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When doing hardware GRO (SHAMPO), the driver puts each data payload of a packet from the wire into one skb fragment. TCP Zero-Copy expects page sized skb fragments to be able to do it's page-flipping magic. With the current way of arranging fragments by the driver, only specific MTUs (page sized multiple + header size) will yield such page sized fragments in a high percentage. This change improves payload arrangement in the skb for hardware GRO by coalescing payloads into a single skb fragment when possible. To demonstrate the fix, running tcp_mmap with a MTU of 1500 yields: - Before: 0 % bytes mmap'ed - After : 81 % bytes mmap'ed More importantly, coalescing considerably improves the HW GRO performance. Here are the results for a iperf3 bandwidth benchmark: +---------+--------+--------+------------------------+-----------+ | streams | SW GRO | HW GRO | HW GRO with coalescing | Unit | |---------+--------+--------+------------------------+-----------| | 1 | 36 | 42 | 57 | Gbits/sec | | 4 | 34 | 39 | 50 | Gbits/sec | | 8 | 31 | 35 | 43 | Gbits/sec | +---------+--------+--------+------------------------+-----------+ Benchmark details: VM based setup CPU: Intel(R) Xeon(R) Platinum 8380 CPU, 24 cores NIC: ConnectX-7 100GbE iperf3 and irq running on same CPU over a single receive queue Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com> Signed-off-by: Tariq Toukan <tariqt@nvidia.com> Link: https://lore.kernel.org/r/20240603212219.1037656-15-tariqt@nvidia.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 reStructuredText 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.