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When we increase the link send window we sometimes observe the following scenario: 1) A packet #N arrives out of order far ahead of a sequence of older packets which are still under way. The packet is added to the deferred queue. 2) The missing packets arrive in sequence, and for each 16th of them an ACK is sent back to the receiver, as it should be. 3) When building those ACK messages, it is checked if there is a gap between the link's 'rcv_nxt' and the first packet in the deferred queue. This is always the case until packet number #N-1 arrives, and a 'gap' indicator is added, effectively turning them into NACK messages. 4) When those NACKs arrive at the sender, all the requested retransmissions are done, since it is a first-time request. This sometimes leads to a huge amount of redundant retransmissions, causing a drop in max throughput. This problem gets worse when we in a later commit introduce variable window congestion control, since it drops the link back to 'fast recovery' much more often than necessary. We now fix this by not sending any 'gap' indicator in regular ACK messages. We already have a mechanism for sending explicit NACKs in place, and this is sufficient to keep up the packet flow. Acked-by: Ying Xue <ying.xue@windriver.com> Signed-off-by: Jon Maloy <jon.maloy@ericsson.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
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.