GPU Boost on NVIDIA’s Tesla K40 GPU

What is GPU Boost?

GPU Boost is a new user controllable feature to change the processor clock speed on the Tesla K40 GPU. NVIDIA is currently supporting 4 selectable Stream Processor clock speeds and two selectable Memory Clock Speeds on the K40.  The base clock for the Stream Processors is 745MHz and the three selectable options are 666 MHz, 810 MHz and 875MHz (finally, a manufacturer not afraid of superstition!). The base Memory Clock frequencies are 3004MHz (default) and 324MHz (idle). Only the effects of tuning the Stream Processor clock are discussed as there is no application performance increase that results from adjusting the Memory Clock. This blog shows the impact of GPU Boost on a seismic imaging application (Reverse Time Migration) and an electromagnetic solver (Finite-difference time-domain).

GPU Boost is useful as not all applications have the same power profile. The K40 has a maximum 235W power capacity. For example, an application that runs at an average power consumption of 180W at the base frequency will have a 55W power headroom. By increasing the clock frequency, the application theoretically can take advantage of the full 235W capacity.

Enabling GPU Boost

GPU Boost is controlled using NVIDIA’s System Management Interface utility (nvidia-smi) with the following commands:

CommandExplanationnvidia-smi –q –d SUPPORTED_CLOCKSShow Supported Clock Frequenciesnvidia-smi –acSet the Memory and Graphics Clock Frequencynvidia-smi –q –d CLOCKShows current modenvidia-smi –racResets all clocksnvidia-smi –acp 0Allows non-root to change clocks

On the K40, an nvidia-smi query to find the supported clock frequencies gives the following output:

[srahim@corsair3 ~]$ nvidia-smi -q -d SUPPORTED_CLOCKS
==============NVSMI LOG==============
Timestamp                           : Mon Mar 10 17:38:22 2014
Driver Version                      : 331.20


Attached GPUs                       : 2
GPU 0000:05:00.0
    Supported Clocks
        Memory                      : 3004 MHz
            Graphics                : 875 MHz
            Graphics                : 810 MHz
            Graphics                : 745 MHz
            Graphics                : 666 MHz
        Memory                      : 324 MHz
            Graphics                : 324 MHz


GPU 0000:42:00.0
    Supported Clocks
        Memory                      : 3004 MHz
            Graphics                : 875 MHz
            Graphics                : 810 MHz
            Graphics                : 745 MHz
            Graphics                : 666 MHz
        Memory                      : 324 MHz
            Graphics                : 324 MHz

 

To set the clock speed to 666MHz, run

[srahim@corsair3 ~]$ sudo nvidia-smi –ac 3004,666
Applications clocks set to "(MEM 3004, SM 666)" for GPU 0000:05:00.0
All done.

If you try to set an unsupported clock speed, nvidia-smi shows a helpful message.

[srahim@corsair3 ~]$ sudo nvidia-smi -ac 3004,888
Specified clock combination "(MEM 3004, SM 888)" is not supported for GPU 0000:05:00.0. Run 'nvidia-smi -q -d SUPPORTED_CLOCKS' to see list of supported clock combinations
Terminating early due to previous errors.

The current clock speed is checked with the following command:

[srahim@corsair3 ~]$ nvidia-smi -q -d CLOCK
==============NVSMI LOG==============


Timestamp                           : Tue Mar 11 10:40:16 2014
Driver Version                      : 331.20


Attached GPUs                       : 2
GPU 0000:05:00.0
    Clocks
        Graphics                    : 324 MHz
        SM                          : 324 MHz
        Memory                      : 324 MHz
    Applications Clocks
        Graphics                    : 666 MHz
        Memory                      : 3004 MHz
    Default Applications Clocks
        Graphics                    : 745 MHz
        Memory                      : 3004 MHz
    Max Clocks
        Graphics                    : 875 MHz
        SM                          : 875 MHz
        Memory                      : 3004 MHz

The GPU Boost settings are not persistent between reboots or driver unloads and should be scripted if persistence is desired. Unless the NVIDIA driver persistence mode is set with nvidia-smi –pm 1, the driver may unload when the GPU is idle.

Unlike Intel’s Turbo Boost, GPU Boost is not on by default. This puts the impetus on the end user or system administrator to take advantage of this feature. An application can programmatically change the Boost clock with NVML if run with appropriate permissions. Run nvidia-smi –acp 0 to grant non-root users permission to change clocks. Two caveats on the uses of GPU Boost from the document “NVIDIA GPU Boost for Tesla” are:

1. An important point to remember is that no matter which clocks the end user selects, if at any time the power monitoring algorithm detects that the application may exceed the 235 W, the GPU comes down to a lower clock level as a precaution. Once the power falls below 235 W the GPU will raise its core clock to the selected clock. This happens automatically and the Tesla K40 does have a few clock levels below the base clock to handle any power digressions.
2. If the workload runs on multiple GPUs and is sensitive to all GPUs running at the same clock, the user may need to try out which particular clock works best for all GPUs.

If you are not running on multiple GPUs or multiple nodes, you should crank up the GPU clock speed to the maximum frequency. Keep the GPU clock at the default value of 745MHz or lower only if there are power consumption concerns.

 

GPU Boost is a relatively simple way to increase performance by up to 30%. It is certainly easier than optimizing a CUDA kernel. Unless you are doing some crazy multi-node MPI application, always boost your clock speed for higher performance!