Sorting versus screening: Search frictions and competing mechanisms

In a market where sellers compete by posting trading mechanisms, we allow for a general search technology and show that its features crucially affect the equilibrium mechanism. Price posting prevails when meetings are rival, i.e., when a meeting by one buyer reduces another buyer’s meeting probability. Under price posting buyers reveal their type by sorting ex ante. Only if the meeting technology is sufficiently non-rival, price posting is not an equilibrium. Multiple buyer types then visit the same sellers who screen ex post through auctions

Performance metrics for consolidated servers

In spite of the widespread adoption of virtualization and consol- idation, there exists no consensus with respect to how to bench- mark consolidated servers that run multiple guest VMs on the same physical hardware. For example, VMware proposes VMmark which basically computes the geometric mean of normalized throughput values across the VMs; Intel uses vConsolidate which reports a weighted arithmetic average of normalized throughput values. These benchmarking methodologies focus on total system through- put (i.e., across all VMs in the system), and do not take into account per-VM performance. We argue that a benchmarking methodology for consolidated servers should quantify both total system through- put and per-VM performance in order to provide a meaningful and precise performance characterization. We therefore present two performance metrics, Total Normalized Throughput (TNT) to characterize total system performance, and Average Normalized Reduced Throughput (ANRT) to characterize per-VM performance. We compare TNT and ANRT against VMmark using published performance numbers, and report several cases for which the VM- mark score is misleading. This is, VMmark says one platform yields better performance than another, however, TNT and ANRT show that both platforms represent different trade-offs in total system throughput versus per-VM performance. Or, even worse, in a cou- ple cases we observe that VMmark yields opposite conclusions than TNT and ANRT, i.e., VMmark says one system performs better than another one which is contradicted by TNT/ANRT performance characterization

A methodology for analyzing commercial processor performance numbers

The wealth of performance numbers provided by benchmarking corporations makes it difficult to detect trends across commercial machines. A proposed methodology, based on statistical data analysis, simplifies exploration of these machines' large datasets

Study of femtosecond laser beam focusing in a direct-write system

En col·laboració amb la Universitat Autònoma de Barcelona (UAB) i la Universitat de Barcelona (UB).Direct-write techniques appear as a versatile option in rapid-prototyping applications because they can directly transfer a custom pattern from a digital file. Lasers are a distinguished tool which allow to perform non-contact direct-write techniques with the ability to add, remove and modify different types of materials. Moreover, they have a high focusing power and offer high spatial resolution when a femtosecond laser is used due to the reduction of thermal effects. Additive and subtractive techniques can be performed in one laser-based direct-write system with minimum variations in the setup. In all cases, properties of the laser beam, such as the beam width or the morphology of the intensity distribution have an effect on the results of the laser processing. The aim of this work is the study of the laser propagation in a specific laser-based direct-write setup. The beam intensity distribution effects are measured at different positions and compared with simulations. The influence of the main parameters, pupil displacement and objective tilt, on the morphological properties of the intensity distribution is analysed. Well defined spots with good reproducibility are obtained. In addition, at comparing the simulation with the experiments, the origin of some morphological properties are reported and they can be used to optimize the setup

The multi-program performance model: debunking current practice in multi-core simulation

Composing a representative multi-program multi-core workload is non-trivial. A multi-core processor can execute multiple independent programs concurrently, and hence, any program mix can form a potential multi-program workload. Given the very large number of possible multiprogram workloads and the limited speed of current simulation methods, it is impossible to evaluate all possible multi-program workloads. This paper presents the Multi-Program Performance Model (MPPM), a method for quickly estimating multiprogram multi-core performance based on single-core simulation runs. MPPM employs an iterative method to model the tight performance entanglement between co-executing programs on a multi-core processor with shared caches. Because MPPM involves analytical modeling, it is very fast, and it estimates multi-core performance for a very large number of multi-program workloads in a reasonable amount of time. In addition, it provides confidence bounds on its performance estimates. Using SPEC CPU2006 and up to 16 cores, we report an average performance prediction error of 2.3% and 2.9% for system throughput (STP) and average normalized turnaround time (ANTT), respectively, while being up to five orders of magnitude faster than detailed simulation. Subsequently, we demonstrate that randomly picking a limited number of multi-program workloads, as done in current pactice, can lead to incorrect design decisions in practical design and research studies, which is alleviated using MPPM. In addition, MPPM can be used to quickly identify multi-program workloads that stress multi-core performance through excessive conflict behavior in shared caches; these stress workloads can then be used for driving the design process further

Identifying Sorting - In Theory

We argue that using wage data alone, it is virtually impossible to identify whether Assortative Matching between worker and firm types is positive or negative. In standard competitive matching models the wages are determined by the marginal contribution of a worker, and the marginal contribution might be higher or lower for low productivity firms depending on the production function. For every production function that induces positive sorting we can find a production function that induces negative sorting but generates identical wages. This arises even when we allow for non-competitive mismatch, for example due to search frictions. Even though we cannot identify the sign of the sorting, we can identify the strength, i.e., the magnitude of the cross-partial, and the associated welfare loss. While we show analytically that standard fixed effects regressions are not suitable to recover the strength of sorting, we propose an alternative procedure that measures the strength of sorting in the presence of search frictions independent of the sign of the sorting.sorting, assortative matching, identification, linked employer-employee data, interpretation of fixed-effects

Identifying Sorting: In Theory

We argue that using wage data alone, it is virtually impossible to identify whether Assortative Matching between worker and firm types is positive or negative. In standard competitive matching models the wages are determined by the marginal contribution of a worker, and the marginal contribution might be higher or lower for low productivity firms depending on the production function. For every production function that induces positive sorting we can find a production function that induces negative sorting but generates identical wages. This arises even when we allow for non-competitive mismatch, for example due to search frictions. Even though we cannot identify the sign of the sorting, we can identify the strength, i.e., the magnitude of the cross-partial, and the associated welfare loss. While we show analytically that standard fixed effects regressions are not suitable to recover the strength of sorting, we propose an alternative procedure that measures the strength of sorting in the presence of search frictions independent of the sign of the sorting.sorting, assortative matching, identification, linked employer-employee data, interpretation of fixed-effects

Sorting and Decentralized Price Competition

We investigate under which conditions price competition in a market with matching frictions leads to sorting of buyers and sellers. Positive assortative matching obtains only if there is a high enough degree of complementarity between buyer and seller types. The relevant condition is root-supermodularity; i.e., the square root of the match value function is supermodular. It is a necessary and sufficient condition for positive assortative matching under any distribution of buyer and seller types, and does not depend on the details of the underlying matching function that describes the search process. The condition is weaker than log-supermodularity, a condition required for positive assortative matching in markets with random search. This highlights the role competition plays in matching heterogeneous agents. Negative assortative matching obtains whenever the match value function is weakly submodular.Competitive Search Equilibrium. Directed Search. Two-Sided Matching. Decentralized Price Competition. Root-Supermodularity.