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There are several reasons for increasing the receive ring sizes: 1. The original ring size of 256 was chosen about 10 years ago when vmxnet3 was first created. At that time, 10Gbps Ethernet was not prevalent and servers were dominated by 1Gbps Ethernet. Now 10Gbps is common place, and higher bandwidth links -- 25Gbps, 40Gbps, 50Gbps -- are starting to appear. 256 Rx ring entries are simply not enough to keep up with higher link speed when there is a burst of network frames coming from these high speed links. Even with full MTU size frames, they are gone in a short time. It is also more common to have a mix of frame sizes, and more likely bi-modal distribution of frame sizes so the average frame size is not close to full MTU. If we consider average frame size of 800B, 1024 frames that come in a burst takes ~0.65 ms to arrive at 10Gbps. With 256 entires, it takes ~0.16 ms to arrive at 10Gbps. At 25Gbps or 40Gbps, this time is reduced accordingly. 2. On a hypervisor where there are many VMs and CPU is over committed, i.e. the number of VCPUs is more than the number of VCPUs, each PCPU is in effect time shared between multiple VMs/VCPUs. The time granularity at which this multiplexing occurs is typically coarser than between processes on a guest OS. Trying to time slice more finely is not efficient, for example, if memory cache is barely warmed up when switching from one VM to another occurs. This CPU overcommit adds delay to when the driver in a VM can service incoming packets. Whether CPU is over committed really depends on customer workloads. For certain situations, it is very common. For example, workloads of desktop VMs and product testing setups. Consolidation and sharing is what drives efficiency of a customer setup for such workloads. In these situations, the raw network bandwidth may not be very high, but the delays between when a VM is running or not running can also be relatively long. Signed-off-by: Shrikrishna Khare <skhare@vmware.com> Acked-by: Jin Heo <heoj@vmware.com> Acked-by: Guolin Yang <gyang@vmware.com> Acked-by: Boon Ang <bang@vmware.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.