Same here with the following env:

• git version: 1.7.2.5
• Debian: 6.0.5
• Java: Sun/Oracle JDK 1.6.0_26

This option exists to restrict parallel SCM operations when you have several dozens, hundreds or thousands of jobs to a limit of maybe 10-50. What's the rationale to enter "1"?

Well, it doesn't matter if you have 1 or 10 concurrent. Sometimes the git command just hangs. Having 10 concurrent pollings just means it needs to happen 10 times before you're completely blocked out. Even if you don't restrict, you will at one point have hundreds or thousands of hanging pollings.

I think the right thing here would be to configure a timeout and if that is reached kill the process.

This is suddenly happening to me too on LTS release 1.554.2 and Ubuntu 12.04 LTS, git version 1.7.9.5. I have verified that it's not a network issue as github is reachable, it seems sometimes the process just hangs with read() (when strace'ing it).

Something similar is happening with me, although it has nothing to do with the "max # of concurrent polling" setting in my case. The ssh process gets hung up when it runs "git-upload-pack". It seems to happen when we're grabbing the same repo on another executor on the same machine at the exact same time. It makes me think that this is the same issue as what you're seeing, but manifested by different means.

Another problem we get when we grab the same repo on the same machine at the same time is https://issues.jenkins-ci.org/browse/JENKINS-24179. I won't go so far as to say that they are the same issue, but it has some potential to be somehow related. If that is the case, then it might be a git/ssh concurrency issue and not so much of a Jenkins issue. But – I can't reproduce this outside of Jenkins yet, so... I can't really say for sure.

I'd be interested to know if the other cases happen to have the same repo accessed at the same time on the same machine.

I am having the same issue.
git 1.7.1
Jenkins 1.492
Git Plugin 1.4.0
RedHat 2.6.32-279.5.2.el6.x86_64

I will go for weeks with no issue then I will have a flurry of them. I would say that it fails about .1% to .01% of the time.

jenkins  18718  0.0  0.0  10136  1240 ?        S    16:59   0:00 git fetch -t origin +refs/heads/*:refs/remotes/origin/*
jenkins  18722  0.0  0.0  60040  3048 ?        S    16:59   0:00 ssh git@bitbucket.org git-upload-pack 'company/REPO.git'
jenkins  29927  0.0  0.0  37336  3640 ?        S    16:01   0:00 git fetch -t origin +refs/heads/*:refs/remotes/origin/*
jenkins  29931  0.0  0.0  60040  3048 ?        S    16:01   0:00 ssh git@bitbucket.org git-upload-pack 'company/REPO2.git'