A Dynamc Load Balancng for Massve Multplayer Onlne Game Server Jungyoul Lm, Jaeyong Chung, Jnryong Km and Kwanghyun Shm Dgtal Content Research Dvson Electroncs and Telecommuncatons Research Insttute Daejeon, Korea {astrod, jaydream,jessekm, shmkh }@etr.re.kr Abstract. On-lne games are becomng more popular lately as the Internet becomes popular, game platforms become dverse and a ubqutous game envronment s supported. Therefore, dstrbuted game server technology s requred to support large numbers of concurrent game users smultaneously. Especally, whle game users are playng games, many unpredctable problems can arse, such as a certan server handles more server loads than recommended because many game users crowded nto a specfc regon of a game world. These knds of stuatons can lead to whole game server nstablty. In ths paper, global dynamc load balancng model and dstrbuted MMOG(Massve Multplayer Onlne Game) server archtecture are proposed to apply our load balancng algorthm. Many dfferent experments were carred out to test for effcency. Also an example of applyng real MMOG applcaton to our research work s shown. 1 Introducton On-lne games are becomng more popular lately as the Internet becomes popular, game platforms become dverse and a ubqutous game envronment s supported. Furthermore, as mult-platform game technology, whch allows one game to be played n a dfferent platform smultaneously, advances, development of dstrbuted game server technology s requred to support large numbers of concurrent game users safely. MMOG(Massve Multplayer Onlne Game), n whch large number of users play a game n the same game world, s a bg genre of heavy server loads because of game event handlng, NPC(Non Player Character) control and persstency n game world management. Generally, MMOG dstrbuted game server system s used to handle large numbers of game users, and to provde the necessary consstency to provde game users the same feelng of the same game world, the bggest computatonal power and network bandwdth s used [1]. In realty, n on-lne games, when a certan server load becomes very bg to the other servers by an unpredctable case, caused by many gamers crowded n a specfc regon of a game world, the whole server can become unstable. Ths can cause response tme delay, game server nstablty etc. and results n an undesrable
stuaton to mantan consstency. In ths case, many dfferent ways of load balancng can solve these problems [1][9][10]. A global load balancng technque whch mnmzes the load dscrepancy by montorng the whole load and a local load balancng technque whch dstrbutes the load to the nearest server usng threshold values are researched for a dynamc load balancng technque[2][4][11]. In ths paper, to support very large numbers of smultaneous game users, we propose a herarchcal structured dstrbuted server archtecture, and global a dynamc load balancng model. The rest of ths paper s organzed as follows: Secton 2 explans MMOG game server system buldng method for dynamc load balancng; Secton 3 defnes a dynamc load balancng model whch can be appled to a dstrbuted server. Secton 4 shows the formaton of suggested dstrbuted server archtecture and a dynamc load balancng model test s carred out. Ths shows an example of a real applcaton appled to ths system. Fnally, secton 5 concludes wth our results and dscusses shortcomngs. 2 Dstrbuted MMOG Server system Fg. 1. MMOG structure that has three network layers 2.1 Server Archtecture Because n Dstrbuted MMOG Server multple game servers support one game, t s mportant to dstrbute the game load effectvely to maxmze effcency. Generally after splttng each game server nto regonal groups, game users belongng to a splt
area and NPC (Non Player Character) are handled. MMOG Server load s proportonate to the user numbers belongng to the splt area. As llustrated n Fg.1, to form Dstrbuted MMOG Server the followng are necessary: World Server dstrbutes and manages a game world; Zone Server handles game processes n a gven game space; Logn Server authentcates users who partcpate n the same game. Also, a Gateway Server s requred, whch handles nternal server changes such as server's regonal processng area change flexbly by controllng the network communcaton between User and server. World Server dvdes game spaces by the number of Zone Servers. It uses a redvdng method of dvdng the dvded areas agan after dvdng nto a maxmum number of 4 game spaces. It s managed to quad tree type. Fg.2 shows the splttng order of game spaces and allocated processng areas accordng to connectng orders of Zone Server. Fg. 2. Game space dvded by the connectng order of Zone Server and management of quad tree 2.2 Dvson and management of game space Users n MMOG partcpate n a game after choosng an avatar that represents tself, and whle playng a game t sends ts nformaton or receves other characters' nformaton through communcaton to the server. Because the MMOG Server restrcts each avatar to exchange nformaton to avatars wthn an AOI (Area Of Interest) t reduces severe network load and the total amount of network transmsson [10][12][13]. For dstrbuted processng of game space form a game world to regular
defned sze cell. If the cell sze s set up to the same sze of AOI radus of an avatar, the AOI of an avatar s defned to be a collecton of 9 cells that conssts of a cell that t belongs to and cells that are adjacent to t as shown n fgure 3. Fg. 3. The nterest area of avatar A s a game space dvded nto 30 cells (the crcle formed at the center of A s the Vsual feld and the 9 shaded cells are the Interest area) 3. Dynamc Load Balancng Model 3.1 Characterstcs of MMOG Server Load The purpose of Dynamc load balancng s to mnmze the load dfferences by evenly controllng the loads between servers by real tme montorng and analyzng the state of the game server. For real tme analyss of the server load, frst t s mportant to understand the crtcal factors that affect the server load of MMOG Server. For real tme analyss of the server load, frst t s mportant to understand the crtcal factors that affect the server load of MMOG Server. Frstly, the process loads n the server, whch are known as the W(work load), due to the management and process of enttes. Secondly, C(communcaton load) due to the nteracton of enttes n boundary regons n multple servers. Lastly, M(management load) due to doman management n the vrtual worlds.[1,3]. Management load s not drectly related to the numbers of users. It s a default load generated by the allocated game space and s a very small load compared to the work load and communcaton load. The boundary domans should be remaned n straght lne so that an average communcaton load can be mnmzed n boundary domans.
Work load and management load of MMOG Server load s generated defntely and proportonally accordng to the numbers of users and scale of the server area, but the communcaton load s vared by how the boundary area s defned. Consequently, ths statement ndcates a method for mnmzng average of C, whch s a prmary factor of loads n communcatons wth other servers under the statement of "dstrbuton of enttes n vrtual world s unpredctable." If we assume that dstrbuton of enttes n vrtual world s unform, then C can be mnmzed when the boundary between partton regons s mantaned n straght lne and such characterstc mproves an applcaton effcency of managng technque and dynamc load balancng model n vrtual world. Fg.4 llustrates the case of 2 to 4 of parttons n vrtual world. Fg. 4. Mnmzaton of average of C 3.2 Server Load Defnton As defned n 3.1, through work load, communcaton load and management load, th server load L can be L = aw + bc + cm wth a + b + c = 1. (1) In ths Formulas, the terms a, b, and c are weght factors W, C, M, respectvely and ther sum must be 1. Through usng Formulas.1, we are able to acqure a process effcency enhancement such as an enhancement of server's response tme f we mantan the managng regon of a server as an optmzed state. The dynamc load balancng model for enhancng the process effcency of a server can be defned by applyng reflexve 4-parttonng method n the prevous secton. If we dvde the vrtual worlds usng 4-parttonng method, we get reflexve load balancng for 4 parttoned regons n maxmum. If dynamc load balancng for maxmum 4 exstng parttoned regons s defned, an effcent model s also defned wthout consderaton of sze of vrtual world or number of servers. Fg.5 llustrates reflexve operaton for dynamc load balancng n reflexve 4 parttonng method. In dynamc load balancng,
t s started wth Depth 0, Depth1,, Depth n, reflexvely, and optmzed load devaton n each depth unforms the load devaton n between servers. Fg. 5. Reflexve operaton of load balancng. Because fndng an optmzed server processng regon s to mantan each partton wth unform loads, ths can be restate that mnmzaton of standard devaton n load of the partton n th server L. Thus, f we set the number of partton n a specfc depth as d ( 4 ) and an average of load L k n k th partton as load devaton SD k for each partton s * L k, then standard SD d * 2 = ( L L ) d wth d ( 4 ) k k k / = 1 (2) At ths tme, * k can be obtaned by mnmzng SD. d * k = 1 2 mn( SD k ) = SD = ( L * L * ) / d k k * (3)
3.3 Smplfed load searchng technque Fg. 6. Smplfed load searchng technque. For actual mplementaton for such dynamc load balancng model, the load devaton of entre vrtual world s nspected and then the partton k * s found usng * Formulas.3. However, t s dffcult to nstantly fnd optmzed partton k when the vrtual world s gettng larger. Thus, we beleve that applcaton of optmzed partton searchng technques that sequentally searches for next optmzed partton based on current partton wll enhance the applcaton possblty and processng effcency of proposed dynamc load balancng model [4]. Fg.6 llustrates a Smplfed load searchng technque based on our dea. 4 Expermental result In ths secton, we present experment for the dynamc load balancng archtecture that ncludes both the dstrbuted MMOG server archtecture n secton 2 and the dynamc load balancng model n secton 3. We used 25 x 25(cell) szed game world and dstrbuted 5000 vrtual clents n a unform dstrbuton, a skewed dstrbuton, and a cluster dstrbuton respectvely [7]. We carred out the experment wth the followng condtons. Because most of Dstrbuted MMOG Server load occurred at network process, management load s gnored by settng c=0 n Formula.1.
Work load and communcaton load s defned as the number of network transmssons created by one avatar. So, when the Gateway Server undertakes network broadcastng, the work load s defned by the number of users wthn splt areas. Also, the communcaton load s the total number of exstng servers (excludng the server that tself exsts n) wthn AOI of each avatar. Therefore, a=0.5, b=0.5, and we expermented th server load as Formula.4. L = 0.5 W + 0. 5 C (4) 4.1 Unform Dstrbuton Wth unform dstrbuton, the measured server load decreased to a narrow range from the pont when dynamc load balancng was appled, but the server response tme that shows real game server performance was ncreased by a narrow range or showed a smlar level. The result of ths experment shows that dynamc load balancng can even ncrease the server load n a condton where users are dstrbuted unformly. 250 Unform Dstrbuton 30.00 225 25.00 average response tme (ms) 200 175 150 125 20.00 15.00 10.00 5.00 load standard devaton 100 0.00 1 9 17 25 33 41 49 57 65 73 81 89 97 105 113 121 129 137 145 153 161 169 177 dynamc load balancng appled tme(sec) average response tme load standard devaton Fg. 7. Changes of Server load evaluaton value (blue lne) and response tme (red lne) when users are dstrbuted unformly
4.2 Skewed Dstrbuton Wth skewed dstrbuton, the measured server load decreased by a large range from the pont where dynamc load balancng was appled, and the server response tme also decreased by a narrow range. It means that dynamc load balancng mproves general server performance when users are concentrated n a specfc game space. Skewed dstrbuton of Game clents s often seen n real game stuaton. 7100 Skewed Dstrbuton 1400.00 6100 1200.00 average response tme (ms) 5100 4100 3100 2100 1000.00 800.00 600.00 400.00 load standard devaton 1100 200.00 100 1 9 17 25 33 41 49 57 65 73 81 89 97 105 113 121 129 137 145 153 161 169 177 0.00 dynamc load balancng appled tme(sec) average response tme load standard devaton Fg.8. Changes of Server load evaluaton value (blue lne) and response tme (red lne) when users are n a skewed dstrbuton 4.3 Clustered Dstrbuton The clustered dstrbuton case showed that server load evaluaton value and server response tme decreased at the same tme, whch means that these results were smlar
to the skewed dstrbuton case. Especally, estmated load evaluaton value and response tme can be verfed that dynamc load balancng model shows better performance wth a skewed dstrbuton than a clustered dstrbuton. The results of ths experment ndcate that, wth a real MMOG game, ths research can be appled effcently n a stuaton where game users were frequently crowded n one area. 460 Clustered Dstrbuton 800.00 415 700.00 average response tme (ms) 370 325 280 235 190 600.00 500.00 400.00 300.00 200.00 load standard devaton 145 100.00 100 1 9 17 25 33 41 49 57 65 73 81 89 97 105 113 121 129 137 145 153 161 169 177 0.00 dynamc load balancng appled tme(sec) average response tme load standard devaton Fg.10. Changes of Server load evaluaton value (blue lne) and response tme (red lne) when users are n a clustered dstrbuton 5 Concluson In ths paper we present the dynamc load balancng archtecture whch ncludes both dstrbuted MMOG server system and dynamc load balancng model. Ths research was appled to real MMOG game and verfed server performance through the smulaton of a manfold stuaton that used massve vrtual clents. As shown n the experment's results, the server system effcency was ncreased when game clents were n a skewed or clustered dstrbuton n a game world that s smlar to a real
game stuaton, and our server archtecture supported server load balancng effcently whch changed n real tme. Also, as shown n Fg.11, ths research was verfed by applyng the research products to a commercal game product. However, although the smplfed load searchng technque n secton 3.3 decreased loads concerned to searchng optmzed world partton, t sometmes exposed the problem of abnormal partton result for local optmzaton problem. Nevertheless, the postve results of our experments show that the load balancng archtecture appled to real MMOG server and can also expand ts effect to general networked vrtual envronment such as mltary tranng, busness etc. In future work, we wll mprove a real-tme load searchng technque and apply varous shapes of cell such as a brck or hexagon. Fg.11. Appled mages n Elma (commercal on-lne MMOG game contents)
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