DB2 Locking, Part 5: Lock Suspensions, Timeouts, and Deadlocks

The longer a lock is held, the greater the potential impact to other applications. When an application requests a lock that is already held by another process, and the lock cannot be shared, that application is suspended. A suspended process temporarily stops running until the lock can be acquired. Lock suspensions can be a significant barrier to acceptable performance and application availability.

When an application has been suspended for a pre-determined period of time, it will be terminated. When a process is terminated because it exceeds this period of time, it is said to timeout. In other words, a timeout is caused by the unavailability of a given resource. For example, consider the scenario depicted in Figure 1.

Figure 1. A Timeout Occurs

If Program 2, holding no other competitive locks, requests a lock currently held by Program 1, DB2 tries to obtain the lock for a period of time. Then it quits trying. This example illustrates a timeout. This timeout scenario is also applicable to row locks, not just page locks.

The length of time a user waits for an unavailable resource before being timed out is determined by the IRLMRWT DSNZPARM parameter. You also can set this period of time by using the RESOURCE TIMEOUT field on the DB2 installation panel DSNTIPI.

When a lock is requested, a series of operations is performed to ensure that the requested lock can be acquired (see Figure 2). Two conditions can cause the lock acquisition request to fail: a deadlock or a timeout.

Figure 2. Processing a Lock Request

A deadlock occurs when two separate processes compete for resources held by one another. DB2 performs deadlock detection for both locks and latches. For example, consider the following processing sequence for two concurrently executing application programs:

Figure 3. A Deadlock Occurs

A deadlock occurs when Program 1 requests a lock for a data page held by Program 2, and Program 2 requests a lock for a data page held by Program 1. A deadlock must be resolved before either program can perform subsequent processing. DB2’s solution is to target one of the two programs as the victim of the deadlock and deny that program’s lock request by setting the SQLCODE to -911. This deadlocking scenario is also applicable to row locks, not just page locks. A graphic depiction of a deadlock is shown in Figure 4.

Figure 4. The Deadlock

The length of time DB2 waits before choosing a victim of a deadlock is determined by the DEADLOK IRLM parameter. You also can set this parameter using the RESOURCE TIMEOUT field on the DB2 installation panel DSNTIPJ.

IDUG NA 2013 in Orlando, Florida

Just a short blog post to let everybody know that I will be in Orlando next week for the IDUG DB2 Tech Conference. I'll be delivering an education seminar (DB2 Developer's Guide Comes Alive!) on Monday and giving two presentations on Thursday...

On Tuesday and Wednesday I'll be roaming around the conference, attending sessions, and looking to say hello to old friends and colleagues.

Hope to see you there!

IDUG NA 2013

DB2 Locking, Part 4: Page and Row Locks

In the first three installments of this series on DB2 locking we have looked ata broad overview of locking (part 1), table and table space locks (part 2) and the difference between locks and latches (part 3). Today we will move ahead and discuss page and row locking.

Page Locking

The types of page locks that DB2 can take are outlined in Table 1. S-locks allow data to be read concurrently but not modified. With an X-lock, data on a page can be modified (with INSERT, UPDATE, or DELETE), but concurrent access is not allowed. U-locks enable X-locks to be queued, whereas S-locks exist on data that must be modified.

Table 1. Page Locks

As with table space locks, concurrent page locks can be acquired but only with compatible page locks. The compatibility matrix for page locks is shown in Table 2.

Table 2. Page Lock Compatibility Matrix

When are these page locks taken? Page locks can be acquired only under the following conditions:

• The DDL for the object requesting a lock specifies LOCKSIZE PAGE or LOCKSIZE ANY.
• If LOCKSIZE ANY was specified, the NUMLKTS threshold or the table space LOCKMAX specification must not have been exceeded. You learn more about these topics later in this section.

Keep in mind, though, that if ISOLATION(RR) was used when the program was bound, the optimizer might decide not to use page locking even if the above criteria are met.

If all these factors are met, page locking progresses as outlined in Table 3. The type of processing in the left column causes the indicated page lock to be acquired for the scope of pages identified in the right column. DB2 holds each page lock until it is released as specified in the ISOLATION level of the plan requesting the particular lock. Page locks can be promoted from one type of lock to another based on the type of processing that is occurring. A program can FETCH a row using a cursor with the FOR UPDATE OF clause, causing a U-lock to be acquired on that row’s page. Later, the program can modify that row, causing the U-lock to be promoted to an X-lock.

Table 3. How Page Locks Are Acquired

Row Locks

The smallest piece of DB2 data that you can lock is the individual row. The types of row locks that DB2 can take are similar to the types of page locks that it can take. Refer back to Table 1 and simply replace Page with Row. So row locks act like page locks, only on a smaller granularity (that is, on rows instead of pages). 

S-locks allow data to be read concurrently but not modified. With an X-lock, you can modify data in that row (using INSERT, UPDATE, MERGE, or DELETE), but concurrent access is not allowed. U-locks enable X-locks to be queued, whereas S-locks exist on data that must be modified.

Once again, concurrent row locks can be acquired but only with compatible row locks. Table 2 works the same way for row locks as it does for page locks. 

When are these row locks taken? Row locks can be acquired when the DDL for the object requesting a lock specifies LOCKSIZE ROW. (Although it is theoretically possible for LOCKSIZE ANY to choose row locks, in practice I have yet to see this happen.) Again, we can use an earlier Table (Table 3) replacing the word page with the word row to see how row locking progresses. The type of processing in the left column causes the indicated row lock to be acquired for the scope of rows identified in the right column. A row lock is held until it is released as specified by the ISOLATION level of the plan requesting the particular lock.

Row locks can be promoted from one type of lock to another based on the type of processing that is occurring. A program can FETCH a row using a cursor with the FOR UPDATE OF clause, causing a U-lock to be acquired on that row. Later, the program can modify that row, causing the U-lock to be promoted to an X-lock.

Page Locks Versus Row Locks

The answer to the question of whether to use page locks or row locks is, of course, “It depends!” The nature of your specific data and applications determine whether page or row locks are most applicable.

The resources required to acquire, maintain, and release a row lock are just about the same as the resources required for a page lock. Therefore, the number of rows per page must be factored into the row-versus-page locking decision. The more rows per page, the more resources row locking will consume. For example, a table space with a single table that houses 25 rows per page can consume as much as 25 times more resources for locking if row locks are chosen over page locks. Of course, this estimate is very rough, and other factors (such as lock avoidance) can reduce the number of locks acquired, and thereby reduce the overhead associated with row locking. However, locking a row-at-a-time instead of a page-at-a-time can reduce contention. Row locking almost always consumes more resources than page locking. Likewise, if two applications running concurrently access the same data in different orders, row locking might actually decrease concurrent data access.

You must therefore ask these questions:

• What is the nature of the applications that access the objects in question? Of course, the answer to this question differs not only from organization to organization, but also from application to application within the same organization.
• Which is more important, reducing the resources required to execute an application or increasing data availability? The answer to this question will depend upon the priorities set by your organization and any application teams accessing the data.

As a general rule of thumb, favor specifying LOCKSIZE PAGE, as page locking is generally the most practical locking strategy for most applications. If you’re experiencing severe contention problems on a table space that is currently using LOCKSIZE PAGE, consider changing to LOCKSIZE ROW and gauging the impact on performance, resource consumption, and concurrent data access. Alternatively, you also might choose to specify LOCKSIZE ANY and let DB2 choose the type of locking to be performed.

Note

Note: A possible alternative to row locking is to specify MAXROWS 1 for the table space and use LOCKSIZE PAGE (or LOCKSIZE ANY), instead of LOCKSIZE ROW. 

DB2 Locking, Part 3: Locks Versus Latches

So far in this series on DB2 locking we have offered up a broad overview of what locking is and then delved into the world of table and table space locks. In this short entry, before we tackles page and row locks, we are going to look at the difference between a lock and a latch.

A true lock is handled by DB2 using the IRLM. The IRLM, or internal resource lock manager, is both a separate subsystem and an integral component of DB2. As its name implies, it manages locks for DB2.

However, whenever doing so is practical, DB2 can lock resources without going to the IRLM. This type of lock is called a latch. True locks are always set in the IRLM. Latches, by contrast, are set internally by DB2, without going to the IRLM.

When a latch is taken instead of a lock, it is handled in the Buffer Manager by internal DB2 code; so the cross-memory service calls to the IRLM are eliminated. Latches are usually held only briefly—for a shorter duration than locks. Also, a latch requires about one-third the number of instructions as a lock. Therefore, latches are more efficient than locks because they avoid the overhead associated with calling an external address space. Latches are used when a resource serialization situation is required for a short time. Both latches and locks guarantee data integrity. 

In subsequent blog entries, any usage of the term lock generically, refers to both locks and latches.

DB2 Locking, Part 2: Table Space and Table Locks

Today's post is the second in our DB2 locking series and it covers the topic of table space and table locks.

Table Space Locks

A table space lock is acquired when a DB2 table or index is accessed. Note that I said accessed, not updated. The table space is locked even when simple read-only access is ­occurring.

Refer to to Table 1 below for a listing of the types of table space locks that can be acquired during the execution of an SQL statement. Every table space lock implies two types of access: the access acquired by the lock requester and the access allowed to other subsequent, concurrent processes.

Table 1. Table Space Locks

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When an SQL statement is issued and first accesses data, it takes an intent lock on the table space. Later in the process, actual S-, U-, or X-locks are taken. The intent locks (IS, IX, and SIX) enable programs to wait for the required S-, U-, or X-lock that needs to be taken until other processes have released competing locks.

The type of table space lock used by DB2 during processing is contingent on several factors, including the table space LOCKSIZE specified in the DDL, the bind parameters chosen for the plan being run, and the type of processing requested. Table 2 provides a synopsis of the initial table space locks acquired under certain conditions.

Table 2. How Table Space Locks Are Acquired

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A table space U-lock indicates intent to update, but an update has not occurred. This is caused by using a cursor with the FOR UPDATE OF clause. A U-lock is non-exclusive because it can be taken while tasks have S-locks on the same table space. More information on table space lock compatibility follows in Table 3.

An additional consideration is that table space locks are usually taken in combination with table and page locks, but they can be used on their own. When you specify the ­LOCKSIZE TABLESPACE DDL parameter, table space locks alone are used as the locking mechanism for the data in that table space. This way, concurrent access is limited and concurrent update processing is eliminated.

Similar in function to the LOCKSIZE DDL parameter is the LOCK TABLE statement. The LOCK TABLE statement requests an immediate lock on the specified table. The LOCK TABLE statement has two forms—one to request a share lock and one to request an exclusive lock.

     LOCK TABLE table_name IN SHARE MODE; 
     LOCK TABLE table_name IN EXCLUSIVE MODE;

You also can issue LOCK TABLE against a specific partition, for example:

     LOCK TABLE table-name 
          PARTITION integer IN SHARE MODE;

A locking scheme is not effective unless multiple processes can secure different types of locks on the same resource concurrently. With DB2 locking, some types of table space locks can be acquired concurrently by discrete processes. Two locks that can be acquired concurrently on the same resource are said to be compatible with one another.

Refer to Table 3 for a breakdown of DB2 table space lock compatibility. A Yes in the matrix indicates that the two locks are compatible and can be acquired by distinct processes on the same table space concurrently. A No indicates that the two locks are incompatible. In general, two locks cannot be taken concurrently if they allow concurrent processes to negatively affect the integrity of data in the table space.


Table 3. Table Space Lock Compatability

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Table Locks

When segmented table spaces are involved, DB2 can use table locks . Table locks are always associated with a corresponding table space lock.

The same types of locks are used for table locks as are used for table space locks. S, U, X, IS, IX, and SIX table locks can be acquired by DB2 processes when data in segmented table spaces is accessed. Table 1 describes the options available to DB2 for table locking. The compatibility chart in Table 3 applies to table locks as well as table space locks.

For a table lock to be acquired, an IS-lock must first be acquired on the segmented table space in which the table exists. The type of table lock to be taken depends on the LOCKSIZE specified in the DDL, the bind parameters chosen for the plan being run, and the type of processing requested. Table 4 (below) is a modified version of Table 2 (shown earlier), depicting the initial types of table spaces and table locks acquired given a certain set of conditions. Table locks are not acquired when the LOCKSIZE TABLESPACE parameter is used.


Table 4. How Table Locks Are Acquired

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Summary

And so concludes Part 2 of our multi-part series on DB2 locking. Be sure to keep watching this blog for additional postings in this series!