[Bug 595] lots of simultaneous ssh's cause sporadic failures

[bugzilla-daemon at mindrot.org]

http://bugzilla.mindrot.org/show_bug.cgi?id=595

           Summary: lots of simultaneous ssh's cause sporadic failures
           Product: Portable OpenSSH
           Version: -current
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ssh
        AssignedTo: openssh-bugs at mindrot.org
        ReportedBy: jcduell at lbl.gov


Openssh seems to fail sporadically if you issue lots of simultaneous
ssh commands.  Take the following program:

    #!/bin/sh

    for NUM in 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15; do
        ssh foo.bar.com echo $NUM &
    done

So, we're running 16 ssh commands at once, each of which just prints out a
different number.

When I run this program, several of the ssh commands fail with 

    ssh_exchange_identification: Connection closed by remote host

Interestingly, when I run 10 or fewer ssh commands, they all work OK, at
least on my linux box (I'm using OpenSSH 3.5p1-6 on Redhat Linux 9).  On 
some other platforms the number is different:  OpenSSH 3.2.3p1 on an IBM 
SP doesn't like more than 8 simultaneous ssh's in the background, while
OpenSSH_3.6.1p1 on Tru64 does around 9 max.

There doesn't seem to be any pattern in terms of which ssh's get 
killed--the first, second and third jobs (ie, those that print 0, 1, and 
2) generally run OK, but which of the following ones die seems to be
random.

This smells like some kind of race condition.

Why on earth would I want to run a dozen ssh jobs simultaneously?  I'm
writing a compiler that needs to ship some files and run some commands 
on a remote server as part of the compilation process.  The latency for
doing this is rather high, so I want to allow users to do a 'make -j' to
parallelize the build, in order to hide the network latency.

I guess for now I'll tell users to run 'make -j N' with N < 6 or so
(which is probably not a bad idea anyway).  But I can't imagine I'll be
the last person to pound on ssh like this...



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