AIX Version 4.3 Differences Guide

• New macros to allow for simple read-write locking, and the INIFADDR lock
macros are changed to use them.
7.9.3.2 Reducing the Contention of Route Lock
Currently, all outgoing IP packets must take the single route lock. On an SMP
machine with multiple adapters, a large number of packets must take this lock, so
it is the hot (critical) lock on output. Reducing the contention on this lock can
increase the throughput on SMP systems with multiple adapters.
There is a global route lock that must be taken any time any route is changed or
used. Each connection keeps a pointer to a cached route, but it still must take the
route lock to check the validity of its cached route. So, the route lock must be
taken for every outgoing packet.
The new design eliminates the requirement to take the global route lock for
outgoing packets in the case where the cached route is valid. This should
improve performance for TCP, where the cached route will usually be valid so the
global lock will not be needed on packet output. For UDP, the cached route often
is not valid, and so the global lock will still be needed for output. But since the
contention on this lock from other code will be reduced, there is also a
performance improvement for UDP.
The global route lock that protects the structure of the routing table has also been
changed to use the same read/write lock in places where the global lock is still
needed.
7.10 TCP Checksum Offload on ATM 155 Mbps PCI Adapter (4.3.2)
The PCI ATM 155 adapter has a hardware TCP/UDP checksum capability. In AIX
4.3.2, the ATM network device driver is modified to use this feature.
In the new ATM 155 adapter device driver, the workload of TCP data checksum
processing is offloaded from the AIX TCP/IP protocol stack to the adapter itself. A
related enhancement automatically remembers the mapping of virtual addresses
to physical addresses for the entire networking buffer pool to save address
translation during networking I/O operations.
TCP checksum offload for ATM reduces the amount of time spent on the main
CPU computing checksums, thereby reducing latency and allowing the system to
handle more work, in particular, to handle more packets in the same amount of
time. This results in performance improvements.
The SpecWeb96 benchmark is published on the 7017-S70 using ATM interfaces.
It benefits from this checksum offload capability.
TCP and the other network layers continue to perform as they do today,
particularly on adapters that do not support checksum offload. All TCP functions
are the same.
In order to improve performance for TCP over ATM, small packets do not have
their checksum processing offloaded since setting up the offload will probably use
as much processor time as computing the checksum. There is no gain, and may
actually be a loss, in offloading checksum processing for these small packets.
176 AIX Version 4.3 Differences Guide