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André Costa

Publications and source records attributed to André Costa.

8 recordsLinked to original sources
arXiv2026

uSTM: A Lightweight and Efficient STM Supporting General Types and Deferred Aborts

Software Transactional Memory (STM) systems allow developers to more easily exploit multicore architectures by wrapping arbitrary sequential code in transactions that are executed concurrently. In recent years, the performance of STM systems has approached that of hand-tuned data structures through techniques that avoid unnecessary aborts and exploit the semantics of underlying data structures. Despite achieving excellent performance, most STM systems do not fully address the concerns they targeted in the first place: safety, usability, and generality. In particular, these systems place restrictions on the data types that may be updated transactionally, such as requiring that these types fit within a word, and can require modification of data layout. Moreover, most STM systems abort transactions in the middle of client code to ensure correctness. This can cause space leaks and other bugs not present in the original code. We present ustm, a novel STM system addressing all of these shortcomings while still maintaining excellent performance, all within ~300 lines of code. uSTM supports general types while maintaining data layout. Aborts are deferred until the end of the transaction, allowing client code within a transaction to terminate normally. To ensure that uSTM guarantees opacity, we implement a novel timestamping algorithm we call split-increment timestamps. We compare the performance of uSTM to a variety of state-of-the-art (SOTA) STM systems, demonstrating that uSTM matches or outperforms the SOTA on a variety of workloads.

cs.DC↗
arXiv2025

Lipschitz geometry and combinatorics of circular snakes

This paper explores the Lipschitz geometric and combinatorial properties of germs of real semialgebraic surfaces (or, more generally, definable in a polynomially bounded o-minimal structure) with circular link (homeomorphic to the circle $\mathbb{S}^1$). We define and investigate the outer Lipschitz geometry of the so-called circular snakes, showing what results in the paper "Lipschitz geometry and combinatorics of abnormal surface germs" (by Andrei Gabrielov and Emanoel Souza) valid to snakes still holds for the circular case. We prove the existence of a canonical decomposition for the Valette link of a circular snake into finitely many segments and nodal zones and establish some necessary and sufficient criteria to determine when it is possible to obtain a snake from a circular snake by "removing" either one of its segments or a Hölder triangle whose Valette link is contained in one of its nodal zones. We construct a combinatorial object associated with a circular snake and prove a realization theorem for this combinatorial object. We also present a weakly outer Lipschitz classification for circular snakes. Finally, we show some results about the combinatorics of binary circular snakes, which is wildly distinct from the corresponding case shown in the work of Gabrielov and Souza.

math.MG↗
arXiv2024

Remarks on Lipschitz geometry on globally conic singular manifolds

We study metric properties of manifolds with conic singularities and present a natural interplay between metrically conic and metrically asymptotically conic behaviour. As a consequence, we prove that a singular sub-manifold is Lipschitz normally embedded, i.e. its inner and outer metric structures are equivalent, in an ambient singular manifold, wheneverthe singularities are conic and the ends of the manifold are asymptotically conic, which answers positively a question raised in our last work.

math.MG↗
arXiv2023

Characterization of Lipschitz Normally Embedded complex curves

The main result of the paper states that a connected complex affine algebraic curve is Lipschitz normally embedded (shortened to LNE afterwards) in $\mathbb{C}^n$ if and only if its germ at any singular point is a finite union of non-singular complex curve germs which are pairwise transverse, and its projective closure is in general position with the hyperplane at infinity. To this aim, we completely characterize complex analytic curves of a compact complex manifold which are LNE (regardless of the given Riemannian structure), and therefore we can relate when a projective algebraic curve is LNE in $\mathbb{CP}^n$ with the property of its general affine traces being LNE in $\mathbb{C}^n$. This allows us to deduce that Lipschitz classification of LNE curves is topological. We describe a complete invariant for the (outer and inner) Lipschitz equivalence of affine LNE curves and show that most such curves cannot be bi-Lipschitz homeomorphic to a plane one.

math.AG↗
arXiv2023

One point compactification and Lipschitz normally embedded definable subsets

A closed subset of $\mathbb{R}^q$, definable in some given o-minimal structure, is Lipschitz normally embedded in $\mathbb{R}^q$ if and only if its one-point compactification is Lipschitz normally embedded in the unit sphere ${\bf S}^q$($ = \mathbb{R}^q \cup \{\infty \}$), i.e. the closure of its image by the inverse of the stereographic projection is Lipschitz normally embedded in ${\bf S}^q$. This implies that any closed connected unbounded definable subset of an Euclidean space is definably inner bi-Lipschitz homeomorphic to a Lipschitz normally embedded definable set.

math.AG↗
arXiv2023

Global Lipschitz geometry of conic singular sub-manifolds with applications to algebraic sets

The main result states that a connected conic singular sub-manifold of a Riemannian manifold, compact when the ambient manifold is non-Euclidean, is Lipschitz Normally Embedded: the outer and inner metric space structures are metrically equivalent. We also show that a closed subset of $\mathbb{R}^n$ is a conic singular sub-manifold if and only if its closure in the one point compactification ${\bf S}^n =\mathbb{R}^n\cup \infty$ is a conic singular sub-manifold. Consequently the connected components of generic affine real and complex algebraic sets are conic at infinity, thus are Lipschitz Normally Embedded.

math.DG↗
arXiv2014

Maintenance of order in a moving strong condensate

We investigate the conditions under which a moving condensate may exist in a driven mass transport system. Our paradigm is a minimal mass transport model in which $n-1$ particles move simultaneously from a site containing $n>1$ particles to the neighbouring site in a preferred direction. In the spirit of a Zero-Range process the rate $u(n)$ of this move depends only on the occupation of the departure site. We study a hopping rate $u(n) = 1 + b/n^α$ numerically and find a moving strong condensate phase for $b > b_c(α)$ for all $α>0$. This phase is characterised by a condensate that moves through the system and comprises a fraction of the system's mass that tends to unity. The mass lost by the condensate as it moves is constantly replenished from the trailing tail of low occupancy sites that collectively comprise a vanishing fraction of the mass. We formulate an approximate analytical treatment of the model that allows a reasonable estimate of $b_c(α)$ to be obtained. We show numerically (for $α=1$) that the transition is of mixed order, exhibiting exhibiting a discontinuity in the order parameter as well as a diverging length scale as $b\searrow b_c$.

cond-mat.stat-mech↗