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Using Release(deallocate) and Painful Lessons to be learned on DB2 locking

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Using Release(deallocate) and Painful Lessons to be learned on DB2 locking

  1. 1. #IDUG Using RELEASE(DEALLOCATE) and Painful Lessons to be Learned on DB2 Locking John Campbell DB2 for z/OS Development Session Code: 6007
  2. 2. #IDUG 2 Agenda • Thread reuse using RELEASE(DEALLOCATE) • Primer on thread reuse with RELEASE bind option • Considerations and limitations • Painful Lessons to be Learned on DB2 Locking • Primer on lock avoidance • CURRENTDATA(YES) versus CURRENTDATA(NO) • Ramifications of lock avoidance for SQL • What should an application programmer do • Finding packages to safely rebind with CURRENTDATA(NO)
  3. 3. #IDUG 3 “Resources”: Static SQL • Static statements • Packages and statements • Parent locks • Index look-aside buffer, dynamic prefetch tracking via sequential detection
  4. 4. #IDUG 4 RELEASE - BIND and REBIND Option • Determines when to release the resources that a program uses • RELEASE(COMMIT) - Releases resources at commit point • RELEASE(DEALLOCATE) - Releases resources when thread terminates (child page/row locks still released at commit) • RELEASE(INHERITFROMPLAN) – Support added by APAR PM07087 • Default behavior • BIND PLAN - COMMIT • BIND PACKAGE - plan value • REBIND PLAN/PACKAGE existing value • DB2Binder Utility for JDBC and SQLJ • DEALLOCATE is default in DB2 10 • COMMIT is default in DB2 9 and earlier releases
  5. 5. #IDUG Package Allocation Program A : “Insert into table 1” Thread allocation Sign on, authorization checking Locate SKCT and SKPT Allocate CT/PT and STMT1 Start Insert STMT1 Lock table space 1 Lock data page End STMT1 “insert into table 2” Allocate STMT2 Start Insert STMT2 Lock table space 2 Lock data page End STMT2 COMMIT ; Program A Stmt 1: Insert into table1 Stmt 2: Insert into table2 SPT01 EDM_SKELTON_POOL SKCT/SKPT V10 Thread storage above the bar V9 EDM thread pool below the bar CT PT SQL Stmt1 SQL Stmt2 I/O
  6. 6. #IDUG 6 Package Allocation and Commit Program A Stmt 1: Insert into table1 Stmt 2: Insert into table2 Commit ; SPT01 EDM_SKELTON_POOL SKCT/SKPT V10 Thread storage above the bar V9 EDM thread pool below the bar CT PT SQL Stmt1 SQL Stmt2 I/O Expensive Operation! Lock TS 1 Lock table1 data page Lock TS 2 Lock table2 data page RELEASE PT, statements Unlock data pages Unlock TS1, TS2 COMMIT
  7. 7. #IDUG 7 Thread Reuse and RELEASE(DEALLOCATE) • Thread reuse eliminates CPU cost of DB2 thread allocation and deallocation • CICS • Protected ENTRYs • Organic reuse of ENTRYs (or POOL) • IMS/TM • Fast Path (IFP) regions • Wait-For-Input (WFI) regions • Pseudo Wait-For-Input (PWFI) regions • DDF • High Performance DBATs • WebSphere Type 2 local connections • Batch with intermediate commits • Use of RELEASE(DEALLOCATE) coupled with effective thread reuse i.e., thread persistence • Further reduces the CPU cost with potential for significant savings (up to 10% plus) • Resources are held until thread deallocation • Without thread reuse RELEASE(COMMIT) vs. RELEASE(DEALLOCATE) is a moot point for discussion
  8. 8. #IDUG 8 Limitations and Considerations • Virtual and real storage • DB2 9 and earlier - DBM1 virtual storage below the 2G bar and real storage • Package information is stored in EDM pool below the bar • DB2 10 after REBIND - real storage usage • Package information is stored in thread storage above the bar in DB2 9 and earlier releases • Accumulated DB2 object control blocks • Virtual, real, potentially CPU cost for scanning the objects built up under the thread • Recommendations • Design for thread reuse for • High volume simple transactions • Complex transactions at a reasonably high rate • Selectively use RELEASE(DEALLOCATE) on high use packages – use % of Total Acctg Class 7 CPU • DBM1 31-bit virtual storage constraint (DB2 9) • Real storage constraint (DB2 10) • Use CICS or WebSphere parameter to periodically clean up and rejuvenate threads (thread deallocation)
  9. 9. #IDUG 9 Considerations for Clean Up • REUSELIMIT (0-10,000) in CICS TS V4R2 - default 1000 • Number of times a thread can be reused before it is terminated • Use default and monitor DB2 storage usage and adjust the number if needed • WebSphere Type 2 connection Aged Timeout - default 0 • The interval in seconds before a physical connection is discarded • Consider setting WAS “aged timeout” to less than 5 min, recommend using 120 secs to reduce exposure of long living threads • DB2 10 High Performance DBATs (threads) • Thread will go inactive every 200 commits • No user control for this value • DB2 11 optimizes RELEASE(DEALLOCATE) execution so that it is consistently better performing than RELEASE(COMMIT)
  10. 10. #IDUG 10 Considerations - Concurrency • More persistent threads with RELEASE(DEALLOCATE) is also trade off with concurrency • BIND/REBIND • SQL DDL • Online REORG which invalidates packages • For RELEASE(DEALLOCATE) some locks are held beyond commit until thread termination • Mass delete locks (SQL DELETE without WHERE clause) • Gross level lock acquired on behalf of a SQL LOCK TABLE • Table space defined with LOCKSIZE TABLESPACE|TABLE • Note: no longer a problem for gross level lock acquired by lock escalation • DO YOUR HOMEWORK BEFORE USING PERSISTENT THREADS WITH RELEASE(DEALLOCATE) BIND OPTION
  11. 11. #IDUG 11 Primer on lock avoidance • Combination of techniques used by DB2 to try to avoid taking a S page/row locks when processing for read only SQL whilst preventing the retrieval of uncommitted data by the application • (1) Page latching (and page p-lock in data sharing) controlled by DB2 to ensure physical consistency of the page • (2) Commit log sequence number (CLSN) – at the page level • DB2 tracks "time" of last update to page (on page) (A) • DB2 tracks "time" of oldest uncommitted activity on every pageset/partition (B) • Non Data Sharing • CLSN = lowest uncommitted RBA for all active transactions for a given pageset • Data Sharing • For non-GBP-dependent page sets, each member uses a local CLSN = lowest uncommitted LRSN for all active transactions for a given pageset • For GBP-dependent page sets, a Global CLSN value is maintained for the entire data sharing group = lowest CLSN value across all members across all page sets (GBP-dependent or not) • If (A) < (B) everything on the page is guaranteed to be committed • Else, check Possibly UNCommitted bits (PUNC bits)
  12. 12. #IDUG 12 Primer on lock avoidance … • Combination of techniques to prevent retrieval of uncommitted data … • (3) Possibly UNCommitted bits (PUNC bits) – at the row level • On each row, a PUNC bit is set when the row is updated • PUNC bits are periodically reset • If successful CLSN check and more than 25% of the rows have the PUNC bit ON • RR scanner • REORG TABLESPACE • If the PUNC bit is not ON, the row/key is guaranteed to be committed
  13. 13. #IDUG 13 CURRENTDATA(YES) versus CURRENTDATA(NO) … • Plans and packages have a better chance for lock avoidance if they are bound with ISOLATION(CS) and CURRENTDATA(NO) • What is CURRENTDATA? • Helps to determine the currency and stability of data returned to an application cursor • Only applies to applications bound with ISOLATION(CS) • What is isolation CURSOR STABILITY(CS)? • Data fetched by a cursor is committed, but if the application process returns to the same page, another application might have since updated, deleted, or inserted qualifying rows • If the cursor is defined as FOR UPDATE OF • Data returned by the cursor is stable and it may not be updated by another transaction while the updatable cursor is positioned on it • If the cursor is defined as FOR READ|FETCH ONLY, or it is implicitly read only (or it is ambiguous) • ISOLATION(CS) ensures that the data returned is committed and the stability of the cursor is determined by the CURRENTDATA option
  14. 14. #IDUG 14 Primer on lock avoidance … • Benefits of lock avoidance • Improved concurrency through less lock collisions • Decrease in lock and unlock activity requests, with an associated decrease in CPU resource consumption and data sharing overhead • V8 improvements • Lock avoidance for non-cursor ‘singleton’ SELECT • In V7, ISOLATION(CS) CURRENTDATA(YES) acquires S page/row lock on the qualified row • In V8, DB2 no longer acquires and hold S page/row lock on the qualified row for ISOLATION(CS) CURRENTDATA(YES or NO) • Overflow lock avoidance when the update of a variable length row in a data page results in a new row that cannot fit in that page i.e., indirect reference • In V7, no lock avoidance on both pointer and overflow • In V8, lock on pointer only • Need to distinguish carefully between eligibility for lock avoidance and actually achieving it
  15. 15. #IDUG 15 Primer on lock avoidance … • BIND option ISOLATION(CS) with CURRENTDATA(NO) could reduce # Lock/Unlock requests dramatically • High Unlock requests/commit could also be possible from • Large number of relocated rows after update of compressed or VL row • Lock/Unlock of pointer record (or page) • Large number of pseudo-deleted entries in unique indexes • Lock/Unlock of data (page or row) in insert to unique index when pseudo-deleted entries exist • Both can be eliminated by REORG Field Name Description QTXALOCK LOCK REQUESTS QTXAUNLK UNLOCK REQUESTS LOCKING ACTIVITY QUANTITY /SECOND /THREAD /COMMIT ------------------------ -------- ------- ------- ------- ... LOCK REQUESTS 521.0M 24.2K 3134.34 1050.75 UNLOCK REQUESTS 478.1M 22.2K 2876.06 964.16 Lock avoidance may not be working effectively if Unlock requests/commit is high, e.g. >5 ROT
  16. 16. #IDUG 16 Primer on lock avoidance … • Effective lock avoidance is very important in data sharing • Global locks propagated beyond IRLM to Coupling Facility are relatively expensive • Global management of lock contention/resolution is very expensive • Effective Lock avoidance is critical to achieving good performance and lower data sharing overhead • Any long-running UR(s) can reduce the effectiveness of lock avoidance by stopping the Global CLSN value from moving forward • Recommendation: Aggressively monitor long-running URs • 'First cut' ROTs: • URs running for a long time without committing: zparm URCHKTH<=5 • Message DSNR035I • URs performing massive update activity without committing: zparm URLGWTH=10(K) • Message DSNJ031I • Need management ownership and process for getting rogue applications fixed up so that they commit frequently based on • Elapsed time and/or • CPU time (no. of SQL update statements)
  17. 17. #IDUG 17 CURRENTDATA(YES) versus CURRENTDATA(NO) … • Switching from CURRENTDATA(YES) to CURRENTDATA(NO) • In general, can be done without any adverse effect to applications • But consider the following scenario: 1. Program A bound with ISOLATION(CS) CURRENTDATA(YES) fetches a row from a cursor with ORDER BY – and the row is read from the base table and not from a work file 2. Program B tries to update the row just read by Program A • The access is not allowed because the CURRENTDATA(YES) option for Program A caused a lock to be acquired on the page/row in the base table 3. Program A then issues a searched UPDATE of the row just fetched from step 1. • In the above example CURRENTDATA(YES) protects the integrity of the data read by Program A in between the FETCH and the searched UPDATE • If Program A was to be rebound with CURRENTDATA(NO) then 1. Program B may (should) be able to access the row just fetched by Program A • This assumes correct timing and that lock avoidance is effective 2. When Program A issues the searched UPDATE, it could wipe out the changes just made by Program B
  18. 18. #IDUG 18 CURRENTDATA(YES) versus CURRENTDATA(NO) … • Switching from CURRENTDATA(YES) to CURRENTDATA(NO) … • Now reconsider the same scenario: 1. Program A bound with ISOLATION(CS) CURRENTDATA(YES) fetches a row from a cursor with ORDER BY, but the row is now read from a work file 2. Program B tries to update the row just read by Program A 1. Access is now allowed as there is no lock on the page/row in the base table 3. Program A then issues a searched UPDATE of the row just fetched from step 1 • In the above example CURRENTDATA(YES) does NOT protects the integrity of the data read by Program A in between the FETCH and the searched UPDATE
  19. 19. #IDUG 19 Ramifications of lock avoidance for SQL • Data may be returned without a lock • All data returned will be committed at the time it is taken from page • But may change by the time the application sees it • DB2 only attempts lock avoidance for ISOLATION(CS) transactions when BOTH of the following are true: • The plan/package is bound CURRENTDATA(NO) • The cursor (if cursor-based FETCH) is declared FOR READ|FETCH ONLY, or has some other attribute (e.g., ORDER BY) that makes it a read-only cursor • Applications that depend on the stability of what is under a read only cursor, should use CURRENTDATA(YES) • Default is CURRENTDATA(NO) • Now lets look at another example
  20. 20. #IDUG 20 Ramifications of lock avoidance for SQL … • Example DECLARE CURSOR AS ... SELECT THIRD_COL ... WHERE CLUST_COL = 'ABC' AND ANOTHER_COL = 'DEF' ORDER BY CLUST_COL; <--- makes cursor read only FETCH ... <--- say this returns row ABC.DEF.GHI without taking a lock UPDATE SET … WHERE CLUST_COL = 'ABC' AND ANOTHER_COL = 'DEF' AND THIRD_COL = 'GHI‘ • What is the issue? • Row may no longer be ABC.DEF.GHI by the time update executes • Another transaction may have gotten in and updated it to ABC.DEF.XYZ • So update will receive "row not found"
  21. 21. #IDUG 21 What should an application programmer do • Use programming techniques which avoid possible data currency exposures and access path dependencies (these sometimes occur regardless of the CURRENTDATA option) • Define cursors with their intended use • Easier to read application code • Number of ambiguous cursors can be reduced • Some queries will not allow the use of a FOR UPDATE cursor due to possible work file usage e.g., with ORDER BY • Use predicates on searched updates to enforce data currency • When a FOR UPDATE cursor cannot be used with a query it is common for application programmers to use a read only with a subsequent searched UPDATE or DELETE • It is important to use WHERE predicates to ensure that data has not changed since your cursor first selected the data • Include all columns which logically determine if the update is necessary, instead of updating based solely on the key • This is an exposure regardless of the CURRENTDATA option, but the use of CURRENTDATA(NO) can increase the exposure opportunity • Consider the use of a timestamp or a version number • Add a timestamp or version number column to a table to record the last update • Select the timestamp or version number column with the cursor • Use it as a predicate on the searched UPDATE or DELETE to ensure that an application is updating the same row
  22. 22. #IDUG 22 Finding packages to safely rebind with CURRENTDATA(NO) • Must recognise that access path dependencies will inevitably occur • CURRENTDATA(YES) will not stop access path dependency and associated exposures • Access path independence and CURRENTDATA(NO) should be the goal for all applications • Can safely rebind packages that use only true read-only cursors with CURRENTDATA(NO)
  23. 23. #IDUG 23
  24. 24. #IDUG John Campbell DB2 for z/OS Development campbelj@uk.ibm.com Session 6007 Using RELEASE(DEALLOCATE) and Painful Lessons to be Learned on DB2 Locking

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