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writes of size (100,208]+N*MCLBYTES. The bug: sosend() hands each mbuf off to the protocol output routine as soon as it has copied it, in the hopes of increasing parallelism (see http://www.kohala.com/~rstevens/vanj.88jul20.txt ). This works well for TCP as long as the first mbuf handed off is at least the MSS. However, when doing small writes (between MHLEN and MINCLSIZE), the transaction is split into 2 small MBUF's and each is individually handed off to TCP. TCP assumes that the first small mbuf is the whole transaction, so sends a small packet. When the second small mbuf arrives, Nagle prevents TCP from sending it so it must wait for a (potentially delayed) ACK. This sends throughput down the toilet. The workaround: Set the "atomic" flag when we're doing small writes. The "atomic" flag has two meanings: 1. Copy all of the data into a chain of mbufs before handing off to the protocol. 2. Leave room for a datagram header in said mbuf chain. TCP wants the first but doesn't want the second. However, the second simply results in some memory wastage (but is why the workaround is a hack and not a fix). The real fix: The real fix for this problem is to introduce something like a "requested transfer size" variable in the socket->protocol interface. sosend() would then accumulate an mbuf chain until it exceeded the "requested transfer size". TCP could set it to the TCP MSS (note that the current interface causes strange TCP behaviors when the MSS > MCLBYTES; nobody notices because MCLBYTES > ethernet's MTU). |
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| contrib | ||
| crypto | ||
| etc | ||
| games | ||
| gnu | ||
| include | ||
| kerberos5 | ||
| kerberosIV | ||
| lib | ||
| libexec | ||
| lkm | ||
| release | ||
| sbin | ||
| secure | ||
| share | ||
| sys | ||
| tools | ||
| usr.bin | ||
| usr.sbin | ||
| COPYRIGHT | ||
| Makefile | ||
| Makefile.alpha | ||
| README | ||
This is the top level of the FreeBSD source directory. This file was last revised on: $Id: README,v 1.11 1997/08/09 14:36:20 jkh Exp $ For copyright information, please see the file COPYRIGHT in this directory (additional copyright information also exists for some sources in this tree - please see the specific source directories for more information). The Makefile in this directory supports a number of targets for building components (or all) of the FreeBSD source tree, the most commonly used one being ``world'', which rebuilds and installs everything in the FreeBSD system from the source tree except the kernel and the contents of /etc. Please see the top of the Makefile in this directory for more information on the standard build targets and compile-time flags. Building a kernel with config(8) is a somewhat more involved process, documentation for which can be found at: http://www.freebsd.org/handbook/kernelconfig.html And in the config(8) man page. The sample kernel configuration files reside in the sys/i386/conf sub-directory (assuming that you've installed the kernel sources), the file named GENERIC being the one used to build your initial installation kernel. The file LINT contains entries for all possible devices, not just those commonly used, and is meant more as a general reference than an actual kernel configuration file (a kernel built from it wouldn't even run). Source Roadmap: --------------- bin System/User commands. contrib Packages contributed by 3rd parties. crypto Export controlled stuff (see crypto/README). etc Template files for /etc games Amusements. gnu Various commands and libraries under the GNU Public License. Please see gnu/COPYING* for more information. include System include files. kerberosIV Kerberos package - also export controlled. lib System libraries. libexec System daemons. lkm Loadable Kernel Modules. release Release building Makefile & associated tools. sbin System commands. secure DES and DES-related utilities - NOT FOR EXPORT! share Shared resources. sys Kernel sources. tools Utilities for regression testing and miscellaneous tasks. usr.bin User commands. usr.sbin System administration commands. For information on synchronizing your source tree with one or more of the FreeBSD Project's development branches, please see: http://www.freebsd.org/handbook/synching.html