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With modern NIC drivers shifting to full page allocations per received frame, we face the following issue: TCP has one per-netns sysctl used to tweak how to translate a memory use into an expected payload (RWIN), in RX path. tcp_win_from_space() implementation is limited to few cases. For hosts dealing with various MSS, we either under estimate or over estimate the RWIN we send to the remote peers. For instance with the default sysctl_tcp_adv_win_scale value, we expect to store 50% of payload per allocated chunk of memory. For the typical use of MTU=1500 traffic, and order-0 pages allocations by NIC drivers, we are sending too big RWIN, leading to potential tcp collapse operations, which are extremely expensive and source of latency spikes. This patch makes sysctl_tcp_adv_win_scale obsolete, and instead uses a per socket scaling factor, so that we can precisely adjust the RWIN based on effective skb->len/skb->truesize ratio. This patch alone can double TCP receive performance when receivers are too slow to drain their receive queue, or by allowing a bigger RWIN when MSS is close to PAGE_SIZE. Signed-off-by: Eric Dumazet <edumazet@google.com> Acked-by: Soheil Hassas Yeganeh <soheil@google.com> Link: https://lore.kernel.org/r/20230717152917.751987-1-edumazet@google.com Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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.