-Member Comparative Relational Networks

The emergence of transitive relations between stimuli that had never been directly paired with one another can be examined through a phenomenon called Transitive Inference (TI). The present experiment explored contextually controlled TI effects in verbally able humans. Specifically, participants were trained in the conditional discriminations A1+B1-, B1+ C1-, C1+D1-, D1+E1-, E1+F1-, F1+G1- and G1+H1- in the presence of a cue (Cue 1), followed by tests for mutual and combinatorial entailment in the presence of either Cue 1 or Cue 2. Note that Cue 1 and Cue 2 had been previously established as functionally equivalent to happier-than and unhappier-than contexts, respectively. Using a performancebased measure of Bimplicit preferences,^ we predicted that successfully demonstrating entailment would yield a performance indicating C1 as more positively valenced than F1. Similarly, if participants learned the discriminations A2+B2-, B2+C2-, C2+D2-, D2+E2-, E2+F2-, F2+G2-, G2+H2- in the presence of Cue 2 only, followed by tests for entailment in the presence of both Cues 1 and 2, we predicted that C2 should be responded to as more negatively valenced than F2. Performances across both conditions supported these predictions, furthering the evidence for the claim that emotional valences can be derived through functionally transitive stimulus-stimulus relations