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SQL Server 2012 : Getting More from Performance Monitor (part 1) - Investigating CPU Problems, Investigating Memory-Related Problems

1/20/2015 3:32:56 AM
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This section builds on the introduction to PerfMon, providing specific counters and prescriptive guidance on acceptable counter thresholds. With so many counters available, it can be difficult to know which to use when; and no single counter is sufficient for making any decisions or recommendations. Typically, a variety of PerfMon counters are used to construct a picture of workload and resource consumption.

NOTE Your aim when troubleshooting is to narrow your focus as quickly as possible to zoom in on the problem. To do this effectively, you should use an iterative approach, whereby each iteration has a specific objective or component, such as disk or memory, to eliminate or incriminate. At the end of each data-gathering cycle and log analysis (an iteration), you should be able to say with some certainty that a particular component of the solution has been ruled in or ruled out as the problem source.

This section looks at hardware, operating system, and SQL Server bottlenecks, considering each major component in order of problem likelihood: memory, disk, and CPU. You’ll also learn about SQL Server performance counters in order to provide a plan for using PerfMon to identify specific SQL Server problem conditions.

Bottlenecks and SQL Server

A bottleneck is any resource that significantly restricts database performance. There will always be bottlenecks of one kind or another — the goal is to ensure that no single component significantly delays the entire transaction processing system. Identifying bottlenecks enables you to prioritize your troubleshooting; there may be numerous problems, but a clear and specific bottleneck provides an area of focus. This section examines some different types of bottlenecks and provides some prescriptive guidance that can help you identify resource contention. It’s normal for an active database server to read and write from disk lots where locking and blocking is part of normal usage patterns; however, when any one resource or component consumes a significant portion of query completion time, this could cause a problem.

SQL Server performance is closely related to server performance because query processing duration is dependent on sufficient memory, disk, and CPU performance. SQL Server depends on each of these resources, so they are listed in order of likelihood of causing a problem; but each should be configured correctly and performing well to service SQL Server and provide optimal transaction throughput for the hardware.

Types of Bottlenecks

Most bottlenecks can be categorized as one of two types: configuration-based or schema-based. Each of these categories can cause bottlenecks within each resource type (CPU, memory, and disk). Although there are many potential problem scenarios, most server-wide or instance-wide bottlenecks tend to be configuration-based, whereas database schema bottlenecks are database design issues, specific to an individual database (common issues may include schema normalization, index selection, and statistics).

Configuration-Based Bottlenecks

SQL Server doesn’t require any specialized knowledge to install, and most of the default values are sufficient for most deployments. When performance and scalability are critical issues, many optimizations can be made — both to the operating system and to SQL Server. Knowing which to change and when is key to getting the most from the hardware and SQL Server itself .

Configuration-based bottlenecks include any operating system configuration, such as memory settings, including /3GB and /PAE; I/O performance tuning, such as disk sector alignment; and HBA queue depth optimization. Additionally, there are many SQL Server configuration-based optimizations, such as disk and log file placement, database auto-growth settings, and any sp_configure options.

Schema-Based Bottlenecks

Schema bottlenecks are application-specific, as they relate to the schema of a specific database (whereas configuration bottlenecks are server-wide or instance-wide). In most cases, the best time to optimize the schema is during application design because schema changes have the least impact on the application when it is still under development. Schema-based bottlenecks illustrate why performance testing must be included as an integral part of software build projects, as it can be incredibly difficult to retrofit performance to an application that’s already live.

Schema-based bottlenecks include normalization problems, whereby the schema is either overnormalized, requiring denormalization, or not fully normalized — i.e., the tables contain duplicate data. Additional schema-based bottlenecks include missing or surplus indexes, missing statistics and poor choice of clustering key (such as using a GUID instead or an incrementing identity column).

Prescriptive Guidance

This section includes details about valuable PerfMon counters to include when troubleshooting each resource type, and prescriptive guidance for “healthy” counter values. The prescriptive guidance can serve as a baseline indicator of problems and highlight any significant resource problems early in the troubleshooting cycle, but you should also use other evidence you gather before making a decision or recommendation to form an action plan. As mentioned earlier, no single item of evidence is usually enough to form a complete picture of a problem.

Each section contains a table with details about the main PerfMon counters for each resource group, a description of what to look for, and a value for a problem condition. Finding a counter value that falls within the problem condition threshold warrants further investigation.

Investigating CPU Problems

The availability of CPU cycles to service SQL Server in a timely manner is critical to database server performance. Configuration-based CPU bottlenecks may include max degree of parallelism, the cost threshold of parallelism, and mis-configured CPU hyperthreading. Changing from default configuration and the optimum setting for each of these configuration options is scenario dependent, it can be challenging to be presecriptive and cover all potential scenarios — there are often edge cases and exceptions.

Kernel Mode and Application Mode

It’s important to recognize the difference between kernel mode consumption and application mode consumption because this concept will provide an important and useful indicator when troubleshooting. It applies to both CPU and memory consumption.

Kernel mode refers to internal Windows operating system operations whereby the kernel has unrestricted access to system hardware, such as the full memory address range, external devices, and so on.

Application mode (also known as user mode) is responsible for everything else, including running applications such as SQL Server. All user-mode applications access hardware resources through the executive, which runs in kernel mode. An application requiring disk I/O submits the request through the kernel-mode executive, which carries out the request and returns the results to the requesting user-mode process.

CPU Performance Counters

SQL Servers suffering from performance problems caused by high CPU usage is a common performance issue. It can be easy to identify the high-consumption Windows process as sqlservr.exe using Task Manager, but the counters shown in Table 1 will provide additional information to assist in troubleshooting further.

The performance data should be captured for at least a few minutes to ensure the sample is representative. If there is an intermittent problem or when gathering a baseline, a longer data capture period will result in more meaningful results.

TABLE 1: Key CPU PerfMon Counters


Common Causes of CPU Problems

This section describes three common causes of high CPU usage conditions:

  • Missing Statistics or Outdated Statistics — The Query Optimizer is dependent on relevant statistics to determine a good execution plan. Therefore, missing or outdated statistics could cause the Query Optimizer to select a sub-optimal plan, causing excessive CPU consumption.
  • Missing Indexes — A lack of useful indexes can result in a high-CPU condition. SQL Server is dependent on meaningful indexes to retrieve data efficiently, and missing indexes often cause excessive CPU utilization. A lack of useful indexes can result in expensive operations, such as hash joins and sorts that could be avoided with improved indexes.
  • Excessive Recompilation — Poor plan reuse can cause a high-CPU condition whereby SQL Server consumes excessive CPU cycles while generating query plans. Recompilations can be caused by ad hoc or dynamic queries or by a lack of memory (procedure cache), causing plans to be dropped from cache.

Investigating Memory-Related Problems

SQL Server performance is closely related to the availability and performance of sufficient memory. SQL Server configuration-related memory settings include the following:

  • sp_configure
    • Min/max server memory
    • AWE Enabled
    • Min memory per query
  • Windows
    • /3GB, /USERVA, /PAE (in 32-bit environments)
  • Lock Pages in Memory privilege

Typically, using the Windows Task Manager doesn’t provide the best measure of the memory consumed by SQL Server. Using PerfMon is a more reliable method of measuring memory consumption, since this includes all types of memory allocation that can be made by SQL Server.

Types of Memory Pressure

SQL Server can suffer from internal or external memory pressure, and understanding how to identify and troubleshoot each will enable more targeted troubleshooting. External memory pressure occurs most often when SQL Server is running on a shared computer and several processes are competing for memory. In this situation, Resource Monitor within SQL Server Operating System (SQLOS) receives a signal from Windows to request that SQL Server reduce its committed memory. This causes SQL Server to recalculate its target commit level, and reduce it if necessary.

Internal memory pressure occurs when multiple SQL Server resources compete with each other for memory. This typically causes SQL Server to shrink the data cache, which can impact server performance. Use the DBCC MEMORYSTATUS command to gain visibility of SQL Server memory consumption.

Virtual Address Space

Every Windows process has its own virtual address space (VAS), the size of which varies according to processor architecture (32-bit or 64-bit) and the operating system edition. The VAS is a fixed-size resource that can be exhausted (even on 64-bit computers) while physical memory is still available.

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