# Jakub Łącki

Jakub Łącki is a research scientist working on graph-mining and large-scale optimization teams. He received his PhD from Univeristy of Warsaw in 2015, advised by Piotr Sankowski. Before joining Google he was a postdoctoral researcher at Sapienza University of Rome, working with Stefano Leonardi.

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Parallel Graph Algorithms in Constant Adaptive Rounds: Theory meets Practice

Hossein Esfandiari

Laxman Dhulipala

Soheil Behnezhad

Warren J Schudy

VLDB 2020

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We study fundamental graph problems such as graph connectivity, minimum spanning forest (MSF), and approximate maximum (weight) matching in a distributed setting. In particular, we focus on the Adaptive Massively Parallel Computation (AMPC) model, which is a theoretical model that captures MapReduce-like computation augmented with a distributed hash table.
We show the first AMPC algorithms for all of the studied problems that run in a constant number of rounds and use only O(n^ϵ) space per machine, where 0<ϵ<1. Our results improve both upon the previous results in the AMPC model, as well as the best-known results in the MPC model, which is the theoretical model underpinning many popular distributed computation frameworks, such as MapReduce, Hadoop, Beam, Pregel and Giraph.
Finally, we provide an empirical comparison of the algorithms in the MPC and AMPC models in a fault-tolerant distriubted computation environment. We empirically evaluate our algorithms on a set of large real-world graphs and show that our AMPC algorithms can achieve improvements in both running time and round-complexity over optimized MPC baselines.
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Massively Parallel Computation via Remote Memory Access

Hossein Esfandiari

Laxman Dhulipala

Soheil Behnezhad

Warren Schudy

SPAA 2019

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We introduce the Adaptive Massively Parallel Computation (AMPC) model, which is an extension of the widely popular Massively Parallel Computation (MPC) model. At a high level, the AMPC model strengthens the MPC model by storing all messages sent within a round in a distributed data store. In the following round all machines are provided with random read access to the data store, subject to the same constraints on the total amount of communication as in the MPC model. Our model is inspired by the previous empirical studies of distributed graph algorithms using MapReduce and a distributed hash table service.
This extension allows us to give new graph algorithms with much lower round complexities compared to the best known solutions in the MPC model. In particular, in the AMPC model we show how to solve maximal independent set in O(1) rounds, and connectivity/minimum spanning tree in O(log log_{m/n} n) rounds, which is an exponential improvement upon the best known algorithms in the MPC model with sublinear space per machine. Our results imply that the 2-Cycle conjecture, the most popular hardness conjecture in the MPC model, does not hold in the AMPC model.
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Optimal Dynamic Strings

Adam Karczmarz

Paweł Gawrychowski

Piotr Sankowski

Tomasz Kociumaka

SODA 2018 (to appear)

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In this paper we study the fundamental problem of maintaining a dynamic collection of strings under the following operations: (a) make_string -- add a string of constant length, (b) concatenate -- concatenate two strings, (c) split -- split a string into two at a given position, (d) compare -- find the lexicographical order (less, equal, greater) between two strings, (e) LCP -- calculate the longest common prefix of two strings.
We develop a generic framework for dynamizing the recompression method recently introduced by Jeż [J. ACM 2016]. It allows us to present an efficient data structure for the above problem, where an update requires only O(log n) worst-case time with high probability, with n being the total length of all strings in the collection, and a query takes constant worst-case time.
On the lower bound side, we prove that even if the only possible query is checking equality of two strings, either updates or queries must take amortized Omega(log n) time; hence our implementation is optimal.
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Round Compression for Parallel Matching Algorithms

Aleksander Mądry

Artur Czumaj

Krzysztof Onak

Piotr Sankowski

Slobodan Mitrović

STOC 2018

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For over a decade now we have been witnessing the success of massive parallel computation (MPC) frameworks, such as MapReduce, Hadoop, Dryad, or Spark. One of the reasons for their success is the fact that these frameworks are able to accurately capture the nature of large-scale computation. In particular, compared to the classic distributed algorithms or PRAM models, these frameworks allow for much more local computation. The fundamental question that arises in this context is though: can we leverage this additional power to obtain even faster parallel algorithms?
A prominent example here is the maximum matching problem—one of the most classic graph problems. It is well known that in the PRAM model one can compute a 2-approximate maximum matching in O(logn) rounds. However, the exact complexity of this problem in the MPC framework is still far from understood. Lattanzi et al. (SPAA 2011) showed that if each machine has n^(1+Ω(1)) memory, this problem can also be solved 2-approximately in a constant number of rounds. These techniques, as well as the approaches developed in the follow up work, seem though to get stuck in a fundamental way at roughly O(log n) rounds once we enter the (at most) near-linear memory regime. It is thus entirely possible that in this regime, which captures in particular the case of sparse graph computations, the best MPC round complexity matches what one can already get in the PRAM model, without the need to take advantage of the extra local computation power.
In this paper, we finally refute that possibility. That is, we break the above O(log n) round complexity bound even in the case of slightly sublinear memory per machine. In fact, our improvement here is almost exponential: we are able to deliver a (2+є)-approximate maximum matching, for any fixed constant є>0, in O((log log n)^2) rounds.
To establish our result we need to deviate from the previous work in two important ways that are crucial for exploiting the power of the MPC model, as compared to the PRAM model. Firstly, we use vertex–based graph partitioning, instead of the edge–based approaches that were utilized so far. Secondly, we develop a technique of round compression. This technique enables one to take a (distributed) algorithm that computes an O(1)-approximation of maximum matching in O(log n) independent PRAM phases and implement a super-constant number of these phases in only a constant number of MPC rounds.
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Round Compression for Parallel Matching Algorithms

Aleksander Mądry

Artur Czumaj

Krzysztof Onak

Piotr Sankowski

Slobodan Mitrović

STOC 2018 (to appear)

Preview abstract
For over a decade now we have been witnessing the success of massive parallel computation (MPC) frameworks, such as MapReduce, Hadoop, Dryad, or Spark. One of the reasons for their success is the fact that these frameworks are able to accurately capture the nature of large-scale computation. In particular, compared to the classic distributed algorithms or PRAM models, these frameworks allow for much more local computation. The fundamental question that arises in this context is though: can we leverage this additional power to obtain even faster parallel algorithms?
A prominent example here is the maximum matching problem---one of the most classic graph problems.
It is well known that in the PRAM model one can compute a 2-approximate maximum matching in O(log n) rounds. However, the exact complexity of this problem in the MPC framework is still far from understood. Lattanzi et al. (SPAA 2011) showed that if each machine has n^(1+Omega(1)) memory, this problem can also be solved 2-approximately in a constant number of rounds. These techniques, as well as the approaches developed in the follow up work, seem though to get stuck in a fundamental way at roughly O(log n) rounds once we enter the (at most) near-linear memory regime. It is thus entirely possible that in this regime, which captures in particular the case of sparse graph computations, the best MPC round complexity matches what one can already get in the PRAM model, without the need to take advantage of the extra local computation power.
In this paper, we finally refute that perplexing possibility. That is, we break the above O(log n) round complexity bound even in the case of slightly sublinear memory per machine. In fact, our improvement here is {\em almost exponential}: we are able to deliver a (2+epsilon)-approximation to maximum matching, for any fixed constant epsilon>0, in O((log log n)^2) rounds.
To establish our result we need to deviate from the previous work in two important ways that are crucial for exploiting the power of the MPC model, as compared to the PRAM model. Firstly, we use vertex--based graph partitioning, instead of the edge--based approaches that were utilized so far. Secondly, we develop a technique of round compression. This technique enables one to take a (distributed) algorithm that computes an O(1)-approximation of maximum matching in O(log n) independent PRAM phases and implement a super-constant number of these phases in only a constant number of MPC rounds.
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Decremental Single-Source Reachability in Planar Digraphs

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In this paper we show a new algorithm for the decremental single-source reachability problem in directed planar graphs. It processes any sequence of edge deletions in O(n log2 n
log log n) total time and explicitly maintains the set of vertices reachable from a fixed source vertex.
Hence, if all edges are eventually deleted, the amortized time of processing each edge deletion is only O(log2 n log log n), which improves upon a previously known O(n1/2) solution. We also show an algorithm for decremental maintenance of strongly connected components in directed planar graphs with the same total update time. These results constitute the first almost optimal (up to polylogarithmic factors) algorithms for both problems.
To the best of our knowledge, these are the first dynamic algorithms with polylogarithmic update times on general directed planar graphs for non-trivial reachability-type problems, for which
only polynomial bounds are known in general graphs.
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