This wait happens when a session wants to access a database block in the buffer cache but it cannot as the buffer is . The two main cases where this can occur are: Another session is reading the block into the buffer (and the session is waiting for that read to complete)Another session holds the buffer in an incompatible mode to our request (that is, some other session is changing the buffer). Wait Time: Normal wait time is 1 second. If the session was waiting for a buffer during the last wait, then the next wait will be 3 seconds. Load Program Status Word (LPSW) is a privileged instruction that loads the Program Status Word (PSW), including the program mode, protection key, and the address of the next instruction to be executed. LPSW is most often used to 'return' from an interruption. This is likely to be an issue with using 'Selection', as the active selection may not actually be what you expect it to be (either due to what your code does/doesnt do, or what a user is doing while your program is running). I would strongly recommend changing the. 2.1.8 Save Data (EEPROM) File This command is available only for splitted Program/Data devices. The buffer is splitted in two different part for these devices: the first part for program memory and the second part for data memory. This command save the content. Parameter. Descriptionfile#This is the file number of the data file that contains the block that Oracle needs to wait for. To find the name of this file enter: select *. This is the block number of the block that Oracle needs to wait for. The block number is relative to the start of the file.
To find the object that this block belongs to enter: select name, kind. Each place in the kernel points to different reason. The p. 3 values are known as reason codes and they are dependent on the version of Oracle. The following table explains the most common Buffer Busy Waits ID's and Meanings. Reason Code (Id)Reason< =8. A block is being read 1. We want to NEW the block but the block is currently being read by another session (most likely for undo). We want to NEW the block but someone else has is using the current copy so we have to wait for them to finish. Trying to get a buffer in CR/CRX mode , but a modification has started on the buffer that has not yet been completed. A modification is happening on a SCUR or XCUR buffer, but has not yet completed 1. CR/CRX scan found the CURRENT block, but a modification has started on the buffer that has not yet been completed. Block is being read by another session and no other suitable block image was found e. CR version, so we wait until the read is completed. This may also occur after a buffer cache assumed deadlock. The kernel can't get a buffer in a certain amount of time and assumes a deadlock. Therefore it will read the CR version of the block. This should not have a negative impact on performance, and basically replaces a read from disk with a wait for another process to read it from disk, as the block needs to be read one way or another. We want the CURRENT block either shared or exclusive but the Block is being read into cache by another session, so we have to wait until their read() is completed. We want to get the block in current mode but someone else is currently reading it into the cache. Wait for them to complete the read. This occurs during buffer lookup. The session wants the block in SCUR or XCUR mode. If this is a buffer exchange or the session is in discrete TX mode, the session waits for the first time and the second time escalates the block as a deadlock and so does not show up as waiting very long. In this case the statistic: . During buffer lookup for a CURRENT copy of a buffer we have found the buffer but someone holds it in an incompatible mode so we have to wait. Finding Blockers: Finding the blocking process can be quite difficult as the information required is not externalized. If P3 parameter value (which is reason code) shows that the . If the wait is due to the buffer being held in an incompatible mode then it should be freed very soon. If not then it is advisable to contact Oracle Support and get 3 SYSTEMSTATE dumps at one minute intervals as the blocking session may be spinning. This information is derived from the view V$WAITSTAT which can be queried in isolation: SELECT time, count, class. FROM V$WAITSTAT. ORDER BY time,count. This shows the class of block with the most waits at the BOTTOM of the list. Oracle Support may also request that the following query be run to show where the block is held from when a wait occurs: SELECT kcbwhdes, why. Rows are accessed by a full table scan. Check the number of FULL tablescanskdsgrp. Get Row Piece. Typically row pieces are fected only in case of chained and Migrated Rows. Row chaining has to be analyed and fixed. Index Range Scankdifxs. Fetching the next or previous row in the index scankdifbk. Fetches the single index row matching the agreement keyktugct. Block cleanout. Additional information regarding which files contain the blocks being waited for can be obtained from the internal view X$KCBFWAIT: SELECT count, file#, name. FROM x$kcbfwait, v$datafile. WHERE indx + 1 = file#. ORDER BY count. ; This shows the file(s) with the most waits (at the BOTTOM of the list). By combining the above information we know what block type(s) in which file(s) are causing waits. The segments in each file can be seen using a query such as: SELECT distinct owner, segment. After a period of time sort the results to see which file & blocks are showing contention: SELECT p. If a particular block or range of blocks keeps showing waits you can try to isolate the object. Reducing Waits / Wait times: As buffer busy waits are due to contention for particular blocks then you cannot take any action until you know which blocks are being competed for and why. Eliminating the cause of the contention is the best option. Check for repeatedly scanned / unselective indexes. Change PCTFREE and/or PCTUSED. Check for 'right- hand- indexes' (indexes that get inserted into at the same point by many processes). Reduce the number of rows per block. Use FREELIST GROUPs (even in single instance this can make a difference). Freelist blocks. Add more FREELISTS. In case of Parallel Server make sure that each instance has its own FREELIST GROUP(s). IBM System/3. 60 architecture - Wikipedia, the free encyclopedia. The IBM System/3. S/3. 60 line of computers. The elements of the architecture are documented in the IBM System/3. Principles of Operation. Various instructions operate on larger units called halfword (2 bytes), fullword (4 bytes), doubleword (8 bytes), quad word (1. Within a halfword, fullword, doubleword or quadword, low numbered bytes are more significant than high numbered bytes; this is sometimes referred to as big- endian. Many uses for these units require aligning them on the corresponding boundaries. Within this article the unqualified term word refers to a fullword. The architecture of System/3. Model 6. 7 extended the architecture and allowed 2. That means that instructions do not contain complete addresses, but rather specify a base register and a positive offset from the addresses in the base registers. In the case of System/3. In some instructions, for example shifts, the same computations are performed for 3. Data formats. Character and integer instructions are mandatory, but decimal and floating point instructions are part of the Decimal arithmetic and Floating- point arithmetic features. Characters are stored as 8- bit bytes. Integers are stored as two's complement binary halfword or fullword values. Packed decimal numbers are stored as 1- 1. Sign values of hexadecimal A, C, E and F are positive and sign values of hexadecimal B and D are negative. Digit values of hexadecimal A- F and sign values of 0- 9 are invalid, but the PACK and UNPK instructions do not test for validity. Zoned decimal numbers are stored as 1- 1. The zone of the rightmost byte is interpreted as a sign. Floating point numbers are only stored as fullword or doubleword values on older models. For all three formats, bit 0 is a sign and bits 0- 7 are a characteristic (exponent, biased by 6. Bits 8- 3. 1 (8- 6. For extended precision, the low order doubleword has its own sign and characteristic, which are ignored on input and generated on output. Instruction formats. Instructions have one of the following formats: RR (two bytes). Generally byte 1 specifies two 4- bit register numbers, but in some cases, e. SVC, byte 1 is a single 8- bit immediate field. RS (four bytes). Byte 1 specifies two register numbers; bytes 2- 3 specify a base and displacement. RX (four bytes). Byte 1 bits 0- 3 specifies either a register number or a modifier; byte 1 bits 4- 7 specifies the number of the general register to be used as an index; bytes 2- 3 specify a base and displacement. SI (four bytes). Byte 1 specifies an immediate field; bytes 2- 3 specify a base and displacement. SS (six bytes). Byte 1 specifies two 4- bit length fields or one 8- bit length field; bytes 2- 3 and 4- 5 each specify a base and displacement. The encoding of the length fields is length- 1. Instructions must be on a two- byte boundary in memory; hence the low- order bit of the instruction address is always 0. Program Status Word (PSW). LPSW is most often used to . Other privileged instructions (e. SSM, STNSM, STOSM, SPKA, etcetera) are available for manipulating subsets of the PSW without causing an interruption or loading a PSW; and one non- privileged instruction (SPM) is available for manipulating the program mask. Interruption system. There are two storage fields assigned to each class of interruption on the S/3. PSW double- word and a new PSW double- word. The processor stores the PSW, with an interruption code inserted, into the old PSW location and then loads the PSW from the new PSW location. This generally replaces the instruction address, thereby effecting a branch, and (optionally) sets and/or resets other fields within the PSW, thereby effecting a mode change. The S/3. 60 architecture defines a priority to each interruption class, but it is only relevant when two interruptions occur simultaneously; an interruption routine can be interrupted by any other enabled interruption, including another occurrence of the initial interruption. For this reason, it is normal practice to specify all of the mask bits, with the exception of machine- check mask bit, as 0 for the . The system stores the device address into the interruption code and stores channel status into the CSW at location 6. X). Program interruption. The Interruption code may be any of. An operation exception. Po. Ops(p. 79) is recognized when a program attempts to execute an instruction with an opcode that the computer does not implement. In particular, an operation exception is recognized when a program is written for an optional feature, e. A privileged operation exception. Po. Ops(p. 79) is recognized when a program attempts to execute a privileged instruction when the problem state bit in the PSW is 1. An execute exception. Po. Ops(p. 79) is recognized when the operand of an EXECUTE instruction is another EXECUTE instruction. A protection exception. Po. Ops(p. 79) is recognized when a program attempts to store into a location whose storage protect key does not match. This normally occurs with an address beyond the capacity of the machine, but it may also occur on machines that allow blocks of storage to be taken offline. A specification exception. Po. Ops(p. 80) is recognized when an instruction has a length or register field with values not permitted by the operation, or when it has an operand address that does not satisfy the alignment requirements of the opcode, e. LH instruction with an odd operand address on a machine without the byte alignment feature. A data exception. Po. Ops(p. 80) is recognized when a decimal instruction specifies invalid operands, e. A fixed- point overflow exception. Po. Ops(p. 80) is recognized when significant bits are lost in a fixed point arithmetic or shift instruction, other than divide. A fixed- point divide exception. Po. Ops(p. 80) is recognized when significant bits are lost in a fixed point divide or Convert to Binary instruction. A decimal overflow exception. Po. Ops(p. 80) is recognized when significant digits are lost in a decimal arithmetic instruction, other than divide. A decimal divide exception. Po. Ops(p. 80) is recognized when significant bits are lost in a decimal divide instruction. The destination is not altered. An exponent overflow exception. Po. Ops(p. 80) is recognized when the characteristic in a floating- point arithmetic operation exceeds 1. An exponent underflow exception. Po. Ops(p. 80) is recognized when the characteristic in a floating- point arithmetic operation is negative and the fraction is not zero. A significance exception. Po. Ops(p. 80) is recognized when the fraction in a floating- point add or subtract operation is zero. A floating- point divide exception. Po. Ops(p. 80. 1) is recognized when the fraction in the divisor of a floating- point divide operation is zero. Supervisor Call interruption. Bits 1. 6- 2. 4 of the External Old PSW are set to 0 and one or more of bits 2. Interruption codes for External interruptions. PSW bit. Type of external interruption. Timer. 25. Interrupt key. External signal 2. Malfunction alert on 3. The most important class of conditions causing a Machine Check is a hardware error such as a parity error found in registers or storage, but some models may use it to report less serious conditions. Both the interruption code and the data stored in the scanout area at '8. Input/Output. It does not discuss the channel cable or connectors, but there is a summary elsewhere and details can be found in the IBM literature. Channels have their own instruction set, and access memory independently of the program running on the CPU. On the smaller models (through 3. CPU program and the channel program. On the larger models the channels are in separate cabinets and have their own interfaces to memory. A channel may contain multiple subchannels, each containing the status of an individual channel program. A subchannel associated with multiple devices that cannot concurrently have channel programs is referred to as shared; a subchannel representing a single device is referred to as unshared. There are three types of channels on the S/3. A byte multiplexer channel is capable of executing multiple CCWs concurrently; it is normally used to attach slow device such as card readers and telecommunications lines. A byte multiplexer channel could have a number of selector subchannels, each with only a single subchannel, which behave like low- speed selector channels. A selector channel has only a single subchannel, and hence is only capable of executing one channel command at a time. It is normally used to attach fast devices that are not capable of exploiting a block multiplexer channel to suspend the connection, such as magnetic tape drives. A block multiplexer channel is capable of concurrently running multiple channel programs, but only one at a time can be active. The control unit can request suspension at the end of a channel command and can later request resumption. This is intended for devices in which there is a mechanical delay after completion of data transfer, e. DASD. The block multiplexer channel was a late addition to the System/3. The block multiplexer channel was an optional feature only on the models 8. The block multiplexor channel was also available on the later System/3. Conceptually peripheral equipment is attached to a S/3. However, the architecture does not require that control units be physically distinct, and in practice they are sometimes integrated with the devices that they control. Similarly, the architecture does not require the channels to be physically distinct from the processor, and the smaller S/3. Peripheral devices are addressed with 1. The high 8 bits identify a channel, numbered from 0 to 6. A device may have multiple cuu addresses. Control units are assigned an address . For example, a CU might be assigned range 2. F or 4. 0- 7. F. The purpose of this is to assist with the connection and prioritization of multiple control units to a channel. For example, a channel might have three disk control units at 2. F, 5. 0- 5. F, and 8.
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