Reading Assignment 2 – Weaving Relations for Cache Performance

Reading Assignment 2 – Weaving Relations forCache PerformanceJosé Filipe Barbosa de Carvalho (jose.carvalho@fe.up.pt)8th November 2007Advanced Database SystemsTechnische Universität Wien,Karlsplatz 13, A-1040 Wien AUSTRIAAbstract: This text wants to resume the fundamental ideas in [1], after a carefulreading. It also presents things that author didn’t well understand and his per-sonnel opinion about it.1 IntroductionThe performance questions in Database Management Systems (DBMSs) are crit-ical, because these systems are an important component of a lot of applications, which rely in DBMS to store dynamic information and perform operations about this infor-mation. Many of the research in Database Management Systems is focused in improve performance issues, like the time and space necessary to do some operations. Tradi-tionally, relational database systems have optimized for I/O (input/output) perfor-mance and organized records sequentially on disk using N-ary Storage Model (NSM), because these operations are the most expensive in time.However, recent research shows that processor cache utilization and perfor-mance is also significant on modern platforms. The document [1] proposes a new approach to organize records on disk pages, the PAX (Partition Attributes Across) model, in comparison with the industrial standard NSM and another research model, the DSM (Decomposition Storage Model). The document outlines the main characte-ristics of these three models, and presents the advantages of PAX in relation to other two, namely explaining why this model has better use of processor cache.2 Important ideas and results of the paperThe paper presents a new model to organize records on disk pages, the PAX model, explaining its advantages in relation to other models. In first parts, in a more theoretical approach the text explains the fundamentals of PAX model, but also of NSM and DSM models (until chapter 3). In last parts text presents research implemen-tations and tests, doing comparisons among them (chapter 4, 5 and 6).Nowadays most of Database Management Systems use NSM model, because it minimizes the communication between processor and the secondary storage, that is generally accepted by industrial and research database community as the major data-base performance bottleneck. NSM stores records continuously starting from the be-ginning of each disk page, and uses an offset table at the end of the page to locate thebeginning of each record (figure1).Figure 1 – The N-ary Storage Model (NSM)However, most queries use only part of each record. This means that we need to read always the complete records from disk, making unnecessary I/O. To resolve this problem a new approach is proposed, the DSM, where one relation is divided in n-relations, one relation per attribute. Each attribute is recorded in a different page, and only the necessaries pages are loaded, accordingly the query (figure 2). But in queries where is necessary join several attributes from a relation, this model spends much more time, with low performance. Because that, this model is not common in modern DBMSs.Figure 2 – The Decomposition Storage Model (DSM)The PAX model has a new approach for layout data records, which can have a better processor cache performance but without the lacks of DSM. As NSM, the PAX model saves the relation in the same page. But it groups all values of a particular attribute together in a minipage (figure 3). During a sequential scan (for example, to do some query), PAX fully utilizes processor cache resources, because on each miss a number of a single attribute’s values are loaded into to cache together (I copy from [1] and underline this sentence because is the major reason for better performance of PAX). And because all parts of one record are in same page, this model doesn’t re-quire more I/O that NSM. To reconstruct a record it is only necessary do a mini-join among minipages, which has a low cost, because it does not have look another pages.Figure 3 – The Partition Attributes Across model (PAX)The paper discusses some design issues in PAX, namely how to divide one page (and the additional data to maintain this structure) and how to implement minipages for fixed and variable-length attributes values. In final part of chapter 3 the text intro-duces two theoretical characteristics that PAX has:•Inter-record spatial locality, that minimizes data cache-related delays when operate over a subset of attributes of record (DSM also has this property);•Low record reconstruction cost, minimizes the delays related with getting multiple fields of same record (NSM also has this property).In chapter 4, the text presents how authors implement the three models. They used the Shore Storage Manager [2] that provides all the features of a modern storage manager, like B-trees and R-trees, ARIES-style recovery and hierarchical locking. It is explained also, in detail, how the three models are implemented, namely record im-plementation, data manipulation algorithms and query operations, and the worries to guarantee a fair competition in tests, like ensure that all models have the same locking type.The fifth chapter makes an analysis of performance of three models, based in tests of real implementations. It is explained the setup and methodology, namely the computer machine and queries used to do the evaluation, and after that, shows the experimental results. Because DSM has a poor performance with several attributes (exponential function in elapsed time, when increases the number of attributes), au-thors focus their attention in NSM vs. PAX “battle” (like in the next chapter). In all results PAX has a better performance, with lower cache data misses that NSM. Of course, the improve rates are variable, and depends of many things. For instance, in queries with more attributes the performance of PAX and NSM converge.The chapter 6 presents more tests, comparing PAX and NSM when running TPC-H decision support workload. PAX are consistently better than NSM, performing queries and updates tested faster. The NSM is only better in load TPC-H databases, because their algorithm of insertion is simpler (and therefore have fewer operations) that PAX.In conclusion, this paper presents a new model to organize records in database management systems and proves that its cache performance (and because this, the overall performance of system) is better than another existent models.3 Things not well understoodI haven’t troubles in understood all the paper. The subject of text is focused in storage models that are introduced in one lesson of this course, some weeks ago. I need to read carefully because many issues of store models research are tricky and difficult to understand at first read.Of course I didn’t know some jargon like “sargable” (appeared in 6th page), some acronyms like DSS (Decision support system) and OLTP (Online transaction processing) or TPC-H benchmarks. But with a simple search in Google or Wikipedia I understood, in a general way, these simple concepts.I also didn’t know the Shore Storage Manager that is used for implementation of the three models. I found the home page of project [2] and download the current re-lease to understand how it works. I read some of its documentation and realize that Shore Storage Manager is a set of libraries developed in C++ at University of Wiscon-sin-Madison, the same place where authors wrote this article.4 Things that I like and I didn’t like in the paperThe paper is an introduction to PAX model, explaining also two existent models to organize data in database management systems: NSM and DSM. I like read this paper because I learn and study more deeply one subject not well understood when it is introduced at classes. The text is focused in prove the better performance of PAX, explaining in a general way the design issues of the three models, hiding (or not giving so much attention) about some complex implementation questions. I like this manner because I don’t like an article with many of implementation details. I always prefer conceptual and design issues than implementation and programming questions.However these low-level issues aren’t my favorite subject. I like most of database topics, but I don’t like so much learn the details like implementation of PAX model to organize data. But ok, it is my personal viewpoint, and maybe this knowledge will be useful in future.I only have one real critic to paper: PAX hasn’t one shortcoming? This paper sounds like advertising. It is natural that their authors magnify their model, but I think that it is natural that PAX have some disadvantages.5 ConclusionThis paper is a good introduction to learn more about how data are organized in da-tabase management systems, and proves how performance issues are important in this area. First engineers and researchers optimized databases for better I/O, and now are optimizing for other issues, like to have better processor cache utilization. The chal-lenges of building a good database management system are really varied and complex.6 Bibliography[1] AILAMAKI et al.; “Weaving Relations of Cache Performance”, VLDB, 2001[2] Shore Project Home Page, consulted in November 2007/shore/。