Knowledge Base

This article provides information on isolating a performance issue on ESX/ESXi. Bad performance can be caused by several different areas: CPU constraints, memory overcommitment, storage latency, or network latency. If one or more of your virtual machines has a bad response time, consider each of these areas to find the bottleneck.

This article includes four main sections:

• CPU constraints
• Memory overcommitment
• Storage latency
• Network Latency

CPU constraints

To determine whether the poor performance is due to a CPU constraint:

1. Use the esxtop command to determine if the ESX/ESXi server is being overloaded. For more information about esxtop, see the Resource Management Guide for your version of ESX/ESXi:
   
   
   • ESXi 5.0
   • ESXi 5.1
   • ESX/ESXi 4.1
   • ESX 4.0
   • ESX 3.5:
     • Update 2 and later
     • Initial Release and Update 1
   • ESX 3.0
     
   1. Examine the load average on the first line of the command output.
      
      A load average of 1.00 means that the ESX/ESXi Server machine’s physical CPUs are fully utilized, and a load average of 0.5 means that they are half utilized. A load average of 2.00 means that the system as a whole is overloaded.
      
      
   2. Examine the %READY field for the percentage of time that the virtual machine was ready but could not be scheduled to run on a physical CPU.
      
      Under normal operating conditions, this value should remain under 5%. If the ready time values are high on the virtual machines that experience bad performance, then check for CPU limiting:
      
      
      • Make sure the virtual machine is not constrained by a CPU limit set on itself
      • Make sure that the virtual machine is not constrained by its resource pool.
        
        For more information, see Impact of virtual machine memory and CPU resource limits (1033115).
        
        
   If the load average is too high, and the ready time is not caused by CPU limiting, adjust the CPU load on the host. To adjust the CPU load on the host, either:
   
   
   • Increase the number of physical CPUs on the host
     
     OR
     
     
   • Decrease the number of virtual CPUs allocated to the host. To decrease the number of virtual CPUs allocated to the host, either:
     
     
     • Reduce the total number of CPUs allocated to all of the virtual machines running on the ESX host. For more information, see Determining if multiple virtual CPUs are causing performance issues (1005362).
       
       OR
       
       
     • Reduce the number of virtual machines running on the host.
       
       
2. If you are using ESX 3.5, determine whether IRQ sharing is an issue. For more information, see ESX has performance issues due to IRQ sharing (1003710).
   

Memory overcommitment

To determine whether the poor performance is due to memory overcommitment:

1. Use the esxtop command to determine whether the ESX/ESXi server's memory is overcommitted. For more information about esxtop, see the Resource Management Guide for your version of ESX/ESXi:
   
   
   • ESXi 5.0
   • ESXi 5.1
   • ESX/ESXi 4.1
   • ESX 4.0
   • ESX 3.5:
     • Update 2 and later
     • Initial Release and Update 1
   • ESX 3.0
     
   1. Examine the MEM overcommit avg on the first line of the command output. This value reflects the ratio of the requested memory to the available memory, minus 1.
      
      Examples:
      
      
      • If the virtual machines require 4 GB of RAM, and the host has 4 GB of RAM, then there is a 1:1 ratio. After subtracting 1 (from 1/1), the MEM overcommit avg field reads 0. There is no overcommitment and no extra RAM is required.
      • If the virtual machines require 6 GB of RAM, and the host has 4 GB of RAM, then there is a 1.5:1 ratio. After subtracting 1 (from 1.5/1), the MEM overcommit avg field reads .5. The RAM is overcommited by 50%, meaning that 50% more than the available RAM is required.
        
        
   If the memory is being overcommited, adjust the memory load on the host. To adjust the memory load, either:
   
   
   • Increase the amount of physical RAM on the host
     
     OR
     
     
   • Decrease the amount of RAM allocated to the virtual machines. To decrease the amount of allocated RAM, either:
     
     
     • Decrease the total amount of RAM allocated to all of the virtual machines on the host
       
       OR
       
       
     • Reduce the total number of virtual machines on the host.
       
       
2. Determine whether the virtual machines are ballooning and/or swapping.
   
   To detect any ballooning or swapping:
   
   
   1. Run esxtop.
   2. Type m for memory
   3. Type f for fields
      
   4. Select the letter J for Memory Ballooning Statistics (MCTL)
   5. Look at the MCTLSZ value.
      
      MCTLSZ (MB) displays the amount of guest physical memory reclaimed by the balloon driver.
      
      
   6. Type f for Field
   7. Select the letter for Memory Swap Statistics (SWAP STATS).
   8. Look at the SWCUR value.
      
      SWCUR (MB) displays the current Swap Usage.
      
      
   To resolve this issue, ensure that the ballooning and/or swapping is not caused by the memory limit being incorrectly set. If the memory limit is incorrectly set, reset it correctly. For more information, see:
   
   
   • Impact of virtual machine memory and CPU resource limits (1033115)
   • Balloon driver retains hold on memory causing virtual machine guest operating system performance issues (1003470)

Storage Latency

To determine whether the poor performance is due to storage latency:

1. Determine whether the problem is with the local storage. Migrate the virtual machines to a different storage location.
2. Reduce the number of Virtual Machines per LUN.
3. Look for log entries in the Windows guests that look like this:
   
   The device, \Device\ScsiPort0, did not respond within the timeout period.

   
4. Using esxtop, look for a high DAVG latency time. For more information, see Using esxtop to identify storage performance issues (1008205).
5. Determine the maximum I/O throughput you can get with the iometer command. For more information, see Testing virtual machine storage I/O performance (1006821).
6. Compare the iometer results for a VM to the results for a physical machine attached to the same storage.
7. Check for SCSI reservation conflicts. For more information, see Analyzing SCSI Reservation conflicts on VMware Infrastructure 3.x and vSphere 4.x (1005009).
8. If you are using iSCSI storage and jumbo frames, ensure that everything is properly configured. For more information, see:
   
   
   • iSCSI and Jumbo Frames configuration on ESX/ESXi (1007654)
   • Enabling IOAT and Jumbo frames (1003712)
   • Configuring Jumbo Frames on a vNetwork Distributed Switch (1010711)
   • Enabling Jumbo Frames for VMkernel ports in a virtual distributed switch (1038827)
     
     
9. If you are using iSCSI storage and multipathing with the iSCSI software initiator, ensure that everything is properly configured. For more information, see these sections of the iSCSI SAN Configuration Guide:
   
   
   • Networking Configuration for Software iSCSI and Dependent Hardware iSCSI
   • Bind iSCSI Ports to iSCSI Adapters
     

1. Ensure that your hardware array and your HBA cards are certified for ESX/ESXi. For more information, see the VMware Hardware Compatibility List.
2. Ensure that the BIOS of your physical server is up to date. For more information, see Checking your firmware and BIOS levels to ensure compatibility with ESX/ESXi (1037257).
3. Ensure that the firmware of your HBA is up to date. For more information, see Slow performance caused by out of date firmware on a RAID controller or HBA (1006696).
4. Ensure that the ESX can recognize the correct mode and path policy for your SATP Storage array type and PSP Path Selection. For more information, see Verifying correct storage settings on ESX 4.x, ESXi 4.x and ESXi 5.0 (1020100).
   

Network latency

Network performance can be highly affected by CPU performance. Rule out a CPU performance issue before investigating network latency.

To determine whether the poor performance is due to network latency:

1. Test the maximum bandwidth from the virtual machine with the Iperf tool. This tool is available from http://sourceforge.net/projects/iperf/.
   
   Note: VMware does not endorse or recommend any particular third-party utility.
   
   
2. 1. While using Iperf, change the TCP windows size to 64 K. Performance also depends also on this value. To change the TCP windows size:
      
      
      1. On the server side, enter this command:
         
         iperf -s

         
      2. On the client side, enter this command:
         
         iperf.exe -c sqlsed -P 1 -i 1 -p 5001 -w 64K -f m -t 10 900M

         

For more information, see http://openmaniak.com/iperf.php.

1. Run Iperf with a machine outside the ESX/ESXi host. Compare the results with what you expect you should have, depending on your physical environment.
2. Run Iperf with another machine outside the ESX/ESXi host on the same VLAN on the same physical switch. If the performance is good, and the issue can only be reproduced with a machine at another geographical location, then the issue is related to your network environment.
3. Run Iperf between 2 VMs on the same ESX server/portgroup/vswitch. If the result is good, you can exclude a CPU, memory or storage issue.

1. Work through the steps in Troubleshooting network performance issues (1004087).
2. If you are using iSCSI storage and jumbo frames, ensure that everything is properly configured. For more information, see:
   
   
   • iSCSI and Jumbo Frames configuration on ESX/ESXi (1007654)
   • Enabling IOAT and Jumbo frames (1003712)
   • Configuring Jumbo Frames on a vNetwork Distributed Switch (1010711)
   • Enabling Jumbo Frames for VMkernel ports in a virtual distributed switch (1038827)
     
     
3. If you are using Network I/O Control, ensure that the shares and limits are properly configured for your traffic. For more information, see Network I/O Resource Management in vSphere 4.1 with vDS (1022585).
4. Ensure that traffic shaping is correctly configured. For more information, see Traffic Shaping Policy in the ESX/ESXi Configuration Guide.