times. in Java Implementation barcode data matrix in Java times.

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times. using jsp toget data matrix 2d barcode with web,windows application Use QR Codes safely Using the tools descri jboss datamatrix 2d barcode bed in the just referenced section, we can make starements conceming the distribution of this IeSpODSe time. Specifically, with the Gamma function, we can estimate the probability that the ~ of a transaction will complete in less than a given time, given the mean time and variance of the processing time. Here a transaction is taken to mean that processing done between sync:hronizing points.

If we have lie systems that will each complete a transaction in t seconds with ~ ability P6 = (.5)1IJIe, then the probability is [(.5)1IJIe)" = .

5 that all will complete the ttansaction in t seconds. Thus, this value of t seconds is the avaage IeSpODSe time for the group of lie systems. For instaDce, ifdRe systems are involved, we wiD want to know that time within wbich the ttaDsadion will complete with probability (.

5)113 = .79. Then we will know that all tbree will have completed their transaction in that time with probability .

79" = .5. (This mgun:aeDt ISSU""N that the RSpODSe times areiDdepeDdeDt.

To the extent that the!e is depeDdence among the .mpcmse times,. this appwach is c:oaservative.

) . Tbe Gamma functiOn gives us these times as a function of the variance of the IapoDSe time. Some typical valDes aregivaa below:.

TAREN. SYNOfRONIZATlON OVERHEAD k. Pm_sing elcmImIs (n.). (.5)"" (P.).

1.25 1.56 1.

83. 1.2 1.25 1.

541.78. "l"1wr(7) 1.4 1.6.

1.8 1.25 1.

41 1.67. 1.24 1.47 1.

65. 2 3 4. .71 .79 .

14. 1.25 1.52 1.

71. 1.25 1.70 In chapter 4 it is not ed that the distnbution of the respcmse time created by random arrivals at tandem queues with random service times is itself random. 1"heIefOIe, its. Chap. 9 Synchronization variance is equal to t jboss data matrix barcodes he square of tbemean response time. RandOm distrilmtion of response times is usually a conservative assumption; if all else fails, it can be used. This is the case of T2/var (7") = 1 in the above table.

However, chapter 4 also describes a technique for estimating the variance of the response time mote accu.tately. The example for a TP system given there yields a ratio of variance to response time-squared equal to .

628. From. the above table, the average response for four processing elements is 1.

83 multiplied by the average response of one element, if random response times are assumed (T 2/var (7") = 1). However, if the normalized variance is .628, then T 2/var (7") = 11.

628 = 1.6; the average four element response time is 1.70 times the average response of one element, or a 7 pereent performance improvement per our estimate.

Thus, it can pay to go through the exercise of more accurately estimating response time variance rather than to casually assume randomness of response times. On the other band, the values in the table above are closely enough clustered to give us a reasonable rule of thumb for synchrouized systems, as follows:. TABLE 9-3. SYNCHRONIZATION OVERHEAD No. of pmcessiDg elemads(~). Additicm to 1be average lespcmse time (~-1)("II). 2S SO Note tbat these result s apply to a transaction wbether there is one synchronizing point or n such points per ttaasac:OOD. To demoDstrate tbis, let. = IlUIDber of processing elements. n:spoDSe time factor for lie processing elements. ke = n" = DU1Ilber of syncb points per traDsaCtion. t,.

1 = average traDsaction-msp time for a siDgle element.. = uansaction-!eSpODSe time for the system. The respcmse time, tro for the system is the sam of the leSpODSe times ~ each synchronizing point:. tr = kerrl (9-1). (9-2). Fault Tolerance Chap. 9 Thus, the re8pODSe tim e, t" is independent of the Dumber of syncbroDizmg points, n., ..

. - . Gamma [Pet ts-12/var (t,l)] is the value of the Gamma function for parameters Pe and t,12/var (t,.

l). The pammeter P. is defined as follows.

If each element completes its transaction within a time t with probability Pe, then the group of elements will complete the traDsaction in that time with probability 0.5:. p. = (0.5)1/,.

.. (9-3). Note that hardware syn servlet Data Matrix 2d barcode cbrcmization as practiced by Stratus results in a variance of zero, since syncbronization points are determined by a common clock. Thus, Ice = 1, and there is DO peIfonnance peualty . ESSAGE QUEUING With message queuing (described in chapter 2 in the section entitled "SmvivabilityMessage Queuing"), a backup process is kept infOIJDed of the statUS of its primary process by queuing those messages that are also ctixected to its primary. In this way, if the primary process fails, the backup can process the queue of old messages to bring itself up-to-date. before taking over the Data Matrix for Java processing functions from the primary. To prevent the backup from sending duplicate messages while it is processing old messages during recovery, the primary process will also sead copies of its output messages to its own backup process. The backup will maintain a count of these so that it vim know when to start teleasing messages.

To prevent the backup queue from becoming too long, the system is "cleaned up" periodically. "Ibis is done by figshing all dirty memoty pages to disk. (This.

of com:se, assumes that memcxy paging for both the primary process and the backup pI9C8SS uses a COIDII1OD miD"oIed disk pair.) At this point. if the backup takes over, it will be in the same state as the primary process.

"l"haefore, its receive queue can be deleted and its message count JeSet. "Ibis is the equivaleDt of the consistency point used with transaction pr0tection. It is igDoJed for peIfornwmce purposes.

since it genaaD.y occurs iDfrequently (eveJY sevenl seccpk to mimnes, depeDding upon the recovay time desired and the maximmn queue leDgtbs). The pmfonnance aualysis oftbese systems is~.

For each inwp0ce8S message sent, tbIee are actually seat: .. One to the cJestinat ion primary process for normal processing. One to the cJestiDariOll backup process fOr replay followiDg a fanure. One to the sender"s backup process so that it can know which messages bave been.

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