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<TITLE>Performance Tuning Tips for Digital Unix</TITLE>
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<H1 ALIGN="CENTER">Performance Tuning Tips for Digital Unix</H1>
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Below is a set of newsgroup posts made by an engineer from DEC in
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response to queries about how to modify DEC's Digital Unix OS for more
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heavily loaded web sites. Copied with permission.
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<HR>
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<H2>Update</H2>
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From: Jeffrey Mogul <mogul@pa.dec.com><BR>
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Date: Fri, 28 Jun 96 16:07:56 MDT<BR>
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<OL>
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<LI> The advice given in the README file regarding the
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"tcbhashsize" variable is incorrect. The largest value
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this should be set to is 1024. Setting it any higher
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will have the perverse result of disabling the hashing
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mechanism.
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<LI>Patch ID OSF350-146 has been superseded by
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<BLOCKQUOTE>
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Patch ID OSF350-195 for V3.2C<BR>
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Patch ID OSF360-350195 for V3.2D
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</BLOCKQUOTE>
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Patch IDs for V3.2E and V3.2F should be available soon.
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There is no known reason why the Patch ID OSF360-350195
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won't work on these releases, but such use is not officially
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supported by Digital. This patch kit will not be needed for
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V3.2G when it is released.
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</OL>
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<HR>
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<PRE>
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From mogul@pa.dec.com (Jeffrey Mogul)
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Organization DEC Western Research
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Date 30 May 1996 00:50:25 GMT
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Newsgroups <A HREF="news:comp.unix.osf.osf1">comp.unix.osf.osf1</A>
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Message-ID <4oirch$bc8@usenet.pa.dec.com>
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Subject Re: Web Site Performance
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References 1
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In article <skoogDs54BH.9pF@netcom.com> skoog@netcom.com (Jim Skoog) writes:
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>Where are the performance bottlenecks for Alpha AXP running the
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>Netscape Commerce Server 1.12 with high volume internet traffic?
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>We are evaluating network performance for a variety of Alpha AXP
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>runing DEC UNIX 3.2C, which run DEC's seal firewall and behind
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>that Alpha 1000 and 2100 webservers.
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Our experience (running such Web servers as <A
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HREF="http://altavista.digital.com">altavista.digital.com</A>
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and <A HREF="http://www.digital.com"
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>www.digital.com</A>) is that there is one important kernel tuning
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knob to adjust in order to get good performance on V3.2C. You
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need to patch the kernel global variable "somaxconn" (use dbx -k
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to do this) from its default value of 8 to something much larger.
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How much larger? Well, no larger than 32767 (decimal). And
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probably no less than about 2048, if you have a really high volume
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(millions of hits per day), like AltaVista does.
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This change allows the system to maintain more than 8 TCP
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connections in the SYN_RCVD state for the HTTP server. (You
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can use "netstat -An |grep SYN_RCVD" to see how many such
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connections exist at any given instant).
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If you don't make this change, you might find that as the load gets
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high, some connection attempts take a very long time. And if a lot
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of your clients disconnect from the Internet during the process of
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TCP connection establishment (this happens a lot with dialup
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users), these "embryonic" connections might tie up your somaxconn
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quota of SYN_RCVD-state connections. Until the kernel times out
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these embryonic connections, no other connections will be accepted,
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and it will appear as if the server has died.
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The default value for somaxconn in Digital UNIX V4.0 will be quite
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a bit larger than it has been in previous versions (we inherited
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this default from 4.3BSD).
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Digital UNIX V4.0 includes some other performance-related changes
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that significantly improve its maximum HTTP connection rate. However,
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we've been using V3.2C systems to front-end for altavista.digital.com
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with no obvious performance bottlenecks at the millions-of-hits-per-day
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level.
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We have some Webstone performance results available at
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http://www.digital.com/info/alphaserver/news/webff.html
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<EM>[The document referenced above is no longer at that URL -- Ed.]</EM>
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I'm not sure if these were done using V4.0 or an earlier version
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of Digital UNIX, although I suspect they were done using a test
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version of V4.0.
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-Jeff
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<HR>
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----------------------------------------------------------------------------
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From mogul@pa.dec.com (Jeffrey Mogul)
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Organization DEC Western Research
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Date 31 May 1996 21:01:01 GMT
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Newsgroups <A HREF="news:comp.unix.osf.osf1">comp.unix.osf.osf1</A>
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Message-ID <4onmmd$mmd@usenet.pa.dec.com>
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Subject Digital UNIX V3.2C Internet tuning patch info
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----------------------------------------------------------------------------
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Something that probably few people are aware of is that Digital
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has a patch kit available for Digital UNIX V3.2C that may improve
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Internet performance, especially for busy web servers.
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This patch kit is one way to increase the value of somaxconn,
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which I discussed in a message here a day or two ago.
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I've included in this message the revised README file for this
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patch kit below. Note that the original README file in the patch
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kit itself may be an earlier version; I'm told that the version
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below is the right one.
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Sorry, this patch kit is NOT available for other versions of Digital
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UNIX. Most (but not quite all) of these changes also made it into V4.0,
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so the description of the various tuning parameters in this README
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file might be useful to people running V4.0 systems.
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This patch kit does not appear to be available (yet?) from
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<A HREF="http://www.service.digital.com/html/patch_service.html"
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>http://www.service.digital.com/html/patch_service.html</A>
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so I guess you'll have to call Digital's Customer Support to get it.
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-Jeff
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DESCRIPTION: Digital UNIX Network tuning patch
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Patch ID: OSF350-146
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SUPERSEDED PATCHES: OSF350-151, OSF350-158
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This set of files improves the performance of the network
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subsystem on a system being used as a web server. There are
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additional tunable parameters included here, to be used
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cautiously by an informed system administrator.
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TUNING
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To tune the web server, the number of simultaneous socket
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connection requests are limited by:
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somaxconn Sets the maximum number of pending requests
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allowed to wait on a listening socket. The
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default value in Digital UNIX V3.2 is 8.
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This patch kit increases the default to 1024,
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which matches the value in Digital UNIX V4.0.
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sominconn Sets the minimum number of pending connections
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allowed on a listening socket. When a user
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process calls listen with a backlog less
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than sominconn, the backlog will be set to
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sominconn. sominconn overrides somaxconn.
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The default value is 1.
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The effectiveness of tuning these parameters can be monitored by
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the sobacklog variables available in the kernel:
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sobacklog_hiwat Tracks the maximum pending requests to any
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socket. The initial value is 0.
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sobacklog_drops Tracks the number of drops exceeding the
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socket set backlog limit. The initial
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value is 0.
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somaxconn_drops Tracks the number of drops exceeding the
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somaxconn limit. When sominconn is larger
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than somaxconn, tracks the number of drops
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exceeding sominconn. The initial value is 0.
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TCP timer parameters also affect performance. Tuning the following
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require some knowledge of the characteristics of the network.
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tcp_msl Sets the tcp maximum segment lifetime.
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This is the maximum lifetime in half
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seconds that a packet can be in transit
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on the network. This value, when doubled,
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is the length of time a connection remains
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in the TIME_WAIT state after a incoming
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close request is processed. The unit is
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specified in 1/2 seconds, the initial
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value is 60.
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tcp_rexmit_interval_min
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Sets the minimum TCP retransmit interval.
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For some WAN networks the default value may
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be too short, causing unnecessary duplicate
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packets to be sent. The unit is specified
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in 1/2 seconds, the initial value is 1.
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tcp_keepinit This is the amount of time a partially
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established connection will sit on the listen
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queue before timing out (<EM>e.g.</EM>, if a client
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sends a SYN but never answers our SYN/ACK).
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Partially established connections tie up slots
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on the listen queue. If the queue starts to
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fill with connections in SYN_RCVD state,
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tcp_keepinit can be decreased to make those
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partial connects time out sooner. This should
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be used with caution, since there might be
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legitimate clients that are taking a while
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to respond to SYN/ACK. The unit is specified
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in 1/2 seconds, the default value is 150
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(ie. 75 seconds).
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The hashlist size for the TCP inpcb lookup table is regulated by:
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tcbhashsize The number of hash buckets used for the
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TCP connection table used in the kernel.
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The initial value is 32. For best results,
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should be specified as a power of 2. For
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busy Web servers, set this to 2048 or more.
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The hashlist size for the interface alias table is regulated by:
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inifaddr_hsize The number of hash buckets used for the
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interface alias table used in the kernel.
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The initial value is 32. For best results,
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should be specified as a power of 2.
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ipport_userreserved The maximum number of concurrent non-reserved,
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dynamically allocated ports. Default range
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is 1025-5000. The maximum value is 65535.
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This limits the numer of times you can
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simultaneously telnet or ftp out to connect
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to other systems.
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tcpnodelack Don't delay acknowledging TCP data; this
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can sometimes improve performance of locally
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run CAD packages. Default is value is 0,
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the enabled value is 1.
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Digital UNIX version:
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V3.2C
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Feature V3.2C patch V4.0
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======= ===== ===== ====
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somaxconn X X X
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sominconn - X X
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sobacklog_hiwat - X -
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sobacklog_drops - X -
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somaxconn_drops - X -
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tcpnodelack X X X
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tcp_keepidle X X X
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tcp_keepintvl X X X
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tcp_keepcnt - X X
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tcp_keepinit - X X
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TCP keepalive per-socket - - X
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tcp_msl - X -
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tcp_rexmit_interval_min - X -
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TCP inpcb hashing - X X
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tcbhashsize - X X
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interface alias hashing - X X
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inifaddr_hsize - X X
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ipport_userreserved - X -
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sysconfig -q inet - - X
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sysconfig -q socket - - X
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</PRE>
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