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For EPOLLET, applications must call sendmsg until they get EAGAIN. Otherwise, there is no guarantee that EPOLLOUT is sent if there was a failure upon memory allocation. As a result on high-speed NICs, userspace observes multiple small sendmsgs after a partial sendmsg until EAGAIN, since TCP can send 1-2 TSOs in between two sendmsg syscalls: // One large partial send due to memory allocation failure. sendmsg(20MB) = 2MB // Many small sends until EAGAIN. sendmsg(18MB) = 64KB sendmsg(17.9MB) = 128KB sendmsg(17.8MB) = 64KB ... sendmsg(...) = EAGAIN // At this point, userspace can assume an EPOLLOUT. To fix this, set the SOCK_NOSPACE on all partial sendmsg scenarios to guarantee that we send EPOLLOUT after partial sendmsg. After this commit userspace can assume that it will receive an EPOLLOUT after the first partial sendmsg. This EPOLLOUT will benefit from sk_stream_write_space() logic delaying the EPOLLOUT until significant space is available in write queue. Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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.