The One Remaining (non-Depracated) Use for rsh
From Dave Falloon
Answered By Jim Dennis, Mike "Iron" Orr, Kapil Hari Paranjape
Hi Answer Guy,
I have a 32 node cluster running Debian 3.0 (Woody). The primary way we
use these machines is in a batch type submission, kind of a fire an forget
thing, via rsh "<command>".
[JimD]
These days the knee jerk response would be: "Don't run rsh; use ssh
instead."
Agreed, the reason for rsh is that this little cluster is all by itself,
accessed through a "choke host" that is pretty well locked down, only a
handful of users can access it on the external interface.
[JimD]
However, compute clusters, on an isolated network segment, (perhaps
with one or more multi-homed ssh accessible cluster controller nodes)
are still a reasonable place for the insecure r* tools (rsh, rlogin,
rcp). (rsync might still be preferable to rcp for some workloads and
filesets).
I crippled PAM a little to allow this ( changed
one line to be sufficient). This cluster is not a super critical farm so if
things go haywire its not a big deal but it would be nice to figure out why
sometimes you can't connect to the nodes, here is the output from one such
attempt:
(503)[dave@snavely] ~$ rsh ginzu
Last login: Thu Jan 16 16:37:22 2003 from snavely on pts/1
Linux ginzu 2.4.18 #1 SMP Fri Aug 2 11:20:55 EDT 2002 i686 unknown
rlogin: connection closed.
(504)[dave@snavely] ~$
This happened once then when I repeated the command it succeeded, with no
error.
[Kapil]
One possible reason for the problem is the assignment of a free pty.
1. You may be running out of pty's if many processes unnecessarily
open them.
This is a definate possibility, and I am recompiling a kernel as we speak to
up this limit to 2048.
[Kapil]
2. Your tweaking of rsh and PAM was not sufficient to give rsh
permission to open a pty.
Would this produce an intermitten connection drop or would it prevent any
connection at all?
[Kapil]
This would also explain the unable to get TTY name error.
So how does the chain of events happen? Is this correct; I rsh to a machine
it, pam looks over its rules and see that it is crippled and should allow
this connection with no passwd, passes this on to login which then tries to
assigned a pty but the pty's are all currently used, then it tries to assign
a TTY because there are no ptys, and in my logs I get the can't get TTY name
error?
[Kapil]
No, there is no separate "TTY" assignment. The "pty/tty" pair is what is
assigned for interactive communication.
Let's see if we can track the sequence of events (the Gang please post
corrections, I am sure I'll go wrong somewhere!):
Client "rsh" request is usually handled on the server by "inetd" which
then passes this request to "tcpd" which then passes the request to
"rshd".
O. However, tcpd may refuse the connection if its host_access rules do
not allow the connection. This refusal could be intermittent depending
on whether the name service system is responding (NIS/DNS whatever).
(This possibility has already been mentioned on the list in greater
detail).
At this point, I looked up the Sun Solaris man page for rshd (none
of the Linux machines here has "rsh" installed!). The following steps
are carried out and failure leads to closing the connection.
A. The server tries to create the necessary sockets for a connection.
B. The server checks the client's address which must be resolvable
via the name service switch specification (default NIS+/etc/hosts).
C. The server checks the server user name which must be verifiable
via the name service switch specification (default NIS+/etc/passwd).
D. The server checks via PAM that the either (the client is in /etc/hosts.equiv
and the client user name is the same as the server user name) or
the client username is in .rhosts.
E. The server tries to acquire the necessary pty/tty's and connects them
to the sockets and the server user's shell (which must exist).
I am a bit confused about the use of PAM but I think it is also used
in steps C and E through the "account" and "session" entries. The
"auth" entry for PAM is used in "D".
So it seems like O,A-E need to be checked on your system. My own
earlier suggestion was only about E but the failure could be
elsewhere.
Temporary failure of the NIS server to respond could affect B and C;
it could even affect E as the "passwd" entry is required to find the
user's shell. Thus, in such situations it is a good idea to run the
name service caching daemon.
If NFS is used for home directories then temporary failure of the NFS
server to respond could affect D as well.
Hope this helps,
Kapil.
[JimD]
So it was a transient (or is an intermittent) problem.
Yup
I have adjusted the /etc/inet.conf by adding the .500 to the rsh line
nowait:
shell stream tcp nowait.500 root /usr/sbin/tcpd /usr/sbin/in.rshd
in order for these machines to allow more jobs to be run at a time.
[JimD]
This adjusts inetd's tolerance/threshold to frequent connections
on a given service. It simply means that inetd won't throttle back
the connections as readily --- it will try to service them even if
they are coming in fast and furious. In this case it will allow up to
500 attempted rsh connections per minute (about 8 per second).
[JimD]
That really doesn't adjust anything about the number of concurrent
jobs that a machine can run --- just the number of times that the
inetd process will accept connections on a give port before treating
it as a DoS (denial of service) attack or networking error, and
throttling the connections.
I adjusted this because we ran into lots of problems with inet dropping
connections, I just wanted to make sure that it behaved like it was supposed
to, ie you didn't know of some immediately relevant bug in this line
[JimD]
In your example this is clearly NOT the problem. It made the
connection and then disconnected you. Thus it wasn't inetd refusing
the connection, but the shell process exiting (or being killed by the
kernel).
[Iron]
Just to clarify, I think Jim is saying that it's not inetd or tcpd refusing
you, because otherwise rlogin wouldn't have started at all, and it (rlogin)
ouldn't have been able to print the "last login:" and kernel version lines.
By the way, when tcpd doesn't like me, it waits a couple seconds (usually
doing reverse DNS lookup), and then I see "Connection closed by foreign host"
with no other messages.
One possibility is that we have everyone's home drive on NFS and if the NFS
was slow to respond that may cause rlogin to find no home directory and
refuse the connection. Is that a realistic possibility?
One interesting turn of events is the message you get in auth.log :
Jan 20 15:41:31 ginzu PAM_unix[31073]: (login) session opened for user dave
by (uid=0)
Jan 20 15:41:31 ginzu login[31073]: unable to determine TTY name, got
/dev/tty6
These machines have no video cards/keyboards/otherinput, really they are
processor/harddrive/ram/NIC and thats all so it would make sense to comment
out the getty lines in inittab for these boxes ... correct?
That would at the very least stop the auth.log and daemon.log spamming, I
think
[Iron]
If inetd is not listening to the port at all and no other daemon is, you'll
get an immediate "Connection refused" error. This is confusing because it
doesn't mean it doesn't like you, it means there's nobody there to answer the
door.
[JimD]
I'd run vmstat processes on the affected nodes (or all of them) for
a day or two --- redirect their output to local files or over the
network (depending one which will have the least impact on your
desired workload) and then write some scripts to analyze and/or graph
them.
I have started collecting info on these machines.
Can you think of why these machines behave like this? Could it be a load
average problem, maybe its network related, is it a setup problem? Any
ideas would be appreciated
[JimD]
It's not likely to be a networking or setup issue. Your networking
seems to work. Things seem to be configured properly for moderate
workloads, so we have to find out which host resources are under the
most pressure. So it's probably a loading problem.
Its not a loading issue the system is pretty good at evening out load across
the pool of machines
[JimD]
(Note I did NOT say "load average" problem. "load average" is simply
a measure of the average number of processes that were in a runnable
(non-blocked) state during each context switch over the last minute,
and five and fifteen minutes. A high load average should NOT result
in processes dying as you've described --- but often indicates a
different resource loading issue. Sorry to split hairs on that point
but this is a case were understanding that distinction is important).
These machines can get a little bagged at times but the login failure
happens regardless of the load of a given host.
[JimD]
As always you should be checking your system logs. Hopefully there'll
be messages therein that will tell you if the kernel killed your
process and why. Otherwise you can always write an "strace" wrapper
around these executables. It will kill your performance, but, if you
can reproduce the problem you'll be able to see what the process died.
After a look in the logs ( I can't believe I didn't do this earlier ), I
found a lot of messages about getty trying to use /dev/tty*, no such device,
which makes sense considering they have no input/output hardware like
video/keyboard, etc.
[JimD]
Some tweaks to the setup might help.
[JimD]
There are basically four resources we're concerned about here:
memory, CPU, process table, and file descriptor table (space and
contention). (I'm not concerned about I/O contention in this case
since that usually causes processes to block --- performance to
go very slowly. It doesn't generally result in processes dying like
you've described here).
[JimD]
vmstat's output will tell you more. You can probably make some guesses
based on your workload profile.
[JimD]
If you're running many small jobs spawning from one (or a small number
of) dispatcher processes (on each node) you might be bumping into
rlimit/ulimit issues. Read the man page for your shell's ulimit
built-in command, and the ulimit(3) man page for more details on that.
Ulimits have been adjusted already we ran into file descriptor limits before
[JimD]
If you're running a few large jobs than its more likely to be a memory
pressure problem --- though we'd expect you'd run into paging/thrashing
issues first. There are cases where you can run out of memory without
doing any signficant paging/swapping (where the memory usage is on
non-swappable kernel memory rather than normal process memory).
[JimD]
By the way, you might want to eliminate tcpd from your configuration
(remove the references to /usr/sbin/tcpd from your inetd.conf file).
This will save you an extra fork()/exec() and a number of file access
operations on each new job dispatched. (The use of rsh already assumed
you've physically isolated this network segment with very restrictive
packet filters and anti-spoofing --- so TCP Wrappers is not useful
in your case and is only costing you some capacity, albeit small).
[JimD]
You might even eliminate rsh/rlogin and go with the even simpler
rexec command!
Some times people will run an interactive job on this cluster, so rsh/rlogin
is still nice to have. We have no real policy about what can or cannot be
run on these machines, like I had said it is more of a playground for our
researchers, than a critical cluster.
[JimD]
It goes without saying that you may wish to eliminate, renice, or
reconfigure any daemons you're running on these nodes. For example,
you can almost certainly eliminate cron and atd from the nodes (since
your goal is to dispatch the jobs from one or a few central
cluster control nodes. They could run a small number of cron/atd
processes and dispatch jobs across the cluster as appropriate.
True, but really it doesn't seem related, I can't see an interaction between
login and cron that would drop your connection. Although it is nice to cut
down bloat where you can.
[JimD]
The klogd/syslogd daemons are worth extra consideration. I'd
strongly consider running syslog under 'nice' and giving it lowest
possible priority. I'd also consider tweaking the syslog.conf to
remove the leading "-" (dashes) from any local log file names
(so that they will be written asynchronously rather than with
fsync() calls after every write to the logs).
[JimD]
I'd even consider eliminating all local files from these configurations
and having these nodes do all their logging over the net (which being
UDP based, might result in some lossage of log messages). However,
that depends heavily on your workload and network topology and
capacity. Basically you might have bandwidth to burn (Gig ethernet,
for example) and this might be a reasonable tradeoff.
We are on a switched full duplex 100 base TX, network. The logs on the
switches report that in the last 3 months we have only went above 80% of the
switches bandwith once, so I think we have enough bandwidth to support logs
over the net.
[JimD]
I'd also consider setting the login shell for these job handling
accounts to ash (or the simplest, smallest shell that can successfully
process your jobs). bash, particularly with version 2.x is a pretty
"resourceful" (read "bloated") program which may not be necessary
unless you're doing some fairly complex shell scripting.
A possibility, but as I had said some of our researchers will run an
interactive job so they want a full shell.
[JimD]
Also in your shell/jobs you might want to make some strategic use of
the exec built-in command. Basically in any case where the shell
or subshell doesn't have a command subsequent to one of your external
binaries --- exec the binary. This saves a fork() system call, and
means that the shell processes are NOT taking up memory, file descriptors,
and entries in the process table just waiting for other executables to
exit.
[JimD]
I'd also eliminate PAM and look for the older r* and login suite.
The traditional /bin/login program does an exec*() system call to
run your shell. The PAM based suite performs a fork() and then an
exec() --- and the /bin/login program remains in order to perform
post logout cleanup. It is quite likely that you are not interested
in these more advanced features provided by PAM's approach.
PAM is overkill but I don't think it is the culprit.
[JimD]
Incidentally, another point to consider is your local filesystems.
You may want to mount as many of them as possible in "read-only" mode
and all of them with the noatime option. Both of these tweaks can
considerably reduce the amount of work the system is doing to maintain
your filesystem consistency and the (rarely used) access time stamps.
[JimD]
You may also want to consider using the older ext2 filesystem rather
than any of the journaling filesystem choices. This depends on your
data integrity requirements, of course, but the journaling done by
ext3, reiserfs, XFS and others does come at a significant cost.
(Note: In some other cases, where intensive use of local filesystems
is part of the workload, XFS or reiserfs might be VASTLY better than
ext2 --- for various complicated reasons).
Reiser is working fine on these nodes. I have found a significant
improvement over ext2 for the majority of tasks run on these boxes.
[JimD]
Depending on your application, you might even want to consider
recompiling it using older, simpler versions of libc/libm (since
many of the advanced features of GNU glibc 2.x may be useless
for your computations). Of course if the application is multi-threaded
then you may needs glibc 2.x' re-entrancy.
Not really possible, in a lot of cases we are running some third party
commercial software which is very closed source.
[JimD]
It's possible that you need to do some kernel tuning. This might
involve writing some magic values into the sysctl nodes under
/proc/sys (or running the systune or Linux powertweak utilities).
It might also involve rebuilding your kernel, possibly with a few
static variables changed or a few scalability patches applied.
[JimD]
(In this case, "scalability" is a loaded term --- since it means
much different things to differing workloads).
I think the real problem is that we are having a bad interaction with some
piece of software and rlogin/login/getty?/init or something and that causes
the connection to be dropped.
[JimD]
You can find numerous hints about Linux kernel performance
tweaking using Google! https://www.google.com/linux
Here's a few links:
-
The C10K problem
- https://www.kegel.com/c10k.html
Written and maintained by Dan Kegel, originally in response
to the infamous "Mindcraft" fiasco wherein Microsoft paid an
"independent" lab to prove that MS Windows was "faster" or
"more scalable" than Linux.
Dan is/was one of the advocates for improving the Linux kernel
in a number of key areas regardless of if Mindcraft's
credibility.
CITI: Projects: Linux scalability - University of Michigan
https://www.citi.umich.edu/projects/linux-scalability
Run by Peter Honeyman (a legendary UNIX programmer).
-
SGI - Developer Central Open Source: Scalability Project
- https://oss.sgi.com/projects/linux-scalability
-
IBM developerWorks: Linux: Linux Kernel Performance & Scalability
- https://www-106.ibm.com/developerworks/linux/library/l-kperf
The problem with all of these links is that they are not focused on
the set of requirements specific to your needs. They are more
concerned with webserver, database, SMP, and single-server scalability
rather than Beowulf style cluster performance.
[JimD]
Of course you could read the information at https://www.beowulf.org
Strictly speaking it doesn't sound like you're really running a Beowulf
cluster --- you're dispatching jobs via rsh rather than distributing
computation load using MPI, PVM or similar libraries. However some of
the same configuration suggestions and performance observations might
still apply.
Most of the tweaking described has already been implemented
[JimD]
In general there isn't any silver bullet to increasing the capacity
of your cluster. You have to find out which resources are being
hit hardest (the bottlenecks), review what is using those resources,
find ways to eliminate as much of that utilization as possible
(removing tcpd, using a simpler/smaller shell, running terminal
processes via exec, changing to a non-journaling filesystem,
eliminating unneeded daemons) and try various tradeoffs that shift
the utilization of a constrained resource (local filesystem I/O vs.
pushing things out to the network, memory/cache and indexed data
structures vs CPU and linear searches).
Capacity is not the objective, I would say reliability ( not necessarily
100%, but better than 50%, for sure
is more my goal.
[JimD]
That's really all there is to performance tuning. Finding what's
using which resources. Finding what you can "not" do. Finding ways
to tradeoff one form of resource consumption with another. Of course
the black magic is in the details (especially when it comes to
poking new values into nodes under /proc/sys/vm/ --- read the
Documentation/sysctl* text files in your Linux kernel sources for some
hints about that)!
Perhaps a more detailed view of the cluster will give you more to work on.
There are really 34 machines to this cluster, one choke node that stands
between the outside world and the inside nodes. 32 machines (identical
hardware), dual Pentium 3 550 MHz's, 256 megs SDRAM (133MHz), single Maxtor
12 GB hardrive (7200 RPM ATA 66), 3com 3c590 ethernet card, identical
kernel's across the board, 2.4.18, SMP. One single sun machine, that
serves, NIS, NFS, and DNS, and home brewed batch server ( keeps track of
jobs and hosts loads and assigns jobs to hosts via rsh ). After searching
some more websites I found that some people have a problem with the
services.byname map in NIS. Could that be an issue here? I have adjusted
the inittab by commenting out the lines:
# <id>:<runlevels>:<action>:<process>
#1:2345:respawn:/sbin/getty 38400 tty1
#2:23:respawn:/sbin/getty 38400 tty2
#3:23:respawn:/sbin/getty 38400 tty3
#4:23:respawn:/sbin/getty 38400 tty4
#5:23:respawn:/sbin/getty 38400 tty5
#6:23:respawn:/sbin/getty 38400 tty6
Because these machines have no video/input.
Thanks for all your help so far. I hope all this new info helps you get a
better idea of whats going on.
[JimD]
You could build a new kernel, setting CONFIG_UNIX98_PTY_COUNT=2048
(apparently their maximal value).
Running a compile with this right now, thanks
[JimD]
I'd also try to eliminate NIS from these systems. I'd look at using
rsync to replicate the various /etc configuration files across the
cluster (passwd, group, hosts, services, et al). Failing that make
sure that you have the nscd (NIS name services cache daemon) properly
configured on the client nodes.
I have been waffling on moving this farm over to Cfengine and losing NIS for
about three months, I think I am going to get started on that right away.
Have you guys used cfengine, or do you have any suggestions for config
management tools?
[JimD]
I'd also try to eliminate NFS, or at least try to minimize it's use
especially for home directories. I'd also eliminate the automounting
if at all possible. This requires that the users work a little
smarter, manually transferring their data/input files down to the
proper nodes, and pulling the results back therefrom.
I have suggested this, but the tools (third party, very badly designed) some
of the research guys use need to write to the home drive, and in order to
take advantage of more than one node that would suggest that the home drive
be in two places at once (NFS, SMB or whatever). The way it works is that
one node will modify a model file, another will immediately pick up the
change and adjust what it is doing and modify the file more until the proper
mathematical model for a given project is found. Then they use that model
to figure out a whole range of useful information, at least thats how its
supposed to work.
[JimD]
If that's not feasible, at least configure these systems so that the
home directories are not automounted, replicate the basic suite of
"dot files" out to them and have a lower mount point provide the shared
data.
I don't think I can.
[JimD]
I'd also be quite wary of configuring the systems to allow NFS crossing
the isolated segment and out into filers on your network. This sounds
like a supremely bad idea allowing anyone with local root access on any
node on the outer network to impersonate any users, dropping files into
their directories which will be executed/sourced by shell session in
the inner network.
NFS traffic never leaves the clusters subnet, think of it as a hole in my
network covered by one node that runs ssh with 6 local accounts. Once you
log in to that firewall node you need to then rsh or ssh out the other
interface to either a node in the cluster or the old sun machine serving
NIS/NFS. All traffic on the local subnet stays on the local subnet. Once a
researcher has a proper model defined they have to rcp/scp that file to the
firewall machine, and then scp (rsync over ssh, or whatever) it to their
destination (rcp is not enabled outside of my protected subnet).
[JimD]
However, I think you're getting closer to the real heard of the problem
by looking into Kapil's suggestion regarding your PTY availability.
I'll know shortly, thanks everyone you guys rock!
Dave