Query and Update Load Shedding With MobiQual

Transcription

1 Dodda Sudeep et al,int.j.computer Technology & Applcatons,Vol 3 (1), Query and Update Load Sheddng Wth MobQual Dodda Sudeep 1, Bala Krshna 2 1 Pursung M.Tech(CS), Nalanda Insttute of Engneerng & Technology, Sddharth Nagar, Sattenapall, Guntur., Afflated to JNTUK, Kaknada, A.P., Inda. 2 Asst. Professor, Department of Computer Scence Engneerng, Nalanda Insttute of Engneerng & Technology, Sddharth Nagar, Sattenapall, Guntur., Afflated to JNTUK, Kaknada, A.P., Inda. sudeep.dodda@gmal.com Abstract - Freshness and accuracy are two key measures of qualty of servce (QoS) n locaton-based, moble contnual queres (CQs). However, t s often dffcult to provde both fresh and accurate CQ results due to (a) lmted resources n computng and communcaton and (b) fast-changng load condtons caused by contnuous moble node movement. Thus a key challenge for a moble CQ system s: How do we acheve the hghest possble qualty of the query results, n both freshness and accuracy, wth currently avalable resources under changng load condtons? In ths paper, we formulate ths problem as a load sheddng one, and develop MobQual a QoS-aware framework for performng both update load sheddng and query load sheddng. The desgn of MobQual hghlghts three mportant features. (1) Dfferentated load sheddng: Dfferent amounts of query and update load sheddng are appled to dfferent groups of queres and moble nodes, respectvely. (2) Per-query QoS specfcatons: The overall freshness and accuracy of the query results are maxmzed wth ndvdualzed QoS specfcatons. (3) Low-cost adaptaton: MobQual dynamcally adapts, wth a mnmal overhead, to changng load condtons and avalable resources.we show that, through a careful combnaton of update and query load sheddng, the MobQual approach leads to much hgher freshness and accuracy n the query results n all cases, compared to exstng approaches. Keywords Load sheddng, Query, MobQual, QOS. 1.INTRODUCTION Today, we are experencng a world where we can stay connected whle on-the-go because there are (1) myrads of affordable moble devces and (2) ever ncreasng accessblty of wreless communcatons for these devces. Combned wth the avalablty of lowcost postonng devces, lke GPS, ths has created a new class of applcatons n the area of moble locatonbased servces (LBSs). Examples nclude locatonaware nformaton delvery and resource management, such as transportaton servces, fleet management, moble games, and battlefeld coordnaton. A key challenge for LBSs s a scalable locaton montorng system capable of handlng large number of moble nodes and processng complex queres over ther postons. Although several moble contnual query (CQ) systems have been proposed to handle long-runnng locaton montorng tasks n a scalable manner [1], [2], the focus of these works s prmarly on effcent ndexng and query processng technques, not on accuracy or freshness of the query results. Accuracy (naccuracy) s defned based on the amount of moble node poston errors found n the query results at the tme of query re-evaluaton. Ths accuracy measure s strongly ted to the frequency of poston updates receved from the moble nodes. Although one can also use a hgher level concept to measure accuracy, such as the amount of contanment errors found n the query results1, ncludng both false postves (ncluson errors) and false negatves (excluson errors), we argue that usng poston update errors for accuracy measure wll provde hgher level of precson. Ths s prmarly because by utlzng the amount of node poston errors as the accuracy measure, one can easly bound the naccuracy by a threshold-based poston reportng scheme [3], [4]. Freshness (staleness), on the other hand, refers to the age of the query results snce the last query reevaluaton. It s dependent on the frequency of query Avalable onlne@ 470

2 Dodda Sudeep et al,int.j.computer Technology & Applcatons,Vol 3 (1), re-evaluatons performed at the server. As moble nodes contnue to move, there are further devatons n moble node postons after the last query re-evaluaton. However, such devatons are not attrbuted to naccuracy. Hence, freshness can be seen as a metrc capturng the post-query-re-evaluaton devatons n moble node postons. It s mportant to note that hgher freshness does no tnecessarly mply hgher accuracy and vce versa. To obtan fresher query results, the CQ server must reevaluate the contnual queres more frequently, requrng more computng resources. Smlarly, to attan more accurate query results, the CQ server must receve and process poston updates from the moble nodes n a hgher rate, demandng communcaton as well as computng resources. However, t s almost mpossble for a moble CQ system to acheve 100% fresh and accurate results due to contnuously changng postons of moble nodes. A key challenge therefore s: How do we acheve the hghest possble qualty of the query results n both freshness and accuracy, n the presence of changng avalable resources and changng workloads of locaton updates and locaton queres? In ths paper, we propose MobQual a resourceadaptve and QoS-aware load sheddng framework for moble CQ systems. MobQual s capable of provdng hgh-qualty query results by dynamcally determnng the approprate amount of update load sheddng and query load sheddng to be performed accordng to the applcaton-level QoS specfcatons of the queres. An obvous advantage of combnng query load sheddng and update load sheddng wthn the same framework s to empower MobQual wth dfferentated load sheddng capablty, that s confgurng query reevaluaton perods and update naccuracy thresholds for achevng hgh overall QoS wth respect to both freshness and accuracy. generally use a threshold to reduce the amount of updates to be sent to the server and to lmt the naccuracy of the query results at the server sde below the threshold. Smaller thresholds result n smaller errors and hgher accuracy, at the expense of a hgher load on the CQ server. Ths s because a larger number of poston updates must be processed by the server, for nstance, to mantan an ndex [5]. When the poston update rates are hgh, the amount of poston updates s huge and the server may randomly drop some of the updates f resources are lmted. Ths can cause unbounded naccuracy n the query results. In MobQual, we use accuracy-conscous update load sheddng to regulate the load ncurred on the CQ server due to poston update processng by dynamcally confgurng the naccuracy thresholds at the moble nodes. Another maor load for the CQ server s to keep the query results up to date by perodcally executng the CQs over the moble node postons. More frequent query re-evaluatons translate nto ncreased freshness n the query results, also at the expense of a hgher server load. Gven lmted server resources, when the rate of query re-evaluatons s hgh, the amount of queres to be re-evaluated s vast and the server may randomly drop some of the re-evaluatons, causng stale query results (low freshness). In MobQual, we utlze freshness conscous query load sheddng to control the load ncurred on the CQ server due to query reevaluatons by confgurng the query re-evaluaton perods. In general, the total load due to evaluatng queres and processng poston updates domnates the performance and scalablty of the CQ server. Furthermore, the tme-varyng processng demands of a moble CQ system entals that update and query load sheddng should be dynamcally balanced and adaptvely performed n order to match the current workload wth the server s capacty. 2.LOAD SHEDDING IN MOBILE CQ SYSTEMS In a moble CQ system, the CQ server receves poston updates from the moble nodes through a set of base statons (see Fgure 1) and perodcally evaluates the nstalled contnual queres (such as contnual range or nearest neghbor queres) over the last known postons of the moble nodes. Snce the moble node postons change contnuously, moton modelng [3], [4] s often used to reduce the number of updates sent by the moble nodes. The server can predct the locatons of the moble nodes through the use of moton models, albet wth ncreasng errors. Moble nodes Avalable onlne@ 471

3 Dodda Sudeep et al,int.j.computer Technology & Applcatons,Vol 3 (1), Fg. 1: Moble CQ system and load sheddng 3.THE MOBIQUAL APPROACH The MobQual system ams at performng dynamc load sheddng to maxmze the overall qualty of the query results, based on per-query QoS specfcatons and subect to processng capacty constrants. The QoS specfcatons are defned based on two factors: accuracy and freshness. In MobQual, the QoS specfcatons are used to decde on not only how to spread out the mpact of load sheddng among dfferent queres, but also how to fnd a balance between query load sheddng and update load sheddng. The man dea s to apply dfferentated load sheddng to adust the accuracy and freshness of queres. Namely, load sheddng on poston updates and query re-evaluatons s done n such a way that the freshness and accuracy of queres are non-unformly mpacted. From the perspectve of query load sheddng, we make two observatons to show that nonunform freshness n the query results can ncrease the overall QoS of the moble CQ system: (1) Dfferent queres have dfferent costs n terms of the amount of load they ncur. (2) Dfferent queres have dfferent tolerance to staleness n the query results. Thus t s more effectve to shed load (by sacrfcng certan amount of freshness) on a costly query than an nexpensve one. Ths s especally benefcal f the costly query happens to be less strngent on freshness, based on ts QoS specfcaton. Bearng these observatons n mnd, n MobQual we employ QoS-aware query load sheddng: We use query re-evaluaton perods as control knobs to perform query load sheddng, where the same amount of ncrease n query re-evaluaton perods for dfferent queres brngs dfferng amounts of load reducton and QoS degradaton wth respect to freshness. have less strngent QoS specfcatons n terms of accuracy. Thus, n MobQual we employ QoS- aware update load sheddng: We use naccuracy thresholds from moton modelng as control knobs to adust the amount of update load sheddng to be performed, where the same amount of ncrease n naccuracy thresholds for dfferent geographcal regons brngs dfferng amounts of load reducton and QoS degradaton wth respect to accuracy. MobQual dynamcally mantans a throttle fracton, whch defnes the amount of load that should be retaned. It performs both update load sheddng and query load sheddng to control the load of the system accordng to ths throttle fracton, whle maxmzng the overall qualty of the query results. MobQual not only strkes a balance between freshness and accuracy by employng both query and update load-sheddng, but also mproves the overall qualty of the results by utlzng perquery QoS specfcatons to capture each query s dfferent tolerance to staleness and naccuracy. 3.1.Problem Formalzaton The obectve of the combned load sheddng problem s to maxmze the overall qualty of We now restate the processng constrant by combnng the load due to query re-evaluaton and update processng. Let zv denote the fracton of the query load retaned for a gven set of re-evaluaton perods {P}. We have: Smlar to query load sheddng, we make two observatons regardng update load sheddng to show that nonunform result accuracy can ncrease the overall QoS. Frst, dfferent geographcal regons have dfferent numbers of moble nodes and queres. Second, dfferent queres have dfferent tolerance to poston errors n the query results. Ths means that sheddng more updates from a regon wth a hgher densty of moble nodes and a lower densty of queres can brng a hgher reducton on the update load and yet have a smaller mpact on the overall query result accuracy. Ths s especally true f the queres wthn the regon Smlarly, let zu denote the fracton of the update load retaned for a gven set of naccuracy thresholds { }. We have: Wth these defntons, we can state the processng constrant as follows: Avalable onlne@ 472

4 Dodda Sudeep et al,int.j.computer Technology & Applcatons,Vol 3 (1), The parameter γ n the above equaton represents the cost of performng update processng wth the settng of,, = є compared to the cost of performng query re-evaluaton wth the settng of,p = τ. In other words, for the deal case the query re-evaluatons costs 1 unt, whereas the updateprocessng costs γ є (0, ] unts. Note that γ s not a system specfed parameter and s learned adaptvely as follows. Let U be the observed cost of update processng and V be the observed cost of query re-evaluaton durng the last adaptaton perod. Then we have γ = (U/zu)/ (V/zv). Ths assumes that the workload does not sgnfcantly change wthn the tme frame of the adaptaton perod. Recall that the load sheddng parameters are confgured after each adaptaton perod, thus yeldng new values for zu and zv (by way of changng P s and s). Thus the combned load sheddng problem s formalzed as follows: 3.2.The MQLS Algorthm The basc prncple of the MQLS algorthm s to start wth the deal case of, P = τ and, = є and ncrementally reduce the load to z tmes that of the deal case by repettvely ncreasng the reevaluaton perod or the naccuracy threshold that gves the smallest qualty loss per unt cost reducton. The algorthm s greedy n nature, snce t takes the mnmum qualty loss per cost step. Concretely, we partton the doman of re-evaluaton perods and naccuracy thresholds nto β segments, such that we ncrease the P s and s n ncrements of sze cv = (τ τ )/β and cu = (є є )/β, respectvely. The MQLS algorthm mantans a mn. heap that stores a qlpc (qualty loss per unt cost3) value for each reevaluaton perod and each naccuracy threshold. The qlpc value of a re-evaluaton perod (or an naccuracy threshold) gves the qualty loss per unt cost for ncreasng t by cv unts (or cu unts). The qlpc value s denoted by S for v query group C and have: u S for sheddng regon A. We The nomnators of the second components n the above equatons represent the changes n the qualty due to the ncrement, whereas the denomnators represent the changes n the cost. Note that the frst components of the above equatons are usedto normalze the costs n the denomnators, so that compared. S s and v S s can be When the MQLS algorthm starts, the current load expendture of the system, whch s the sum of the load due to update and query load sheddng approprately weghted by γ, s above our load budget mposed by the throttle fracton z. The algorthm teratvely pops the topmost element of the mn. heap and dependng on whether we have a re-evaluaton perod or naccuracy threshold makes the ncrement usng ether cv or cu. The qlpc value of the popped element s updated based on above equatons and s put back nto the heap unless no further ncrements are possble. The algorthm runs untl the load expendture of the system s wthn the budget or all the re-evaluaton perods and naccuracy thresholds ht ther maxmum value. In the latter case the load cannot be shed to meet the processng constrant and random droppng of ncomng updates as well as delay n query re-evaluatons wll unavodably take place. 4.CONCLUSION In ths paper we presented MobQual, a load sheddng system amed at provdng hgh qualty query results n moble CQ systems. MobQual has three unque propertes. Frst, t uses per-query QoS specfcatons that characterze the tolerance of queres to staleness and naccuracy n the query results, to maxmze the overall QoS of the system. Second, t effectvely combnes query load sheddng and update u Avalable onlne@ 473

5 Dodda Sudeep et al,int.j.computer Technology & Applcatons,Vol 3 (1), load sheddng wthn the same framework, through the use of dfferentated load sheddng concept. Fnally, the load sheddng mechansms used by MobQual are lghtweght, enablng quck adapton to changes n the workload. REFERENCES [1] B. Gedk, K.-L. Wu, P. S. Yu, and L. Lu, Processng movng queres over movng obects usng moton adaptve ndexes, IEEE Transactons on Knowledge and Data Engneerng, vol. 18, no. 5, [2] H. Hu, J. Xu, and D. Lee, A generc framework for montorng contnuous spatal queres over movng obects, n Proceedngs of ACM SIGMOD, [3] O. Wolfson, P. Sstla, S. Chamberlan, and Y. Yesha, Updatng and queryng databases that track moble unts, Dstrbuted and Parallel Databases, vol. 7, no. 3, [4] A. Cvls, C. S. Jensen, and S. Pakalns, Technques for effcent road-network-based trackng of movng obects, IEEE Transactons on Knowledge and Data Engneerng, vol. 17, no. 5, [5] S. Saltens, C. S. Jensen, S. T. Leutenegger, and M. A. Lopez, Indexng the postons of contnuously movng obects, n Proceedngs of ACM SIGMOD, AUTHORS PROFILE Dodda Sudeep, Pursung M.Tech(CS) from Nalanda Insttute of Engneerng & Technology,Sddharth Nagar, Sattenapall, Guntur Afflated to JNTUK, Kaknada, A.P., Inda. My research Interests are computer networks, Data Mnng. Bala Krshna, workng as Asst. Professor, Department of Computer Scence Engneerng at Nalanda Insttute of Engneerng & Technology,Sddharth Nagar, Sattenapall, Guntur Afflated to JNTUK, Kaknada, A.P., Inda. My research Interests are Data Mnng and Computer Networks. Avalable onlne@ 474