In older versions of Xemn, each DomU could effectively use the system time of the Dom0. That was remove after Xen 4 or so. In versions after that issues of clock drift need to be addressed. I was investigating this and came across a discussion of best practices about setting the Limux clocksource on DomU’s. It seems there has been some performance issues with using “xen” and a recommendation to use “tsc” but it’s not entirely straightforward.

• Xen 4.2 TSC_MODE HOW-TO
  • gives this handy code: xm debug-key s; xm dmesg | tail
  • which I improved as: sudo xl debug-key s; sudo xl dmesg | grep mode=
• (CMP402) Amazon EC2 Instances Deep Dive (2015)
  • recommends “TSC as clocksource”
• Performance Tuning EC2 Instances (2014)
  • recommends “tuning clocksource to TSC”
• Two frequently used system calls are ~77% slower on AWS EC2 (2017)
  • raises concerns about unexpected backward clock drift
• Running a database on EC2? Your clock could be slowing you down. (2018)
  • helpful for getting Xen hypervisor version from inside guest: sudo dmesg | grep -i ‘xen version’
  • testing for clocksource acceleration: strace -e gettimeofday,clock_gettime — date >/dev/null
  • getting current clocksource: cat /sys/devices/system/clocksource/clocksource0/current_clocksource
  • setting clocksource to tsc: echo tsc | sudo tee /sys/devices/system/clocksource/clocksource0/current_clocksource
• How do I manage the clock source for EC2 instances running Linux? (2020)
  • “For EC2 instances launched on the AWS Xen Hypervisor, it’s a best practice to use the tsc clock source. Other EC2 instance types, such as C5 or M5, use the AWS Nitro Hypervisor. The recommended clock source for the AWS Nitro Hypervisor is kvm-clock.”
  • recommends setting GRUB_CMDLINE_LINUX in grub2: “clocksource=tsc tsc=reliable”

Let’s suppose one is tasked with a complex set of calculations where the input data loads from a database and the output data saves to a database. This has come up a number of times in my consulting work. There are 2 approaches:

1. Use language of choice, read data, perform calculations, write data
2. Work entirely within the database

This latter has the obvious benefits of skipping the data transfer times. This is also where it’s easy to use the first approach, only to run into problems as the amount of data is increased.

Now for simpler tasks I’ve often chosen approach 2. A simpler task, in this case, is when the computation is question can be expressed as an INSERT or UPDATE query. This works when one needs to produce statistics like mean and standard deviation that can be easily expressed as aggregates on the input data. And when something is more complicated than that I have chosen option 1.

One issue with more complicated programming within a database is that there is no agreed upon common portable language. I have client whose databases include MariaDB and SQL Server and thus there is no way to develop code that will work on both of those platforms. I was thinking the other day, wouldn’t it be great if there was an agreed upon embedded language, in addition to whatever legacy languages are support(PL/SQL, T-SQL). Maybe Lua would be a good choice. It’s already available to Postgresql as pllua-ng. Perhaps Python would be good as well.

MariaDB can understand a subset of PL/SQL and (currently small) T-SQL which is nice. Postgresql can understand a subset of PL/SQL. Although I don’t expect Oracle or SQL Server to make changes in this regard, it would be nice if a team created a language that could be added to both that could be used to write portable procedural code.

I don’t like JSON, or YAML. Moving on.

Actually I’ve been exploring binary interchange formats. I am not the first to note that converting data from binary, to text, and back, is often suboptimal, especially for the not-uncommon use case of floating point numbers. With some projects I’ve been working with there is a lot of interchange with delimited files (CSV, TSV, PSV) and JSON. These suffer from the creation of the text stream only to have it parsed back into binary formats. These formats can be useful because they are human readable so being able to inspect them is helpful in a development stage but for interchange they are suboptimal.

Here are some alternatives I’ve been investigating:

• MessagePack – lots of language support
• CBOR – lots of language support
• RION – good design goals, only available for JAVA
• BSON – used by MongoDB and not much else

I’ve been using some of these a lot for some psychometrics software developmont work.

• TAM: Test Analysis Modules (3.3-10 released 2019年08月23日)
• sirt: Supplementary Item Response Theory Models (3.7-40 released 2019年11月04日)
• mirt: Multidimensional Item Response Theory (1.31 released 2019年09月15日)
• eRm: Extended Rasch Modeling (1.0-0 released 2019年06月06日)
• catR: Generation of IRT Response Patterns under Computerized Adaptive Testing (3.16 released 2018年07月28日)
• mirtCAT: Computerized Adaptive Testing with Multidimensional Item Response Theory (1.9.3 released 2019年06月28日)

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• DNC Blasts Facebook For Allowing Trump Ad Blitz To Spread False Claims

Using systemd-resolved opens up some options that may be easier to configure then in the past.

Enabling DNSSEC:

• systemd-resolve –status
  • look for “DNSSEC setting: no”
• sudo mkdir -p /etc/systemd/resolved.conf.d
• sudo nano /etc/systemd/resolved.conf.d/dnssec.conf
  • enter:
    • [Resolve]
      DNSSEC=true
• sudo systemctl restart systemd-resolved
• systemd-resolve –status
  • look for “DNSSEC setting: yes”

It’s not so simple as “cat /etc/resolv.conf” anymore. DNS is now handled by systemd-resolved in Ubuntu and Debian and their derivatives. This is easy to spot if you do check resolv.conf and it contains “nameserver 127.0.0.53”. The correct command now is “systemd-resolve –status”.

I’ve drafted but not published a long list of problems I’ve experienced with Ubuntu 18.04. In short it’s been one of the worst desktop releases of Ubuntu I’ve used. My experience on servers has been fine. Anyway I haven’t looked into the details of these patches but:

• libwebkit2gtk-4.0-37:amd64 (2.22.6-0ubuntu0.18.04.1)
• libjavascriptcoregtk-4.0-18:amd64 (2.22.6-0ubuntu0.18.04.1)

These package releases seem to have fixed some of my issues that have been present since May 2018. In particular Liferea has been broken on my desktop for that long. Better late than never I guess.

I’ve often run a wired router at the front of my network (well immediately behind the modem). The only other topology has been a linux server at the front of the network. Then I like a network behind that where a wireless router can live. I keep my wireless router turned off when not in use, which is an uncommon setup it seems. Anyway, for 1 network I’m managing I’m considering replacing the linux system with a wired router. Wired routers are not super popular for SOHO uses as mos tpeople seem to prefer a wireless router to do everything. Because there is not a huge market options can be limited. Here are some products I’ve investigated.

• Specs:
  • 4 gigabit ethernet ports
  • cost <= 100ドル
• TP-Link TL-R600VPN
  • I’ve had good experiences with TP-Link wired routers in the past. However, this router is believed to be vulnerable to VPNFilter malware so that makes it too risky at this time.
  • TPLink routers affected by VPNfilter malware?
• Ubiquiti ER-X
  • I’ve had no first-hand experiences with Ubuquiti products.
  • POE passthrough support.
• Cisco RV042G
• Grandstream GS-GWN7000
• Cisco RV130
• Research
  • VPNFilter Devices at risk
  • VPNFilter Update – VPNFilter exploits endpoints, targets new devices
  • VPNFilter-affected Devices Still Riddled with 19 Vulnerabilities
  • Over 200,000 MikroTik Routers Compromised in Cryptojacking Campaign
  • FBI: Kindly Reboot Your Router Now, Please
• Networking Companies Country of Operation
  • TP_Link – China
  • MicroTik – Latvia
  • Cisco – United States
  • Ubiquiti – United States
  • GrandStream – United States
  • TrendNet – United States
• Other Products
  • TRENDnet TW100-S4W1CA – 10/100 only
  • D-Link – no gigabit products in price range
  • Netgear – no gigabit products in price range
  • Linksys LRT214 – outside price range
• Further research
  • There are many more options above 100ドル.

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