Datasheet

Intel
®
82599 10 GbE Controller—- Software Clarifications
52
3.0 - Software Clarifications
1 While in TCP Segmentation Offload, Each Buffer Is
Limited to 64 KB
The 82599 supports 256 KB TCP packets; however, each buffer is limited to 64 KB since the data length
field in the transmit descriptor is only 16 bits. This restriction increases driver implementation
complexity if the operating system passes down a scatter/gather element greater than 64 KB in length.
This can be avoided by limiting the offload size to 64 KB.
Investigation has concluded that the increase in data transfer size does not provide any noticeable
improvements in LAN performance. As a result, Intel network software drivers limit the data transfer
size in all drivers to 64 KB.
Please note that Linux* operating systems only support 64 KB data transfers.
2 RSC Performance Tradeoff
The RSC feature is used to merge receive frames into the same descriptor structure with a shared
header, improving receiving packet performance.
It should be noted that if small Rx data buffers are used (2 KB), RSC may involve a high rate of partial
cache line PCIe transactions, which have a performance penalty from a memory access perspective.
In overloaded systems (more than 2 x 10 Gb/s LAN ports traffic load) the use of RSC may adversely
affect Rx data throughput. Therefore, there is a performance tradeoff regarding the usage of the RSC
feature.
To improve throughput in overloaded systems, the user can use large receive data buffers (larger than
2 KB or may opt to turn of RSC.
Table 3-1 Summary of Software Clarifications
Software Clarification Status
1. While in TCP Segmentation Offload, Each Buffer Is Limited to 64 KB N/A
2. RSC Performance Tradeoff N/A
3. Serial Interfaces Programmed by Bit Banging N/A
4. Identify Network Adapter Port by Blinking LED N/A
5. PF/VF Drivers Should Configure Registers That Are not Reset by VFLR N/A