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  man pages->FreeBSD man pages -> zero_copy_sockets (9)              



NAME    [Toc]    [Back]

     zero_copy, zero_copy_sockets

SYNOPSIS    [Toc]    [Back]

     options ZERO_COPY_SOCKETS

DESCRIPTION    [Toc]    [Back]

     The FreeBSD kernel includes a facility for eliminating data copies on
     socket reads and writes.

     This code is collectively known as the zero copy sockets code, because
     during normal network I/O, data will not be copied by the CPU at all.
     Rather it will be DMAed from the user's buffer to the NIC (for sends), or
     DMAed from the NIC to a buffer that will then be given to the user

     The zero copy sockets code uses the standard socket read and write semantics,
 and therefore has some limitations and restrictions that programmers
 should be aware of when trying to take advantage of this functionality.

     For sending data, there are no special requirements or capabilities that
     the sending NIC must have.  The data written to the socket, though, must
     be at least a page in size and page aligned in order to be mapped into
     the kernel.  If it does not meet the page size and alignment constraints,
     it will be copied into the kernel, as is normally the case with socket

     The user should be careful not to overwrite buffers that have been written
 to the socket before the data has been freed by the kernel, and the
     copy-on-write mapping cleared.  If a buffer is overwritten before it has
     been given up by the kernel, the data will be copied, and no savings in
     CPU utilization and memory bandwidth utilization will be realized.

     The socket(2) API does not really give the user any indication of when
     his data has actually been sent over the wire, or when the data has been
     freed from kernel buffers.  For protocols like TCP, the data will be kept
     around in the kernel until it has been acknowledged by the other side; it
     must be kept until the acknowledgement is received in case retransmission
     is required.

     From an application standpoint, the best way to guarantee that the data
     has been sent out over the wire and freed by the kernel (for TCP-based
     sockets) is to set a socket buffer size (see the SO_SNDBUF socket option
     in the setsockopt(2) man page) appropriate for the application and network
 environment and then make sure you have sent out twice as much data
     as the socket buffer size before reusing a buffer.  For TCP, the send and
     receive socket buffer sizes generally directly correspond to the TCP window

     For receiving data, in order to take advantage of the zero copy receive
     code, the user must have a NIC that is configured for an MTU greater than
     the architecture page size.  (E.g., for alpha this would be 8KB, for
     i386, it would be 4KB.)  Additionally, in order for zero copy receive to
     work, packet payloads must be at least a page in size and page aligned.

     Achieving page aligned payloads requires a NIC that can split an incoming
     packet into multiple buffers.  It also generally requires some sort of
     intelligence on the NIC to make sure that the payload starts in its own
     buffer.  This is called ``header splitting''.  Currently the only NICs
     with support for header splitting are Alteon Tigon 2 based boards running
     slightly modified firmware.  The FreeBSD ti(4) driver includes modified
     firmware for Tigon 2 boards only.	Header splitting code can be written,
     however, for any NIC that allows putting received packets into multiple
     buffers and that has enough programability to determine that the header
     should go into one buffer and the payload into another.

     You can also do a form of header splitting that does not require any NIC
     modifications if your NIC is at least capable of splitting packets into
     multiple buffers.	This requires that you optimize the NIC driver for
     your most common packet header size.  If that size (ethernet + IP + TCP
     headers) is generally 66 bytes, for instance, you would set the first
     buffer in a set for a particular packet to be 66 bytes long, and then
     subsequent buffers would be a page in size.  For packets that have headers
 that are exactly 66 bytes long, your payload will be page aligned.

     The other requirement for zero copy receive to work is that the buffer
     that is the destination for the data read from a socket must be at least
     a page in size and page aligned.

     Obviously the requirements for receive side zero copy are impossible to
     meet without NIC hardware that is programmable enough to do header splitting
 of some sort.  Since most NICs are not that programmable, or their
     manufacturers will not share the source code to their firmware, this
     approach to zero copy receive is not widely useful.

     There are other approaches, such as RDMA and TCP Offload, that may potentially
 help alleviate the CPU overhead associated with copying data out
     of the kernel.  Most known techniques require some sort of support at the
     NIC level to work, and describing such techniques is beyond the scope of
     this manual page.

     The zero copy send and zero copy receive code can be individually turned
     off via the kern.ipc.zero_copy.send and kern.ipc.zero_copy.receive sysctl
     variables respectively.

SEE ALSO    [Toc]    [Back]

     sendfile(2), socket(2), ti(4), jumbo(9)

HISTORY    [Toc]    [Back]

     The zero copy sockets code first appeared in FreeBSD 5.0, although it has
     been in existence in patch form since at least mid-1999.

AUTHORS    [Toc]    [Back]

     The zero copy sockets code was originally written by Andrew Gallatin
     <gallatin@FreeBSD.org> and substantially modified and updated by Kenneth
     Merry <ken@FreeBSD.org>.

FreeBSD 5.2.1			 June 23, 2002			 FreeBSD 5.2.1
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