Re: Using --filter-hosts breaks parallel

[Ole Tange]

On Sat, Nov 23, 2013 at 1:19 AM, Ole Tange <ole@tange.dk> wrote:
> On Wed, Nov 20, 2013 at 11:11 AM, Adam Lindberg <hello@alind.io> wrote:
>> Running the following command results in a crash
> [...]
>> $ parallel --filter-hosts --controlmaster -j 128 --nonall --tag --slf 
>> servers 'ps aux | grep [o]psworks | wc -l’

I have worked a bit on this. The problem seems to be that
--filter-hosts starts 4 ssh connections to each server in parallel. If
the connections are proxied through a single machine (e.g. using SSH's
ProxyCommand or ControlMaster) then this single machine's ssh daemon
may be overloaded and reject some ssh connections.

The problem only arises when there are a lot of machines (e.g. if you
have 3 machines then it will never happen), and only when you do not
connect directly (i.e. no proxy = no problems).

My experiments show that putting a delay (--delay 0.1) in for every
ssh command makes the problem much smaller. The same is true if the
connections are retried (--retries 3).

The problem by using these is that it makes --filter-hosts slower, and
if you have many hosts and you connect directly to these hosts, then
you will be paying a price without getting any benefit.

I have chosen safety over speed, so --filter-hosts ought to work
better now - albeit slower: 0.4 seconds per host + 18 seconds if one
or more hosts are down.


/Ole