My personal answer, to whether Hyper-Threading works under Linux.

I have been exploring this subject, through a series of experiments written in Python, and through what I learned when I was studying the subject of System Hardware, at Concordia University.

When a person uses a Windows computer, this O/S provides all the details of scheduling processes and threads. And arguably, it does well. But when a person is using Linux, the kernel makes all the required information available, but does not take care of optimizing how threads are scheduled, specifically. It becomes the responsibility of the application, or any other user-space program, to optimize how it will take up threads, using CPU affinity, or using low-level C functions that instruct the CPU to replace a single line in the L1 cache

In the special case when a person is writing scripts in Python, because this is an interpreted language, the program which is actually running, is the Python interpreter. How well the scheduling of threads works in that case, depends on how well this Python interpreter has been coded to do so. In addition, how well certain Python modules have been coded, has a strong effect on how efficiently they schedule threads. It just so happens that I’ve been lucky, in that the Python versions I get from the Debian repositories, happen to be programmed very well. By other people.

Dirk

 

System Maintenance Today

As a follow-up to this test yesterday, I just performed a reboot of the home-computer that acts as my Web-server, and that I name ‘Phoenix’ on my LAN. Unlike how it was yesterday, the reboot took place effortlessly tonight, requiring less than 5 minutes to complete, and undertaken around 21h20.

Again, my purpose was just to determine, whether a reboot would work, and again, there was a plausible reason why it might not have. Luckily, this one worked on the first try.

Continue reading System Maintenance Today

Linux Support for RTL8723BE Almost Perfect Now

In This Posting, I was continuing a long thread of comments, which began by saying, Linux support for the Realtek WiFi Chipset, that is served by the (linux) RTL8723BE kernel module, was dodgy at first. I was also experiencing problems with this, which required certain patches on my part.

At the same time, my kernel has been upgraded to version ‘4.4.0-30-generic’, on the affected laptop I name ‘Klystron’, which means that kernel-crackers have also been updating the in-tree kernel modules, including the modules RTL8723BE…

What I find is that by now, the behavior of that WiFi-chipset is stable. There was only one problem with it, which I had lastly been reporting, which stated that after the laptop resumes from Suspended-to-RAM, i.e. from a low-power mode that stops it in mid-session and continues to supply very small amounts of current to the DRAM – still performing refresh-cycles – the newly-connected WiFi was experiencing problems again.

Well I found that changing a script which I had written, which was located in the directory

/lib/systemd/system-sleep

To a newer version, still based on the same principles, gives improved performance:


#!/bin/bash
#
# fixing https://bbs.archlinux.org/viewtopic.php?id=173487

case "$1" in
  pre)
    date +%s > /tmp/suspend.log
    ;;
  post)
    was=`cat /tmp/suspend.log`
    now=`date +%s`
    # time shifts for 68 hours
    if [ $now -gt `expr $was + 244800` ]; then
      date -s "`date -R --date="68 hours ago"`"
    fi
    sleep 20; /etc/init.d/nmbd restart
    ;;
  *)
    ;;
esac

 

The most important difference in the new script, is the unconditional execution of the script command “sleep 20“. My original intention was, that the script should wait for the network connection to be reestablished, before also restarting the ‘nmbd‘ daemon.

But, I observe that the behavior of the kernel, is not to create a new WiFi connection, until all the scripts located in ‘/lib/systemd/system-sleep‘, which run asynchronously, have exited. This means, that my WiFi also does not come back up, until this one script has finished, which will be more than 20 seconds after I Resume the laptop.

I do not believe strongly anymore, that the actual restart of the ‘nmbd‘ daemon accomplishes much.

But apparently, the simple fact that the WiFi connection waits for 20 seconds before being re-established, is what finally makes my WiFi stable – after a Resume From Standby ! :)

This observation would also seem to confirm, that the last misbehavior of this chip-set was mainly due to the fact that it physically has two antennae attached, and that it makes a selection about which one to use, when the WiFi-connection is (re-)established. Apparently, the physical WiFi connection was not yet stable, ~half a second~ after the Resume. For which reason the Kernel Module has been choosing the weaker antenna, whereas after more than 20 seconds have elapsed, the correct antenna is chosen.

Dirk