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Patch ID OSF350-146 has been superseded by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