diff --git a/docs/man/docker-run.1.md b/docs/man/docker-run.1.md index 7e3875afcc..cbb835b87b 100644 --- a/docs/man/docker-run.1.md +++ b/docs/man/docker-run.1.md @@ -82,24 +82,38 @@ option can be set multiple times. **-c**, **--cpu-shares**=0 CPU shares (relative weight) - You can increase the priority of a container -with the -c option. By default, all containers run at the same priority and get -the same proportion of CPU cycles, but you can tell the kernel to give more -shares of CPU time to one or more containers when you start them via **docker -run**. + By default, all containers get the same proportion of CPU cycles. This proportion +can be modified by changing the container's CPU share weighting relative +to the weighting of all other running containers. -The flag `-c` or `--cpu-shares` with value 0 indicates that the running -container has access to all 1024 (default) CPU shares. However, this value -can be modified to run a container with a different priority or different -proportion of CPU cycles. +To modify the proportion from the default of 1024, use the **-c** or **--cpu-shares** +flag to set the weighting to 2 or higher. -E.g., If we start three {C0, C1, C2} containers with default values -(`-c` OR `--cpu-shares` = 0) and one {C3} with (`-c` or `--cpu-shares`=512) -then C0, C1, and C2 would have access to 100% CPU shares (1024) and C3 would -only have access to 50% CPU shares (512). In the context of a time-sliced OS -with time quantum set as 100 milliseconds, containers C0, C1, and C2 will run -for full-time quantum, and container C3 will run for half-time quantum i.e 50 -milliseconds. +The proportion will only apply when CPU-intensive processes are running. +When tasks in one container are idle, other containers can use the +left-over CPU time. The actual amount of CPU time will vary depending on +the number of containers running on the system. + +For example, consider three containers, one has a cpu-share of 1024 and +two others have a cpu-share setting of 512. When processes in all three +containers attempt to use 100% of CPU, the first container would receive +50% of the total CPU time. If you add a fouth container with a cpu-share +of 1024, the first container only gets 33% of the CPU. The remaining containers +receive 16.5%, 16.5% and 33% of the CPU. + +On a multi-core system, the shares of CPU time are distributed over all CPU +cores. Even if a container is limited to less than 100% of CPU time, it can +use 100% of each individual CPU core. + +For example, consider a system with more than three cores. If you start one +container **{C0}** with **-c=512** running one process, and another container +**{C1}** with **-c=1024** running two processes, this can result in the following +division of CPU shares: + + PID container CPU CPU share + 100 {C0} 0 100% of CPU0 + 101 {C1} 1 100% of CPU1 + 102 {C1} 2 100% of CPU2 **--cap-add**=[] Add Linux capabilities diff --git a/docs/sources/reference/commandline/cli.md b/docs/sources/reference/commandline/cli.md index da4daec88b..abe752260c 100644 --- a/docs/sources/reference/commandline/cli.md +++ b/docs/sources/reference/commandline/cli.md @@ -817,7 +817,8 @@ container at any point. This is useful when you want to set up a container configuration ahead of time so that it is ready to start when you need it. -Please see the [run command](#run) section for more details. +Please see the [run command](#run) section and the [Docker run reference]( +/reference/run/) for more details. #### Examples diff --git a/docs/sources/reference/run.md b/docs/sources/reference/run.md index 268ade05ee..f2400ecfc4 100644 --- a/docs/sources/reference/run.md +++ b/docs/sources/reference/run.md @@ -405,7 +405,9 @@ container: -m="": Memory limit (format: , where unit = b, k, m or g) -memory-swap="": Total memory limit (memory + swap, format: , where unit = b, k, m or g) - -c=0 : CPU shares (relative weight) + -c, --cpu-shares=0 CPU shares (relative weight) + +### Memory constraints We have four ways to set memory usage: - memory=inf, memory-swap=inf (not specify any of them) @@ -423,24 +425,40 @@ We have four ways to set memory usage: It is not allowed to use more than L bytes of memory, swap *plus* memory usage is limited by S. -The operator can increase the priority of this container with -the `-c` option. By default, all containers run at the same priority and -get the same proportion of CPU cycles, but you can tell the kernel to -give more shares of CPU time to one or more containers when you start -them via Docker. +### CPU share constraint -The flag `-c` or `--cpu-shares` with value 0 indicates that the running -container has access to all 1024 (default) CPU shares. However, this value -can be modified to run a container with a different priority or different -proportion of CPU cycles. +By default, all containers get the same proportion of CPU cycles. This proportion +can be modified by changing the container's CPU share weighting relative +to the weighting of all other running containers. -E.g., If we start three {C0, C1, C2} containers with default values -(`-c` OR `--cpu-shares` = 0) and one {C3} with (`-c` or `--cpu-shares`=512) -then C0, C1, and C2 would have access to 100% CPU shares (1024) and C3 would -only have access to 50% CPU shares (512). In the context of a time-sliced OS -with time quantum set as 100 milliseconds, containers C0, C1, and C2 will run -for full-time quantum, and container C3 will run for half-time quantum i.e 50 -milliseconds. +To modify the proportion from the default of 1024, use the `-c` or `--cpu-shares` +flag to set the weighting to 2 or higher. + +The proportion will only apply when CPU-intensive processes are running. +When tasks in one container are idle, other containers can use the +left-over CPU time. The actual amount of CPU time will vary depending on +the number of containers running on the system. + +For example, consider three containers, one has a cpu-share of 1024 and +two others have a cpu-share setting of 512. When processes in all three +containers attempt to use 100% of CPU, the first container would receive +50% of the total CPU time. If you add a fouth container with a cpu-share +of 1024, the first container only gets 33% of the CPU. The remaining containers +receive 16.5%, 16.5% and 33% of the CPU. + +On a multi-core system, the shares of CPU time are distributed over all CPU +cores. Even if a container is limited to less than 100% of CPU time, it can +use 100% of each individual CPU core. + +For example, consider a system with more than three cores. If you start one +container `{C0}` with `-c=512` running one process, and another container +`{C1}` with `-c=1024` running two processes, this can result in the following +division of CPU shares: + + PID container CPU CPU share + 100 {C0} 0 100% of CPU0 + 101 {C1} 1 100% of CPU1 + 102 {C1} 2 100% of CPU2 ## Runtime privilege, Linux capabilities, and LXC configuration