Design and Implementation of an Energy Efficient Multimedia Playback System

Transcription

1 Desgn and Implementaton of an Energy Effcent Multmeda Playback System Zhjan Lu, John Lach, Mrcea Stan, Kevn Skadron, Departments of Electrcal and Computer Engneerng and Computer Scence, Unversty of Vrgna Charlottesvlle, VA 94 {zl4j, jlach, Abstract Moble devces capable of multmeda playback have become popular consumer tems, makng technques for energy management durng multmeda decodng ncreasngly mportant. In ths paper, we model the multmeda decodng process as a dscrete-tme system excted by random nput sequences representng the ncomng stream and therefore, unlke many exstng technques, do not make any assumptons about the workload streams. Usng ths novel stochastc process model, we can apply formal methods to analyze the decodng system and desgn a feedback-based on-lne dynamc voltage/frequency scalng (DVS) algorthm to effectvely reduce the energy consumpton durng multmeda playback. We mplemented our technque n a laptop computer equpped wth a DVS enabled processor, and results reveal good performance for real-world vdeo clps across a wde range of vdeo compresson formats compared wth exstng technques.. Introducton Whle the contnuously ncreasng computaton power provded by technology scalng enables more complex moble applcatons, energy consumpton becomes a lmtng factor. Snce multmeda playback s among the domnant applcatons n moble devces, t s mportant to desgn energy effcent multmeda playback systems. Fortunately, due to the varatons n multmeda decodng complexty, the requred computaton power changes durng the playback, and dynamc voltage/frequency scalng (DVS) s a powerful technque to explot ths opportunty to reduce runtme power by lowerng down the crcut speed (and thereby power) whenever possble. However, DVS multmeda systems should be carefully desgned to avod causng large degradaton n playback qualty. In ths paper, we systematcally analyze the desgn of such a system, propose a new desgn technque and valdate our desgn through a prototype system on a DVS enabled laptop computer. Ths work s supported n part by the Natonal Scence Foundaton under grant Nos. EHS-456, CCF-49765, CCR (Career) and EHS Desgnng a good multmeda playback system usng DVS nvolves trade-offs among multple contradctng objects and s subject to practcal constrants. Technques proposed n [5, 6, requre perfect predctons for decode executon tmng, whch s not possble n some multmeda compresson formats. The methods n [7, 8 explot bufferng to mprove the energy effcency of DVS. However, smaller buffer szes mght ncrease the frame deadlne mss rate, whle larger buffer szes not only ncrease hardware costs but also ncrease the playback latency, whch s unacceptable n many real-tme applcatons. In general, a practcal soluton has to seek the best trade-off between power consumpton, playback qualty and hardware resources and, n general, has to assume no specfc nformaton about the ncomng multmeda streams. Power (W) Normalzed cpu clock frequency Fgure. MPEG decodng power at dfferent CPU speed. The DVS desgn presented n ths paper provdes a good balance between all these competng factors. We frst create a model for the avalable desgn space. Then we search the desgn space wth two desred propertes n mnd: speed schedule unformty and system stablty. Power consumpton s a convex functon of crcut speed. As an example, n Fgure, the MPEG decode power consumpton of a laptop computer s shown at dfferent CPU speeds (.e., frequency and voltage settngs). Though each frame can be decoded at a dfferent speed due to computaton varatons, decodng multple frames at a unform speed (.e., the average decodng speed) provdes better energy effcency accordng to Jensen s nequalty [9. Therefore, n an energy effcent

2 Decode tme (ms) /f (a) I frame P frame B frame S frame Average frame dsplay tme (ms) H.64 (844x48) SVQ3 (64x344) MPEG4 (7x48) MPEG (7x48).5.5 (b) /f Average dther tme per frame (ms) H.64 (844x48) SVQ3 (64x344) MPEG4 (7x48) MPEG (7x48) (c) /f Fgure. Tmng characterstcs of MPEG vdeo playback. DVS desgn, a unform speed schedule s preferable. On the other hand, due to the complexty n many modern multmeda codng formats, a good DVS soluton should not assume any pror knowledge or rely on any accurate predctons about the decodng process. Therefore, the decode speed decsons should be solely dependent on the results of the prevous frames. Any system workng n ths way s n fact a closed loop feedback system for whch stablty s an mportant property. The rest of the paper s organzed as follows. In Secton, we present some vdeo playback tmng characterstcs from actual measurements of vdeo clps wth varous compresson formats. Based on these observed tmng characterstcs, n Secton 3, we frst propose a generc archtecture to model the dynamcs of a varable speed vdeo decodng system. Then we dscuss the desgn of the speed controller based on speed unformty and system stablty, and we propose an enhanced feedback based speed control scheme. In Secton 4, we descrbe our mplementaton of varous DVS technques for vdeo playback on a DVS enabled platform. Fnally, Secton 5 presents the performance comparson of dfferent DVS technques on a set of clps wth varous vdeo formats and shows the effectveness of the new technque proposed n ths paper.. Tmng characterstcs of MPEG workloads In an MPEG playback applcaton, there are two major tasks: frame decodng and frame dsplay. Though dfferent vdeo compresson formats have qute dfferent mplementaton detals, there are several common steps n frame decodng: a. Huffman decodng, b. IDCT and moton estmaton/compensaton, and c. frame dtherng (.e., convertng the decoded frame from the YUV plane to the RGB plane). The decoded frame n RGB format s then sent to a graphcs devce that fnshes the dsplay task. There are several frame types dependng on the operatons needed to encode/decode a frame: I (ntra-coded) frame, P (predctve coded) frame, B (bdrectonally predctve-coded) frame and S (sprte) frame (coded usng global moton estmaton) n MPEG4. Fgure (a) plots the decodng tme (ncludng dtherng) of dfferent frame types versus the nverse of decode speed (.e., CPU clock frequency). Ths fgure ndcates that the decodng tme of a frame s strongly dependent of the decodng speed and can be modeled as t = k u + m, where t s the decode tme, u s the CPU speed, and k and m are model parameters representng computaton tme and memory access tme, respectvely, durng decodng. k s usually several tmes larger than m, whch mples that MPEG decodng s a computaton-domnated applcaton. On the other hand, frame dsplay tme s almost nsenstve to CPU speed as llustrated by Fgure (b). Ths suggests that energy savng opportuntes are avalable durng frame dsplay snce one can lower the CPU frequency and voltage when the frame s sent to the screen wthout affectng playback qualty. Choet al. [6 made a smlar observaton n ther study. They also reported that the tme spent n frame dtherng s also nsenstve to CPU speed. We plotted the frame dtherng tme at dfferent CPU speeds n Fgure (c) and found that the dtherng tme n the vdeo clps we tested s actually strongly dependent on CPU speed. 3. Desgn and analyss of feedback based speed control 3.. Model of DVS multmeda decode systems Most exstng onlne DVS decodng schemes [5, 6, 7, 8 determne the requred decodng speed for future frames accordng to the decode tmng nformaton of prevous frames. In ths paper, we frst propose a dynamc system model, shown n Fgure 3, to represent a desgn space contanng many of these exstng schemes. In order to explan ths model more clearly, we ntroduce the concept of frame slack tme, defned as the tme nterval between when the frame decodng begns and the frame deadlne (the tme to dsplay the frame). The slack tme for frame n s the maxmum avalable tme for ts de-

3 { k( n), m( n)} u( n) k( n) s( n + ) = s( n) + T + m( n) u( n) Speed decson u( n) r c( n) Frame decoder c( n) s( n) Control rule = c( n) = y( s( n), s( n ), s( n ),...) Fgure 3. A generc dynamc system model for onlne DVS MPEG decodng. codng, denoted by s(n). As dscussed n Secton, the decodng tme of a frame can be expressed as k(n) u(n) + m(n). Let T represent the frame dsplay nterval durng playback, and we can update the slack tme for the next frame by s(n+) = s(n)+t [ k(n) u(n) +m(n), as represented by the Frame decoder block n Fgure 3. By modelng the decodng tme for a frame as a random varable, an MPEG stream can be represented by a sequence composed of random varables k(n) and m(n). Therefore, the MPEG decodng process can be modeled as a dscrete tme system excted by a stochastc nput sequence as shown n Fgure 3. The feedback path n Fgure 3 represents a generc approach to decode future frames based on prevous frames, as used n many exstng onlne DVS MPEG decodng schemes. Ths generc model makes t possble for us to explore the desgn space more systematcally. The major dfference n varous onlne schemes les n the parameter r and the control functon y(). For example, n the frame based schemes [5, 6, c(n) = s(n) m (n) k (n) and r =, where k (n) and m (n) are the predctons for k(n) and m(n) of frame n, based on prevous decodng results of s(n),s(n ),... In the scheme proposed by Im and Ha [7, r = WCET and c(n) = s(n) where WCET denotes the worst-case frame decodng tme. When a formal PI controller s adopted as the control rule as the one proposed by Lu et al. [8, the control functon y() has the form: c(n) = n and r =, where k k p p(s(n) s )+k = s() s and k are the proportonal and ntegral gans, respectvely, and s s the setpont of the controller nput. 3.. Desgn of an enhanced feedback control scheme wth speed unformty Snce unform speed provdes better energy savngs when the decodng throughput s matched wth that of dsplay, speed unformty s an deal property for any DVS scheme. Keepng ths n mnd and usng the dynamc model shown n Fgure 3, t s possble to desgn a better feedback based speed control scheme for MPEG decodng. Assumng that the system descrbed n Fgure 3 s operated under a constant speed, the expected value of random varable s(n) (.e., the frame slack tme), denoted by S(n), satsfes the followng relaton: [ S(n + ) = E[s(n + ) = E s(n) + T [ k(n) u(n) + m(n) = E[s(n) = S(n) [ snce E T [ k(n) u(n) + m(n) =, whch s requred by the system steady state condton that the decodng throughput s matched wth the dsplay throughput. Therefore, n the desred DVS schedule, both the decodng speed and the average value of frame slack tme are constant. Snce the frame slack tme s observable n the system, we propose to use the average slack tme to determne the decodng speed,.e., a control rule c(n) = y(s(n)) and r =. Usng ths control rule, the feedback system can be descrbed by a dfference equaton: s(n + ) = s(n) + [T [y(s(n))k(n) + m(n) () Assumng that {k(n), m(n)} are IID (ndependent, dentcal dstrbuton) random sequences and E[k(n) = K,E[m(n) = M, one can apply the expected value functon on both sdes of Equaton (). S(n + ) = S(n) + [T [y(s(n))k + M In the steady state, S(n + ) = S(n) = S and T = y(s)k + M, and t follows that the steady decodng speed s U = y(s) = K T M The expected value of s(n) can only be estmated from the decodng results {s(n),s(n ),...}. We adopt a smple movng average functon, s (n) = s(n)+s(n +...+s(n +)) as the estmaton of E[s(n), n whch s the wndow wdth of the movng operaton and s (n) s also a random varable. Durng runtme, even f the decodng speed s correctly chosen for frame n (.e., u(n) = K T M ), u(n + ) could be dfferent due to the dfference between s (n + ) and s (n). The varaton of s (n) can be estmated usng the standard devaton of s (n) = s (n) s (n ). Let σ() denote the standard devaton functon, and t can be shown that: σ( s (n)) = σ(t [ k(n) U = σ (k(n)) U + m(n)) + σ (m(n)) σ(k(n)) U because σ(k(n)) s usually much larger than σ(m(n)), as found from the tmng measurements of some MPEG clps Frames n a clp are decoded n a predefned order specfed by GOP (Group of Pctures). Therefore two consecutve frames are not necessarly statstcally ndependent. The IID assumpton only serves to reduce the analyss complexty. () (3)

4 n Secton. Therefore, the varaton n speed decson u(n) = u(n) u(n ) due to the varatons n frame decodng tme can be estmated accordng to u(n) = y(s (n) : σ ( u(n)) y (s (n)) y (s (n) σ ( s (n)) (4) If we can assume that the varatons of the amount of computaton n frame decodng s proportonal to the average amount of computaton, we have σ(k(n)) K. It follows that σ( s (n)) s ndependent of s (n) (.e., current speed) because of Equaton () and (3). We would lke to choose a functon y(x) such that σ ( u(n)) s also ndependent of the current decodng speed or s (n). Accordng to Equaton (4), ths requres that: y (x) y (x) = C where C s a constant. Thus, the control functon y() n Fgure 3 has the form c(n) = y(s (n)) = as (n)+b, where a and b are two constant parameters to be determned. In other words, we have derved a smple lnear control rule to determne the next frame decodng speed as c(n) = y(s (n)) = as (n)+b and u(n) = c(n) = as (n) + b. Essentally, ths s a P (proportonal) controller. The parameters a and b can be determned accordng to practcal constrants such as avalable buffer sze and decodng speed (clock frequency) range. For example, we can conservatvely set the full speed when the frame slack tme s less than the dsplay perod and set the mnmum speed when the dsplay buffer s full. Let B denote the number of avalable dsplay buffer szes, we have Max(s (n)) = (B + ) T. These settngs requre that: { a T + b = umax ; a [(B + )T + b = u mn. where u max and u mn are the maxmum and mnmum decodng speeds provded by the system. The values of a and b can be determned accordngly and obvously a < Stablty analyss of the proposed feedback control system In the above analyss, we assume the nput sequences {k(n), m(n)} are statonary (.e., K and M are ndependent of tme). Durng move playback, we fnd that some clps show strong phased behavors. In order to provde both good playback qualty and low energy consumpton, (5) (6) E + D( n) + [ c( n) C(z) [ ( + ) = [ ( ) [ ( ) E s n E s n K E c n [ ( ) = [ '( ) c E c n l E s n E S(z) F(z) [ s( n) [ s '( n) [ ( ) + [ ( ) + + [ ( + ) E s n E s n E s n E[ s'( n) = Fgure 4. Modelng the transent behavors of the proposed closed loop feedback system. A pulse nput D(n) models the dsturbance caused by the change n the requred decode computaton power. the feedback system should be able to adapt the speed decson to the new set of K and M. Ths requres the system to have a fast response tme and be stable. The proposed feedback control system can be analyzed usng well establshed lnear system analyss technques (e.g., transfer functons). In order to do so, we frst lnearze the proposed control functon (Equaton (5)) usng the frst order Taylor expanson: c(n) c(n ) + l c [s (n) s (n ) and (7) l c = y (E [s a (n )) = E [u(n ). Second, nstead of drectly analyzng the sgnals shown n Fgure 3, we focus on the sgnal dfference at consecutve tme steps, e.g., s(n) = s(n) s(n ), c(n) = c(n) c(n ), etc. In the steady state, the decodng rate matches the dsplay rate, and E[ s(n) =. When the requred computaton power s changed durng the playback (.e., the values of K (E[k(n)) and M (E[m(n)) change), E[ s(n) wll become non-zero and break the steady state. If the system s stable, E[ s(n) wll converge to zero agan and the system reaches a new steady state. The block dagram n Fgure 4 models ths dynamc process. The changes n K and M are modeled by njectng a dsturbance sgnal D(n) (usually a pulse sgnal) nto the system, as shown n Fgure 4. One mportant dfference between Fgure 3 and Fgure 4 s that the sequences n Fgure 4 are formed by applyng the expected value functon to those stochastc sequences n Fgure 3, because only expected values of those random varables are amable for analyss usng transfer functons. Other dfferences between these two fgures nclude:. the blocks Frame decoder and speed decson n Fgure 3 are combned nto one block denoted by S(z), and. one more block s added to model the movng average functon ntroduced n the proposed feedback system. It can be verfed that the transfer functons n Fgure 4 are: S(z) = K z, C(z) = l c, and F(z) = E

5 z +z z. Consequently, the relaton between D(n) and E[ s(n) can be created usng ther Z transforms. Z{E[ s(n)} S(z) Z{D(n)} = S(z)C(z)F(z) Kz = z ( Klc )z + Klc (z +z ) Therefore the characterstc equaton for the closed loop system n Fgure 4 s z ( Kl c )z + Kl c (z +z ) = (8) The system s stable f and only f the roots of Equaton (8) are wthn the unt crcle. The magntude of the roots determnes the convergng speed (.e., response tme) of the system. Fgure 5 shows the root wth the maxmal magntude of Equaton (8) at dfferent values of Kl c and. As ndcated by the fgure, as the movng averagng wndow sze () ncreases, the system tends to be unstable and reacts more slowly. Ths s expected as the new feedback sgnals are weghted less wth larger wndow szes. From (7), we have Kl c K a u max u mn Bu mn u mn, because of a = umax umn BT at u mn = from (6) and assumng K T. Thus, the stablty condton for the system under study can be wrtten as: u max u mn Bu mn < P (9) where there are roots on the unt crcle when Kl c = P n Equaton (8). Strctly speakng, condton (9) only guarantees the stablty of the lnearzed closed loop system shown n Fgure 4 and, consequently, the stablty of the orgnal system when the system s operatng near the steady state. However, condton (9) does not necesstate the global stablty of the orgnal system due to the non-lnear control functon (Equaton (5)). The study of the global stablty of the closed loop non-lnear system s outsde the scope of ths paper. In practce, we found that condton (9) s suffcent to ensure stablty n our expermental system Real-tme guarantee Hgh vdeo playback qualty s accomplshed by ensurng that frames are decoded before ther deadlnes. In a feedback based DVS decodng system (e.g., the system shown n Fgure 3), real-tme guarantees are acheved by the control functon c(n). Specfcally, for the proposed control functon n Equaton (5), we fnd that when =, no frames wll mss ther deadlnes f the worst-case decodng tme at full speed s less than the dsplay nterval T and: B u max u mn u mn () n whch B s the dsplay buffer sze. When >, condton () no longer provdes hard realtme guarantee. But t stll provdes helpful reference for choosng the correct desgn parameter Trade-off analyss on desgn parameters For a practcal DVS system provdng varable speed between u max and u mn and a target dsplay rate (or dsplay nterval T ), there are two desgn parameters for the proposed feedback based decodng system: dsplay buffer sze B and movng average wndow sze. Accordng to Equatons (4) and (5), the decodng speed varaton can be rewrtten as σ ( u(n)) aσ ( s (n)) where a s the control functon parameter n Equaton (5) and σ ( s (n)) s defned n Equaton (3). Equatons (6) and (3) ndcate that larger B and result n smaller a and σ ( s (n)), respectvely, and therefore smaller speed varaton, whch s preferable for energy savngs. In addton, larger dsplay buffer szes help satsfy the stablty condton (9) and reduce frame deadlne msses as mpled by Inequalty (). On the other hand, there are dsadvantages for larger values of B and. Larger dsplay buffer szes not only ncrease hardware costs, but they also ncrease the playback latency because B + frames are decoded ahead of dsplayng n the worst-case. Ths s especally undesrable n some realtme multmeda applcatons. The dsadvantages of larger values of can be seen from Fgure 5. A larger tends to ncrease the magntude of the roots of the characterstc equaton of the feedback system, thereby ncreasng the system response tme. It also reduces the stablty range of Kl c, causng the system to be unstable. 4. Implementaton on a hardware platform Our prototype platform s a Compaq notebook computer equpped wth a moble AMD Athlon XP DVS enabled processor. The CPU clock frequency can be dynamcally scaled to 4 dscrete speeds between 665MHz and 53MHz by wrtng the voltage/frequency settng to a model specfc regster (MSR). Durng the experments, we pull out the notebook battery and put a small sense resstance (mω) n seres wth the laptop. The voltage drop on the sense resstance s amplfed and sampled at a 5K sample rate by a desktop computer equpped wth a data acquston card and LabVIEW software. The vdeo playback software s mplemented usng two processes and runs under the Lnux operatng system. One process of the software s responsble for fetchng coded frames from the hard dsk, decodng the frame and puttng the decoded frame n the dsplay buffer. The other process s responsble for dsplayng frames on the screen by fetchng a frame pcture from the buffer and sendng t to the X server. The dsplay process s n sleep mode most of tme and s perodcally woken up by a tmer that repeatedly

6 = = 3 = 5 Imag Axs Klc: Magntude Imag Axs Klc: Magntude Imag Axs Klc: Magntude Real Axs.5.5 Klc.5.5 Real Axs.5.5 Klc.5.5 Real Axs.5.5 Klc (a) (b) (c) Fgure 5. Roots wth the maxmal magntude as functons of Kl c. (a) =. (b) = 3. (c) = 5. The plots on the left show the locatons of the roots n the unt crcle and those on the rght show the magntude of the roots. tmes out at the dsplay nterval (.e., the nverse of the playback rate). Frame deadlne msses occur when the dsplay process tres to fetch a frame pcture from an empty buffer. When the dsplay process s woken up, the decode process s suspended and resumed after the dsplay process s fnshed. At the begnnng of decodng a new frame, the decode process determnes the decodng speed accordng to the specfc DVS scheme and changes the CPU frequency/voltage usng a system call. The open source codec lbrary ffmpeg [ s used n the decode process. Thus, vdeo clps wth a wde range of dfferent formats can be played n ths software. 5. Expermental results We mplemented varous DVS MPEG decodng schemes n our hardware platform and tested them on a set of eght vdeo clps wth dfferent compresson formats, ncludng MPEG/4, H.64 and SVQ (QuckTme move format). Among the clps, sx are move tralers downloaded from the Internet [, 3, 4. The other two, (fs3 and fs4), are home-made moves. The varous DVS schemes mplemented are smlar to those examned n [8 plus the new feedback scheme ntroduced n ths paper: Full speed The CPU always decodes frames at full speed just as n the systems wthout DVS capabltes. The CPU becomes dle untl the decoded frame s dsplayed. Therefore the dsplay buffer sze n the scheme s. Ideal perod Usng proflng nformaton for the vdeo streams, the processor calculates the correct decodng speed such that the decodng tme for each frame s exactly equal to the dsplay perod and rounds up ths calculated speed to the avalable dscrete speed provded by the hardware. Ths scheme s equvalent to the one n [6 when ther predctons about frame decode tmng are always accurate. Optmum In ths scheme, an off-lne schedulng algorthm s developed to fnd the best schedule such that no frame wll mss ts deadlne whle total energy consumpton s mnmzed. The profled tmng nformaton for all frames has to be used to fnd the optmal schedule, and ths scheme sets up an achevable energy consumpton lower bound wth DVS. Ths scheme was frst proposed n [8 and later revsted n [. Panc Ths scheme s smlar to that proposed by Im et al. [7, whch tres to spend the avalable slack tme n decodng the next frame and assumes the decodng tme of the next frame s equal to WCET. Agan the calculated speed s rounded up to the next avalable dscrete speed. The dsplay buffer sze for ths scheme s 5. Dead-zone based feedback PI controller Ths scheme specfes a regon (dead-zone) for the number of decoded frames n the buffer. A PI controller s used to pull the system back to ths regon f the current number of frames n the buffer s out of the dead-zone. Followng the desgn n [8, n our mplementaton, the dead-zone s between 3 and 8 frames. When the system s wthn the dead-zone, the CPU s operated at a speed calculated usng the decode tmng nformaton from the prevous several frames [8. The dsplay buffer sze for ths scheme s. Lnear slack speed control Ths s the enhanced feedback DVS control scheme proposed n ths paper n whch the decodng speed for the next frame s a lnear functon of the averaged avalable slack tme as ndcated by Equaton ( 5). In our mplementaton, the averagng wndow sze s fxed to be 3, as t s a good trade-off between speed varaton and system response tme as dscussed n Secton 3. We also tested ths technque wth dfferent dsplay buffer szes and found that a fve-frame buffer would be a good desgn trade-off. Usng = 3, B = 5 and u max =.,u mn =.435 (the frequency range of our hardware platform), one can verfy that both the stablty condton (9) and real-tme condton () are satsfed. As revealed n Secton, the tme needed to dsplay a

7 frame on the screen s nsenstve to the CPU speed. Therefore, for the sake of far energy consumpton comparsons, n all of the above schemes, we scale the CPU speed to a fxed low frequency when the dsplay process tres to send the decoded frame to the screen. Energy consumpton deal perod panc p control lnearslack_buf5 optmum schedule msson game skeleton exorcsm fantastc kngdom fs3 fs4 Fgure 6. Total system energy consumpton for vdeo playback wth dfferent DVS schemes. The energy consumptons are normalzed to those for the full speed schedule. Fgure 6 plots the measured total system energy consumpton durng vdeo playback. As one mght expect, all DVS schemes can save sgnfcant energy consumpton compared to the case wthout DVS, and the optmum scheme acheves mnmum energy consumpton. The proposed lnear slack scheme wth buffer sze 5 performs roughly equal to or better than the rest DVS schemes n terms of energy savng. Table. Number of frames mssng deadlne wth dfferent DVS schemes vdeo clps panc (buf5) lnear slack (buf5) PI controller (buf) msson game skeleton 9 exorcsm fantastc kngdom 77 4 fs3 3 3 fs4 7 6 In the theoretcal analyss carred out n Secton 3, we assume that the worst-case full speed frame decodng tme s less than the dsplay nterval. However, durng practcal vdeo play back, there are some frames exhbtng extremely long decodng tmes. Therefore, a sgnfcant number of frames mght mss ther deadlnes. The Table lsts the number of frames mssng ther deadlne n each vdeo clp for dfferent runtme DVS schemes. Though both schemes have the same dsplay buffer sze, the lnear slack scheme does a much better job than the panc scheme to hde those extreme large frames. The PI controller scheme can further reduce the number of deadlne msses at the cost of twce the buffer sze. Gven that the total number of frames n each clp s 3, the deadlne mss rate of the lnear slack scheme s around % for most clps. 6. Concluson Although many low-power multmeda playback technques have been proposed, many of them suffer from dealzed assumptons or practcal constrants. The work descrbed n ths paper seeks a balanced desgn by frst creatng a model for the avalable desgn space, then dentfyng a preferable archtecture based on desrable system behavors, and fnally analyzng the desgn trade-offs usng formal methods. The mplementaton overhead of ths new DVS technque s ultra-low and can be easly mplemented n both software and hardware based decodng systems. We mplemented the proposed archtecture on a prototype hardware platform and compared t wth exstng technques. Expermental results show that the proposed new desgn acheves equal or better energy effcency than the exstng technques, brngs close to deal playback qualty (about a % deadlne mss rate), and only requres a moderate buffer sze (5-frame buffer). Therefore, the proposed new DVS multmeda decodng archtecture s ndeed a good trade-off among energy, playback qualty, hardware cost and playback latency. References [ FFmpeg project. [ Move tralers. com/enu/ndex.html. [3 Move tralers. [4 Move tralers. qucktme/gude/hd/. [5 K. Cho, K. Dantu, W. Cheng, and M. Pedram. Frame-based dynamc voltage and frequency scalng for a mpeg decoder. In Proceedngs of Internatonal Conference on Computer Aded Desgn, pages 73 37, November. [6 K. Cho, R. Soma, and M. Pedram. Off-chp latency-drven dynamc voltage and frequency scalng for an mpeg decodng. In Proceedngs of 4st Desgn Automaton Conference, pages , June 4. [7 C. Im and S. Ha. Dynamc voltage schedulng wth buffers n low- power multmeda applcatons. ACM Transactons on Embedded Computng Systems (TECS), 3(4):686 75, November 4. [8 Z. Lu, J. Lach, M. Stan, and K. Skadron. Reducng multmeda decode power usng feedback control. In Proc. of Internatonal Conference on Computer Desgn, pages , October 3. [9 W. Rudn. Real and Complex Analyss. McGraw-Hll, 987. [ Y. Tan, P. Malan, Q. Qu, and Q. Wu. Workload predcton and dynamc voltage scalng for mpeg decodng. In Proceedngs of Asa and South Pacfc Desgn Automaton Conference, pages 9 96, January 6.