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Windows Server 2012 : Comprehensive Performance Analysis and Logging (part 5) - Resolving performance bottlenecks - Resolving memory bottlenecks

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Resolving performance bottlenecks

Generally, a bottleneck is any condition that keeps a computer from performing at its best. Bottlenecks can also apply to situations in which one resource is preventing another resource from performing optimally. For example, if a system doesn’t have enough physical memory, it doesn’t matter whether it has a fast processor or a slow processor. The system will still perform poorly because it doesn’t have enough physical memory available and must rely heavily on the paging file, reading and writing to disk frequently.

Memory is usually the main bottleneck on both workstations and servers. It is the resource you should examine first to try to determine why a system isn’t performing as expected. But memory isn’t the only bottleneck. The processor, disk subsystem, and networking components are also sources of potential performance bottlenecks.

Resolving memory bottlenecks

Windows applications use a lot of memory. If you install a server with the minimum amount of memory required, it isn’t going to perform at its optimal level. The reason for this is that a server’s memory requirements depend on many factors, including the services, components, and applications that are installed on the server, as well as the server’s configuration.

Computers use both physical and virtual memory. Physical memory is represented by the amount of random access memory (RAM) installed. Virtual memory is memory written to a paging file on disk. Reading from and writing to the paging file involve the disk subsystem, and it is much slower than accessing physical memory. Because of this, you don’t want a system to have to use the paging file too frequently.

Before you set out to monitor memory usage, you should check to ensure that the computer has the recommended amount of memory for the operating system and the applications it is running. After you’ve done this, you can determine how the system is using memory and check for problems. Look closely at the amount of memory available and the amount of virtual memory being used. If the server has very little available memory, you might need to add memory to the system. In general, you want the available memory to be no less than 5 percent of the total physical memory on the server. If the server has a high ratio of virtual memory being used to total physical memory on the system, you might need to add physical memory as well.

Look at the way the system is using the paged pool and nonpaged pool memory. The paged pool is an area of system memory for objects that can be written to disk when they aren’t used. The nonpaged pool is an area of system memory for objects that can’t be written to disk. If the size of the paged pool is large relative to the total amount of physical memory on the system, you might need to add memory to the system. If the size of the nonpaged pool is large relative to the total amount of virtual memory allocated to the server, you might want to increase the virtual memory size.

Look at the way the system is using the paging file. A page fault occurs when a process requests a page in memory and the system can’t find it at the requested location. If the requested page is elsewhere in memory, the fault is called a soft page fault. If the requested page must be retrieved from the paging file on disk, the fault is called a hard page fault. Most processors can handle large numbers of soft faults. Hard faults, however, can cause significant delays. If there are a high number of hard page faults, you might need to increase the amount of memory or reduce the size of the system cache.

Counters you can use to check for memory bottlenecks include the following:

  • Memory\Available Bytes Records the number of bytes of physical memory available to processes running on the server. When there is less than 5 percent of memory free, the system is low on memory and performance can suffer. The server might page excessively to disk to try to keep up with resource demands. Memory is critically short if there is 128 megabytes (MBs) or less of memory free; in this case, the system might page excessively to disk and try to borrow memory from running processes to keep up with resource demands. If the system is very low on memory, it could also point to a possible memory leak.

  • Memory\Committed Bytes Records the number of bytes of committed virtual memory. This represents memory that has been paged to disk and is in use. If a server is using too much virtual memory relative to the total physical memory on the system, you might need to add physical memory.

  • Memory\Commit Limit Shows the total physical and virtual memory available. As the number of committed bytes grows, the paging file is allowed to grow up to its maximum size, which can be determined by subtracting the total physical memory on the system from the commit limit. If you set the initial paging file size too small, the system will repeatedly extend the paging file, and this requires system resources. It is better to set the initial page size as appropriate for typical usage or simply use a fixed paging file size. For a fixed paging file, set the size to at least two times the size of RAM.

  • Memory\Page Faults/Sec Records the average number of page faults per second. It includes both hard and soft page faults. Soft faults result in memory lookups. Hard faults require access to disk.

  • Memory\Pages/Sec Records the number of memory pages that are read from disk or written to disk to resolve hard page faults. It is the sum of Memory\Pages Input/Sec and Memory\Pages Output/Sec.

  • Memory\Pages Input/Sec Records the rate at which pages are read from disk to resolve hard page faults. Hard page faults occur when a requested page isn’t in memory and the computer has to go to disk to get it. Too many hard faults can cause significant delays and hurt performance.

  • Memory\Pages Output/Sec Records the rate at which pages are written to disk to free up space in physical memory. If the server has to free up memory too often, this is an indicator that there isn’t enough physical memory (RAM) on the system.

  • Memory\Pool Paged Bytes Represents the size in bytes of the paged pool. The paged pool is an area of system memory for objects that can be written to disk when they aren’t used. If the size of the paged pool is large relative to the total amount of physical memory on the system, you might need to add memory to the system. If this value slowly increases in size over time, a kernel-mode process might have a memory leak.

  • Memory\Pool Nonpaged Bytes Represents the size in bytes of the nonpaged pool. The nonpaged pool is an area of system memory for objects that can’t be written to disk. If the size of the nonpaged pool is large relative to the total amount of virtual memory allocated to the server, you might want to increase the virtual memory size. If this value slowly increases in size over time, a kernel-mode process might have a memory leak.

  • Paging File\%Usage Records the percentage of the paging file currently in use. If this value approaches 100 percent for all instances, you should consider either increasing the virtual memory size or adding physical memory to the system. This will ensure that the server has additional memory if it needs it, such as when the server load grows.

  • Paging File\%Usage Peak Records the peak size of the paging file as a percentage of the total paging file size available. A high value can mean that the paging file isn’t large enough to handle increased load conditions.

  • Physical Disk\%Disk Time Records the percentage of time that the selected disk spent servicing read and write requests. Keep track of this value for the physical disks that have paging files. If you see this value increasing over several monitoring periods, you should more closely monitor paging-file usage and you might consider adding physical memory to the system.

  • Physical Disk\Avg Disk Queue Length Records the average number of read and write requests that were waiting for the selected disk during the sample interval. Keep track of this value for the physical disks that have paging files. If you see this value increasing over time and the Memory\Page Reads/Sec is also increasing, the system is having to perform a lot of paging-file reads.

  • Physical Disk\Avg Disk Sec/Transfer Records the length in seconds of the average disk transfer. Track this value for the physical disks that have paging files in conjunction with Memory\Pages/Sec. Memory\Pages/Sec tracks the number of reads and writes for the paging file. If you multiply the Physical Disk\Avg Disk Sec/Transfer by the Memory\Pages/Sec value, you have an excellent indicator of how much of the disk access time is being used by paging. Use the result to help you decide whether to move the paging files to faster disks or add physical memory to the system.

Resolving processor bottlenecks

After you’ve eliminated memory as a potential bottleneck, you should examine the system’s processor usage to determine whether there are any potential bottlenecks. Processor bottlenecks can occur if a process’s threads need more processing time than is available. This, in turn, causes the processor queue to grow because threads have to wait to get processing time. As a result, the system response suffers and the system appears sluggish or nonresponsive.

Excess interrupts are another common reason for processor bottlenecks. Each time drivers or disk subsystem components, such as hard disk drives or network components, generate an interrupt, the processor has to stop what it is doing to handle the request because requests from hardware take priority. However, poorly designed drivers and components can generate false interrupts, which tie up the processor for no reason. System boards or components that are failing can generate false interrupts as well.

TROUBLESHOOTING: Rule out processor affinity as an issue on multiprocessor systems

On multiprocessor systems, you might need to rule out processor affinity as a cause of a processor bottleneck. By using processor affinity, you can set a program or process to use a specific processor to improve its performance. Assigning processor affinity, however, can block access to the processor for other programs and processes.

If a system’s processors are the performance bottleneck, adding memory, drives, or network connections won’t overcome the problem. Instead, you might need to upgrade the processors to faster clock speeds or add processors to increase the server’s upper capacity. You could also move processor-intensive applications, such as Microsoft Exchange Server, to another server.

Counters you can use to check for processor bottlenecks include the following:

  • System\Processor Queue Length Records the number of threads waiting to be executed. These threads are queued in an area shared by all processors on the system. If this counter has a sustained value of 10 or more threads, you might need to upgrade the processors to faster clock speeds or add processors to increase the server’s upper capacity.

  • Processor\%Processor Time Records the percentage of time the selected processor is executing a nonidle thread. You should track this counter separately for all processor instances on the server. If the %Processor Time values for all instances are high (above 75 percent) while the network interface and disk input/output (I/O) throughput rates are relatively low, you might need to upgrade the processors to faster clock speeds or add processors to increase the server’s upper capacity.

  • Processor\%User Time Records the percentage of time the selected processor is executing a nonidle thread in User mode. User mode is a processing mode for applications and user-level subsystems. A high value for all process instances might indicate that you need to upgrade the processors to faster clock speeds or add processors to increase the server’s upper capacity.

  • Processor\%Privileged Time Records the percentage of time the selected processor is executing a nonidle thread in Privileged mode. Privileged mode is a processing mode for operating system components and services, allowing direct access to hardware and memory. A high value for all processor instances might indicate that you need to upgrade the processors to faster clock speeds or add processors to increase the server’s upper capacity.

  • Processor\Interrupts/Sec Records the average rate, in incidents per second, that the processor received and serviced hardware interrupts. Compare this value to your baselines. If this value changes substantially (I mean by thousands of interrupts) without a corresponding increase in activity, the system might have a hardware problem. To resolve this problem, you must identify the device or component that is causing the problem. Start with devices that have drivers you’ve updated recently.

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