[Bug 782092] Re: ssh server extremly unresponsive

[Martin Konopka]

I now have acquired enough evidence and confirm all my writings above.
It implies that there may be a scheduling issue in the linux kernel of ubuntu 10.10.
I will try to check if there is similar issue also with Natty.

(I have 15 years of experience in performing CPU intensive tasks on UNIX/Linux workstations
and servers and this is most likely for the first time when I experience the reported kind
of the odd behaviour.)

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https://bugs.launchpad.net/bugs/782092

Title:
  ssh server extremly unresponsive

Status in “linux” package in Ubuntu:
  New
Status in “openssh” package in Ubuntu:
  Incomplete

Bug description:
  I describe the issue in few steps below.

  1.) I want to transfer files from machine A (with ubuntu 11.04, but
  this is not important) to machine B (ubuntu 10.10; this is important
  to say) using scp.

  2.) Machine B is under constant computational load due to a user
  program (a simulation) such that most of its CPU power is consumed by
  the program but it uses less then half of total (12 GB) memory and
  there is not any significant usage of disk and no other application is
  running. (Even no user logged into a GNOME session.)

  3.) I issue the command scp on machine A; I immediatelly get prompt to
  give password. However the transfer does not start immediatelly after
  that. I must wait for some time, say half a minute or even full minute
  or more (I have not measured it.) just to see the start of the
  tranfer.

  Few additional remarks:

  Until recently machine B had had installed ubuntu 10.04 (Lucid Lynx,
  64-bit) and there was no such problem even at higher computational
  loads. Similarly there was no such problem even in more distant past
  when the machine was running under Linux Mint Helena (based upon
  Ubuntu 9.10).

  The long delays when doing scp occur if machine B was not exposed to
  an scp or ssh demand for a longer period of time. (Typically I face
  the problem in the morning when.) If an scp transfer is finally
  accomplished then I can start a new one which already works without
  delay.

  Machine B is equipped with an Intel i7 CPU, 12 GB of DDR3 and it is a
  quite powerful workstation. All cores of CPU are set to run at maximum
  frequency.

  I attach a file with content shown by the program top. (Grabbed in
  situation when a user is logged into GNOME session.)

  ProblemType: Bug
  DistroRelease: Ubuntu 10.10
  Package: openssh-server 1:5.5p1-4ubuntu5
  ProcVersionSignature: Ubuntu 2.6.35-28.50-generic 2.6.35.11
  Uname: Linux 2.6.35-28-generic x86_64
  Architecture: amd64
  Date: Fri May 13 10:39:53 2011
  InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
  ProcEnviron:
   LANGUAGE=en_GB:en
   PATH=(custom, user)
   LANG=en_GB.utf8
   SHELL=/bin/bash
  SourcePackage: openssh