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[Launchpad Bug Tracker <1706735@xxxxxxxxxxxxxxxxxx>]

This bug was fixed in the package lmbench - 3.0-a9-1.1ubuntu0.1

---------------
lmbench (3.0-a9-1.1ubuntu0.1) xenial; urgency=medium

  * d/p/dynamic-tcp-backlog.dpatch: Dynamically increase the TCP backlog on
    high core-count systems. (LP: #1706735)

 -- dann frazier <dann.frazier@xxxxxxxxxxxxx>  Mon, 16 Oct 2017 15:37:03
-0600

** Changed in: lmbench (Ubuntu Xenial)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1706735

Title:
  lmbench tcp lib backlog reservation too small, can cause hang.

Status in lmbench package in Ubuntu:
  Fix Released
Status in lmbench source package in Xenial:
  Fix Released
Status in lmbench source package in Zesty:
  Fix Released

Bug description:
  [Impact]
  Network related tests will hang on high-core count systems.

  lib_tcp reserves backlog of 100. If there are more then 100 threads on
  the system, the backlog is pretty much guaranteed to fail. The clients
  will then fail to connect and lmbench will hang. Potential patch below
  works around this by reserving backlog for at least 4x number of
  processors reported in the system, assuming that common use case will
  be to run lmbench with as many threads as there are cpus reported from
  the OS. Alternatively, backlog reservation can be made into a config
  option.

  [Test Case]
  ubuntu@ubuntu:~$ /usr/lib/lmbench/bin/lat_select -P 128 tcp
  connect: Connection timed out
  connect: Connection timed out
  connect: Connection timed out
  connect: Connection timed out
  connect: Connection timed out
  connect: Connection timed out
  connect: Connection timed out
  connect: Connection timed out
  connect: Connection timed out
  connect: Connection timed out

  With the patch:
  ubuntu@ubuntu:~/lmbench$ /usr/lib/lmbench/bin/lat_select -P 128 tcpSelect on 200 tcp fd's:
  XX.XXXX microseconds

  [Regression Risk]
  The patch is careful to preserve the previous behavior for lower core count systems. Perhaps if there were a bug in the platform's sysconf(_SC_NPROCESSORS_ONLN) function, we could end up with a regression that skewed results.

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