Localizing Defects in Multithreaded Programs by Mining Dynamic Call Graphs

Writing multithreaded software for multicore computers confronts many developers with the difficulty of finding parallel programming errors. In the past, most parallel debugging techniques have concentrated on finding race conditions due to wrong usage of synchronization constructs. A widely unexplored issue, however, is that a wrong usage of non-parallel programming constructs may also cause wrong parallel application behavior. This paper presents a novel defect-localization technique for multithreaded shared-memory programs that is based on analyzing execution anomalies. Compared to race detectors that report just on wrong synchronization, this method can detect a wider range of defects affecting parallel execution. It works on a condensed representation of the call graphs of multithreaded applications and employs data-mining techniques to locate a method containing a defect. Our results from controlled application experiments show that we found race conditions, but also other programming errors leading to incorrect parallel program behavior. On average, our approach reduced in our benchmark the amount of code to be inspected to just 7.1% of all methods

Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb-Pb collisions at root s(NN)=2.76TeV

Peer reviewe

Measurement of K*(892)(+/-) production in inelastic pp collisions at the LHC

The first results on K⁎(892)± resonance production in inelastic pp collisions at LHC energies of s=5.02, 8, and 13 TeV are presented. The K⁎(892)± has been reconstructed via its hadronic decay channel K⁎(892)→±KS0+π± with the ALICE detector. Measurements of transverse momentum distributions, pT-integrated yields, and mean transverse momenta for charged K⁎(892) are found to be consistent with previous ALICE measurements for neutral K⁎(892) within uncertainties. For pT>1 GeV/c the K⁎(892)± transverse momentum spectra become harder with increasing centre-of-mass energy from 5.02 to 13 TeV, similar to what previously observed for charged kaons and pions. For pT<1 GeV/c the K⁎(892)± yield does not evolve significantly and the abundance of K⁎(892)± relative to K is rather independent of the collision energy. The transverse momentum spectra, measured for K⁎(892)± at midrapidity in the interval 0 < pT<15 GeV/c, are not well described by predictions of different versions of PYTHIA 6, PYTHIA 8 and EPOS-LHC event generators. These generators reproduce the measured pT-integrated K⁎±/K ratios and describe well the momentum dependence for pT<2 GeV/c

KS0KS0 and KS0K± femtoscopy in pp collisions at √s=5.02 and 13 TeV

Femtoscopic correlations with the particle pair combinations K0 SK0 S and K0 SK± are studied in pp collisions at √s = 5.02 and 13 TeV by the ALICE experiment. At both energies, boson source parameters are extracted for both pair combinations, by fitting models based on Gaussian size distributions of the sources, to the measured two-particle correlation functions. The interaction model used for the K0 SK0 S analysis includes quantum statistics and strong final-state interactions through the f0(980) and a0(980) resonances. The model used for the K0 SK± analysis includes only the final-state interaction through the a0 resonance. Source parameters extracted in the present work are compared with published values from pp collisions at √s = 7 TeV and the different pair combinations are found to be consistent. From the observation that the strength of the K0 SK0 S correlations is significantly greater than the strength of the K0 SK± correlations, the new results are compatible with the a0 resonance being a tetraquark state of the form (q1, q2, s, s), where q1 and q2 are u or d quarks

Transverse-momentum and event-shape dependence of D-meson flow harmonics in Pb–Pb collisions at √sNN = 5.02 TeV

The elliptic and triangular flow coefficients v2 and v3 of prompt D0, D+, and D∗+ mesons were measured at midrapidity (|y| < 0.8) in Pb–Pb collisions at the centre-of-mass energy per nucleon pair of √sNN = 5.02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decays in the transverse momentum interval 1 < pT < 36 GeV/c in central (0–10%) and semi-central (30–50%) collisions. Compared to pions, protons, and J/ψ mesons, the average D-meson vn harmonics are compatible within uncertainties with a mass hierarchy for pT 3 GeV/c, and are similar to those of charged pions for higher pT. The coupling of the charm quark to the light quarks in the underlying medium is further investigated with the application of the event-shape engineering (ESE) technique to the D-meson v2 and pT-differential yields. The D-meson v2 is correlated with average bulk elliptic flow in both central and semi-central collisions. Within the current precision, the ratios of per-event Dmeson yields in the ESE-selected and unbiased samples are found to be compatible with unity. All the measurements are found to be reasonably well described by theoretical calculations including the effects of charm-quark transport and the recombination of charm quarks with light quarks in a hydrodynamically expanding medium

Measurement of charged jet suppression in Pb-Pb collisions at √sNN = 2.76 TeV

A measurement of the transverse momentum spectra of jets in Pb-Pb collisions at sNN−−−√=2.76 TeV is reported. Jets are reconstructed from charged particles using the anti-kT jet algorithm with jet resolution parameters R of 0.2 and 0.3 in pseudo-rapidity |η|<0.5. The transverse momentum pT of charged particles is measured down to 0.15 GeV/c which gives access to the low pT fragments of the jet. Jets found in heavy-ion collisions are corrected event-by-event for average background density and on an inclusive basis (via unfolding) for residual background fluctuations and detector effects. A strong suppression of jet production in central events with respect to peripheral events is observed. The suppression is found to be similar to the suppression of charged hadrons, which suggests that substantial energy is radiated at angles larger than the jet resolution parameter R=0.3 considered in the analysis. The fragmentation bias introduced by selecting jets with a high pT leading particle, which rejects jets with a soft fragmentation pattern, has a similar effect on the jet yield for central and peripheral events. The ratio of jet spectra with R=0.2 and R=0.3 is found to be similar in Pb-Pb and simulated PYTHIA pp events, indicating no strong broadening of the radial jet structure in the reconstructed jets with R<0.3.

Lambda(+)(C) production in pb-pb collisions at root S-NN=5.02 TeV

A measurement of the production of prompt +cbaryons in Pb–Pb collisions at √sNN=5.02 TeV with the ALICE detector at the LHC is reported. The +cand −cwere reconstructed at midrapidity (|y| <0.5) via the hadronic decay channel +c→pK0S(and charge conjugate) in the transverse momentum and centrality intervals 6 <pT<12 GeV/cand 0–80%. The +c/D0ratio, which is sensitive to the charm quark hadronisation mechanisms in the medium, is measured and found to be larger than the ratio measured in minimum-bias pp collisions at √s=7TeV and in p–Pb collisions at √sNN=5.02 TeV. In particular, the values in p–Pb and Pb–Pb collisions differ by about two standard deviations of the combined statistical and systematic uncertainties in the common pTinterval covered by the measurements in the two collision systems. The +c/D0ratio is also compared with model calculations including different implementations of charm quark hadronisation. The measured ratio is reproduced by models implementing a pure coalescence scenario, while adding a fragmentation contribution leads to an underestimation. The +cnuclear modification factor, RAA, is also presented. The measured values of the RAAof +c, D+sand non-strange D mesons are compatible within the combined statistical and systematic uncertainties. They show, however, a hint of a hierarchy (RD0AA<RD+sAA<R +cAA), conceivable with a contribution from coalescence mechanisms to charm hadron formation in the medium

Λ c+ production in Pb–Pb collisions at s NN =5.02 TeV

AA measurement of the production of prompt Λ c + baryons in Pb–Pb collisions at s NN =5.02 TeV with the ALICE detector at the LHC is reported. The Λ c + and Λ‾ c − were reconstructed at midrapidity (|y|&lt;0.5) via the hadronic decay channel Λ c + →pK S 0 (and charge conjugate) in the transverse momentum and centrality intervals

Coherent J/psi photoproduction at forward rapidity in ultra-peripheral Pb-Pb collisions at root s(NN)=5.02 TeV

The ALICE collaboration performed the first rapidity-differential measurement of J/psi coherent photoproduction in ultra-peripheral Pb\u2013Pb collisions at a center-of-mass energy sqrt(sNN) = 5.02 TeV. The J/psi is detected via its dimuon decay in the forward rapidity region (-4.0 < y < -2.5) for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 750 \u3bcb 121. The cross section for J/psi coherent production is presented in six rapidity bins. The results are compared with theoretical models for J/psi coherent photoproduction. These comparisons indicate that gluon shadowing effects play a role in the photoproduction process. The ratio of psi' to J/psi coherent photoproduction cross sections was measured and found to be consistent with that measured for photoproduction off protons