Thompson's Technological Insight

Spotify - How to remove song recommendation from playlist shuffle.

noreply@blogger.com (Thompson Ng) — Tue, 23 Dec 2025 01:06:43 +0000

As a premium user, Spotify should not include recommended songs in your playlist. However, they had included a "Smart Shuffle" which may show the following when you click on Shuffle

If you do not have the above screen, look for the "diamond" shuffle icon. If you see that, it means Smart Shuffle which will include recommendation to your playlist at shuffle mode.

To turn it off, click on the shuffle button and you will see "Smart Shuffle" recommendation removed message.

And, if you click the shuffle button again, it will have the normal shuffle icon, which does not include song recommendation.

The shuffle is used to toggle between normal shuffle and Smart Shuffle mode.

awk - Unexpected outcome from awk when there is multiple delimiter

noreply@blogger.com (Thompson Ng) — Sat, 16 Aug 2025 04:44:00 +0000

For example,

$ echo "tom+|dick+|and+|harry" | awk -F '+|' '{print $2}'
|dick

$ echo "tom | dick | and | harry" | awk -F ' | ' '{print $2}'
|

In the above example, one may expect the output as dick. However, both awk output seems unexpected.

The reason is - awk -F (as least more recent version of Linux) perform regex. The above

+ mean matches character before + one or more times

| mean OR operator

as a result, above output is given due to regex matching.

If you want to literal matching of the delimiter, you need to escape the regex operator with \\

$ echo "tom+|dick+|and+|harry" | awk -F '\\+\\|' '{print $2}'
dick

$ echo "tom | dick | and | harry" | awk -F ' \\| ' '{print $2}'
dick

Reference

https://www.computerhope.com/jargon/r/regex.htm

OpenLDAP - Modifying cn=config attribute

noreply@blogger.com (Thompson Ng) — Tue, 06 May 2025 13:02:00 +0000

You could modify the cn=config attribute with the following steps

1) Log in as root

2) Create a ldif file (e.g ldap_log_level.ldif ) with the following entry. For example, I am adding olcLogLevel to any

dn: cn=config
changetype: modify
add: olcLoglevel
olcLogLevel: any

2) Then run

ldapmodify -H ldapi:/// -Y EXTERNAL -f ~/ldap_log_level.ldif

-H Specify URI(s) referring to the ldap server(s)

-Y Specify the SASL mechanism to be used for authentication

-f file

LDAPI is an Abbreviation of Using LDAP over IPC Mechanisms. UNIX-domain socket.

EXTERNAL is use EXTERNAL mechanism for SASL. If log in as root, this will authenticate the root user to read and make changes to ldap configuration. This is most commonly use to modify cn=config root entry

Example output

$ ldapmodify -H ldapi:/// -Y EXTERNAL -f ~/ldap_log_level.ldif
SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
SASL SSF: 0
modifying entry "cn=config"

3) You could verify the addition with

cat /etc/openldap/slapd.d/cn=config.ldif

To delete an attribute, you can update the ldif file as

dn: cn=config
changetype: modify
delete: olcLoglevel

and run this command

ldapmodify -H ldapi:/// -Y EXTERNAL -f ~/ldap_log_level.ldif

OpenLDAP - Debug logging

noreply@blogger.com (Thompson Ng) — Thu, 10 Apr 2025 13:03:00 +0000

While it may be possible to make OpenLDAP log to syslog, I am unable to do it so far with OLC

See https://www.openldap.org/doc/admin24/slapdconf2.html

So, the next alternative is log to stdout. To do so, you could run

slapd -d 256 -d 128

which defines logging as Stats and ACL respectively

You will see log output at the console

63b51bda conn=1014 op=0 BIND dn="cn=user,dc=abc,dc=com" method=128
63b51bda conn=1014 op=0 BIND dn="cn=user,dc=abc,dc=com" mech=SIMPLE ssf=0
63b51bda conn=1014 op=0 RESULT tag=97 err=0 text=
63b51bda conn=1014 op=1 SRCH base="uid=ldapuser2,ou=people,dc=abc,dc=com" scope=2 deref=3 filter="(&(objectClass=*)(uid=ldapuser2))"
63b51bda => access_allowed: search access to "uid=ldapuser2,ou=people,dc=abc,dc=com" "entry" requested

You could refers to https://www.openldap.org/doc/admin24/slapdconf2.html for debug level information.

Windows Defenders - Windows cannot remove Microsoft.Windows.SecHealthUI_10.0.19041.1023_neutral__cw5n1h2txyewy because the current user does not have that package installed

noreply@blogger.com (Thompson Ng) — Sat, 10 Aug 2024 10:03:00 +0000

I hate McAfee, not only it constantly changing my browser search engine to the useless Secure Search, it "remove" or "deregister" my Windows Defenders when I uninstall McAfee.

Once McAfee is uninstalled, I could not open Windows Defender and it complains with the following

Amazingly, the uninstall only destroy the installation of the user who uninstall McAfee. Other PC users are able to access Windows Defender.

Anyhow, after reading multiple blogs, I tried to open the PowerShell as administrator and run the following command

Get-AppxPackage *Microsoft.Windows.SecHealthUI* -AllUser | Reset-AppxPackage

But, it returns with error

Reset-AppxPackage : Package was not found.
Windows cannot remove Microsoft.Windows.SecHealthUI_10.0.19041.1023_neutral__cw5n1h2txyewy because the current user does not have that
package installed. Use Get-AppxPackage to see the list of packages installed. At line:1 char:60 + ... xPackage *Microsoft.Windows.SecHealthUI* -AllUser | Reset-AppxPackage + ~~~~~~~~~~~~~~~~~ + CategoryInfo : NotSpecified: (:) [Reset-AppxPackage], COMException + FullyQualifiedErrorId : System.Runtime.InteropServices.COMException,Microsoft.Windows.Appx.PackageManager.Commands.ResetAppxPackageCommand

Next, I do note that some blogs suggested the following action

Add-AppxPackage -Register -DisableDevelopmentMode “C:\Windows\SystemApps\Microsoft.Windows.SecHealthUI_cw5n1h2txyewy\AppXManifest.xml”

but that path does not exist in my system - including "C:\Windows\SystemApps\Microsoft.Windows.SecHealthUI". Duh - each installation uses different installation location, doesn't it?

After days of research, I found the post How to solved error with Windows defender when update from Win10 to Win11. Although this post started with Reset-AppxPackage recommendation, a User suggested the following

add-appxpackage -disabledevelopmentmode -register ((Get-AppxPackage Microsoft.SecHealthUI -allusers).InstallLocation + '\AppxManifest.xml')

This is effectively the similar command to the other Add-Appxpackage command, except the AppxManifest.xml is resolved based on Get-AppxPackage.

And the more important thing - it works!!! I can now open Microsoft Defender UI!!

Ending Note - The suggestions from other blog may work if I knew Get-AppxPackage Microsoft.SecHealthUI would return InstallLocation of Windows Defender.

Linux - passwd "The password is too similar to the old one"

noreply@blogger.com (Thompson Ng) — Mon, 01 Jul 2024 14:50:00 +0000

At times, if you are

the VM owner
does not really care about security on a this particular VM
need to change the password or keep it in sync with other password you are using
you had sudo access

You could bypass the error "The password is too similar to the old one" by running

sudo passwd <username>

This is the root user way to reassign password for user and it does not check password similiarity.

Windows 11 - Restore old Right-click Context menu

noreply@blogger.com (Thompson Ng) — Fri, 14 Jun 2024 02:00:00 +0000

I really hate the new Windows 11 right click context menu. It literally makes me to perform extra click to perform my task

To restore the old right lick context menu, perform the following

1. Type cmd at Search to launch Command Prompt

2. Add the following registry

reg.exe add "HKCU\Software\Classes\CLSID\{86ca1aa0-34aa-4e8b-a509-50c905bae2a2}\InprocServer32" /f /ve

3) Restart Windows Explorer from Task Manager if you do not want to restart your machine.

4) Once restarted, you will see the old good right click context menu

Alternatively, you could always hold shift and right click to launch the old context menu without adding the registry key.

Reference

https://answers.microsoft.com/en-us/windows/forum/all/restore-old-right-click-context-menu-in-windows-11/a62e797c-eaf3-411b-aeec-e460e6e5a82a

Oracle Virtual Box - "Failed to get device handle and/or partition ID" error

noreply@blogger.com (Thompson Ng) — Tue, 21 May 2024 10:59:00 +0000

If you upgraded to Windows 11 recently and you are hit by the error

"Failed to get device handle and/or partition ID"

in Oracle Virtual Box, this may not be related to anything about disk. You should go to your Virtual Box log and check the underlying error.

And if you see "Attempting fall back to NEM: VT-x is not available" when you scroll up the log, this is almost the hint that Hyper-V is enabled and prevented your VM to start up

To fix this, you may want to first check if Hyper-V is turned on. One good check is

1) Open Command Prompt or PowerShell as Administrator

2) Run systeminfo

The last output of systeminfo is about Hyper-V Requirement.

If you see " A hypervisor has been detected. Features required for Hyper-V will not be displayed."

it means that Hyper-V is enabled.

Otherwise, you should see something like the following

If Hyper-V is turned on, you will need to turned it off as follows

1) Disable Hyper-V feature

Open Command Prompt or PowerShell as Administrator and run

dism.exe /Online /Disable-Feature:Microsoft-Hyper-V

2) Disable Windows Hypervisor Platform and Virtual Machine Platform

a) Press Win + R to open the Run dialog.

b) Type optionalfeatures and press Enter.

c) In the Windows Features dialog, uncheck Hyper-V, Windows Subsystem for Linux., Windows Hypervisor Platform and Virtual Machine Platform.

3) Disable Hyper-V via Boot Configuration Data (BCD)

Open Command Prompt or PowerShell as Administrator and run

bcdedit /set hypervisorlaunchtype off

4) Ensure No Conflicting Services are Running

a) Press Win + R to open the Run dialog.

b) Type services.msc and press Enter.

c) Look for services like Hyper-V Virtual Machine Management and set their Startup type to Disabled.

5) Make sure Turn on Virtualization Based Security is configured and disabled

Go to

Local Group Policy Editor > Computer Configuration > Administrative Templates > System > Device Guard > Turn on Virtualization Based Security

and set this to Disabled

6) Finally, restart your computer.

7) After restart, you could use systeminfo to check if Hyper-V had been turned off successfully.

1) I had restarted at least 2 times before systeminfo shows Hyper-V is turned off.

2) You may want to take a look at your BIOS and check if there is any Hyper-V feature listed in Secure Boot. You need to turn off Secure Boot as well.

3) You may also need to set Turn on Virtualization Based Security to Not Configured and reboot again.

Window 11 - "This is no longer located in" error when deleting a directory

noreply@blogger.com (Thompson Ng) — Tue, 21 May 2024 10:13:00 +0000

In my previous Windows 7 - this is no longer located post, I mentioned 1 method to delete these trouble directories.

There is an alternative way to do this. As a recap, below is the error message

You can simply run

rd /s C:\Path\To\Bad\Directory

and say Y to the command. It will remove the directory.

rd command is a remove directory Windows command. /s means deletes a directory tree (the specified directory and all its subdirectories, including all files).

WebEx - Code Block with Syntax Highlighting in message

noreply@blogger.com (Thompson Ng) — Wed, 10 Jan 2024 06:13:00 +0000

WebEx provide a very nice feature to message code block with syntax highlighting - it is call Markdown.

To enable Markdown, do the following

1) Click on the T icon

2) Click on the M icon

3) Click on the Markdown enabled checkbox

Or to simplify all step

1) Click "Ctrl+m"

Once the T icon become M (below is example), Markdown is enabled

Once you have markdown, you could provide code block with syntax highlighting as follow

Below is an example

``` C++

int main() {

cout << "Hello World!";

return 0;

}

```

Note the following

` - this is a back-tick and NOT single quote

Use 'Shift + Enter' for new line

language is optional - currently it supports only 16 languages: markup, css, clike, javascript, c, c++, c#, docker, git, java, json, objective-c, perl, powershell, python and swift.

Python - Wait for TCP ack before sending next data

noreply@blogger.com (Thompson Ng) — Wed, 13 Dec 2023 08:49:00 +0000

In usual case, it is not likely you need to control how socket send message, and most of the time, it is OK to send stream of data without waiting for acknowledgement for receiver.

However, in some rare case, you may want to wait for TCP ack'ed before sending the next data out to destination. You could use fcntl.ioctl (https://docs.python.org/3/library/fcntl.html) to perform this requirement.

In sockio.h (https://github.com/torvalds/linux/blob/master/include/uapi/linux/sockios.h), it define

We can use the above SIOCOUTQ to queue the file descriptor and get the result. If the result is not zero, it means there are bytes unsent in the queue.

Below is a sample code

Linux - "cannot execute: required file not found" error

noreply@blogger.com (Thompson Ng) — Sat, 21 Oct 2023 11:13:00 +0000

In Bash, if you are running a script, for example installation, and it throws error

$./my_installer
-bash: ./my_installer: cannot execute: required file not found

and you are certain that all files requires exist. You may want to check what SHELL was used to write the install script.

If you are using BASH but the script is written by KSH and KSH is not installed, this error will happen.

PowerShell - Increase screen buffer

noreply@blogger.com (Thompson Ng) — Fri, 15 Sep 2023 23:49:00 +0000

If you are facing problem when you cat a long file and the PowerShell does not show the earlier lines, you may want to check if you have a short Screen Buffer

You could check the Screen Buffer Size by

1) Right click at the PowerShell and click on Properties at the popup menu

2) Navigate to Layout tabe and chang the height of the Screen Buffer Size

No restart of the PowerShell is required.

Wireshark - How to find Connection Timeout TCP packets?

noreply@blogger.com (Thompson Ng) — Thu, 17 Aug 2023 08:49:00 +0000

Connection Timeout in TCP happens after TCP has send multiple attempts of SYN packets but did not received a respond from the remote destination. Based on this, we could try to filter tcpdump by SYN flag and locate those that has multiple one directional packet.

The steps in Wireshark as follows

1) Filter by 'tcp.flags eq 0x02' (only SYN flag set)

2) Then select Statistics -> Conversations

3) Then,

a) Select on "Limit to display filter". Limit to display filter will only show conversations matching the current display filter

b) Select TCP tab

c) Sort by Packets

Those that had more than 1 SYN packets are likely retransmission TCP packets

4) To drill in, right click on the "Apply as Filter" -> "Selected" > "A <->B"

5) You will see a packet trace on the TCP retransmission of SYN packet. That mean Connection Timeout.

FFMPEG - Fast Forwarding a Video 100 times

noreply@blogger.com (Thompson Ng) — Wed, 19 Jul 2023 05:43:00 +0000

To create a fast forward video, you can create a filter to change the presentation timestamp (PTS) of each video frame.

Below is a general command - Note - This method drops frame to achieve the speed up. And, I applied the above to a video only file.

ffmpeg -i input.mkv -filter:v "setpts=0.5*PTS" output.mkv

where

-filter:v mean filter apply to video stream
setpts is the set presentation timestamp filter
0.5*PTS means 2X the speed.

Now, to speed thing up in 100x, you can run

ffmpeg -i input.mkv -filter:v "setpts=PTS/100" output.mkv

PTS/100 means speeding the video up by 100x

C:\ffmpeg-2021-11-03-git-08a501946f-full_build\bin>ffmpeg.exe -i "..\..\TimeLapses - Pentas Lack Of Water\pantas_no_audio.mp4" -filter:v "setpts=PTS/100" pantas_ff100x.mp4

built with gcc 11.2.0 (Rev1, Built by MSYS2 project)

configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libglslang --enable-vulkan --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint

libavutil 57. 7.100 / 57. 7.100

libavcodec 59. 12.100 / 59. 12.100

libavformat 59. 8.100 / 59. 8.100

libavdevice 59. 0.101 / 59. 0.101

libavfilter 8. 16.101 / 8. 16.101

libswscale 6. 1.100 / 6. 1.100

libswresample 4. 0.100 / 4. 0.100

libpostproc 56. 0.100 / 56. 0.100

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '..\..\TimeLapses - Pentas Lack Of Water\pantas_no_audio.mp4':

Metadata:

major_brand : isom

minor_version : 512

compatible_brands: isomiso2avc1mp41

encoder : Lavf59.8.100

Duration: 02:54:08.33, start: 0.010000, bitrate: 13865 kb/s

Stream #0:0[0x1](eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709, progressive), 1920x1080, 13862 kb/s, SAR 1:1 DAR 16:9, 30 fps, 30 tbr, 90k tbn (default)

Metadata:

handler_name : VideoHandle

vendor_id : [0][0][0][0]

Stream mapping:

Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))

Press [q] to stop, [?] for help

[libx264 @ 000001fd9e812780] using SAR=1/1

[libx264 @ 000001fd9e812780] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2 AVX512

[libx264 @ 000001fd9e812780] profile High, level 4.0, 4:2:0, 8-bit

[libx264 @ 000001fd9e812780] 264 - core 164 r3075 66a5bc1 - H.264/MPEG-4 AVC codec - Copyleft 2003-2021 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=12 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00

Output #0, mp4, to 'pantas_ff100x.mp4':

Metadata:

major_brand : isom

minor_version : 512

compatible_brands: isomiso2avc1mp41

encoder : Lavf59.8.100

Stream #0:0(eng): Video: h264 (avc1 / 0x31637661), yuv420p(tv, bt709, progressive), 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 30 fps, 15360 tbn (default)

Metadata:

handler_name : VideoHandle

vendor_id : [0][0][0][0]

encoder : Lavc59.12.100 libx264

Side data:

cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A

frame= 3137 fps=4.0 q=-1.0 Lsize= 72397kB time=00:01:44.46 bitrate=5677.2kbits/s dup=0 drop=310358 speed=0.134x

video:72359kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.053391%

[libx264 @ 000001fd9e812780] frame I:13 Avg QP:22.04 size:265781

[libx264 @ 000001fd9e812780] frame P:791 Avg QP:24.62 size: 64773

[libx264 @ 000001fd9e812780] frame B:2333 Avg QP:27.59 size: 8317

[libx264 @ 000001fd9e812780] consecutive B-frames: 0.8% 0.1% 0.2% 98.9%

[libx264 @ 000001fd9e812780] mb I I16..4: 9.2% 61.0% 29.8%

[libx264 @ 000001fd9e812780] mb P I16..4: 0.9% 1.1% 0.1% P16..4: 46.6% 16.5% 13.8% 0.0% 0.0% skip:21.1%

[libx264 @ 000001fd9e812780] mb B I16..4: 0.1% 0.1% 0.0% B16..8: 38.4% 3.0% 0.6% direct: 1.4% skip:56.4% L0:48.8% L1:43.1% BI: 8.0%

[libx264 @ 000001fd9e812780] 8x8 transform intra:55.6% inter:74.7%

[libx264 @ 000001fd9e812780] coded y,uvDC,uvAC intra: 41.9% 64.9% 21.8% inter: 14.0% 22.3% 0.8%

[libx264 @ 000001fd9e812780] i16 v,h,dc,p: 42% 21% 18% 19%

[libx264 @ 000001fd9e812780] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 26% 10% 41% 3% 4% 6% 3% 4% 4%

[libx264 @ 000001fd9e812780] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 30% 12% 9% 7% 9% 10% 8% 8% 8%

[libx264 @ 000001fd9e812780] i8c dc,h,v,p: 52% 19% 25% 5%

[libx264 @ 000001fd9e812780] Weighted P-Frames: Y:2.0% UV:1.3%

[libx264 @ 000001fd9e812780] ref P L0: 36.3% 10.3% 39.2% 14.0% 0.2%

[libx264 @ 000001fd9e812780] ref B L0: 65.0% 27.7% 7.3%

[libx264 @ 000001fd9e812780] ref B L1: 86.8% 13.2%

[libx264 @ 000001fd9e812780] kb/s:5668.69

See FFMPEG - Speeding up/slowing down video

Wireshark - Beware of false positive marking by Wireshark - TCP Keep alive

noreply@blogger.com (Thompson Ng) — Thu, 29 Jun 2023 04:45:00 +0000

Wireshark tries to provide intelligent information for the user, but no matter what, it is based on characteristic of the packet mentioned in TCP Analysis

For example, it is very possible that Wireshark mark TCP Keep-alive wrongly.

Look at the below, Wireshark mark these packets are TCP keep-alive packet and acknowledgement. At first glace, they look legitimate.

But if you understand how TCP keep-alive work under Linux, these packets does not falls into the TCP keep alive packet pattern.

Based on TCP Man Page, it said

So, TCP keep alive packet will only start to send after a default of 7200 seconds (2 hours). Next, it will only send 9 probes for an interval of 75 seconds apart.

For the above example, those packets was not send after 2 hours of idle connection and they are send at 60 seconds apart. Wireshark marked them as TCP keep alive packet likely because of their payload of 1.

To demonstrate a genuine TCP keep alive packets, below is a genuine TCP keep alive mechanism in action

You can see that

There is a gap of 2 hours between the earlier packet and the TCP keep alive packet
Each packet are sent at 75 seconds interval
Each packet is 0 in bytes
There are only 9 probes
After 9 probes, the connection is close with RST flag

In summary, you should verify Wireshark analysis if you are in doubt

Java - How to map Linux Thread ID to Java Thread ID

noreply@blogger.com (Thompson Ng) — Wed, 03 May 2023 15:28:00 +0000

When you are looking at Top output, you will something see Java processes eating up CPU. The next question you will ask, what is happening to the Java process? Usually, I will do the following

1) Identify the process

In the following example, I saw CPU usage over 170% of one of the Java process. The process ID is 11795

2) Find out the Linux thread

What next? Let's check the Linux threads within the process. I run

top -H -p 11795

where -H is thread mode and -p is the pid

In the Top output, you will now see the threads that are using the CPU. 11797 and 11798 is the suspicious thread

3) Find out the Java stack trace

Next, you will need the Java stack trace of the troubled process. In this case, you need jstack

I run

jstack 11795 > jstack_output1.txt

the above will print out the Java stack trace to a file.

4) Mapping Linux thread id to Java thread id

This is the most important part. First, if you look at the jstack output, you will see the following

especially, you will see nid=0x????

nid mean native thread id - OS

tid mean Java thread id

The remaining thing to do is to map Linux thread id 11797 to Hex value. And Google will tell you it is 0x2E15

Now, if you search the jstack output file, you should see a match in nid. For me, they are

GC task threads that eating the CPU.

Windows 10 - Could not find this item when deleting directory

noreply@blogger.com (Thompson Ng) — Sat, 15 Apr 2023 09:26:00 +0000

If you encounter "Could not find this item" and you could not delete the folder no matter what you do from the Windows Explorer, you may want to try rd command.

Windows being Windows, there are many reason for you to encounter this issue (ranging from invalid fold path to invalid files within the directory).

To solve this issue, you could

Open Command Prompt (cmd) from Windows Search Bar
Navigate to the parent directory of the troubled folder
Run

rd /s "the trouble folder"

The directory including the sub directory will be removed

See Microsoft rd command

Linux - What is the best way to identify currently used Linux Shell?

noreply@blogger.com (Thompson Ng) — Wed, 29 Mar 2023 13:56:00 +0000

In Linux, there is a convenience environment variable SHELL which may tell what is the current shell. However, please do not be confused by it, because, $SHELL only tells us the about the SHELL of the user (and likely the SHELL used when the user log in) and does not necessary tell the runtime shell the user is currently using.

In such runtime case, "ps -p $$" is a better choice

Below is an example

$ echo $SHELL
/bin/bash
$ ps -p $$
PID TTY TIME CMD
6364 pts/0 00:00:00 bash
$ ksh
$ echo $SHELL
/bin/bash
$ ps -p $$
PID TTY TIME CMD
26970 pts/0 00:00:00 ksh

As you can see, $SHELL consistently say it is /bin/bash even after I had change to ksh shell during runtime. But, ps -p $$ provide consistent result.

ps is the command to list process

-p mean Select by process ID

$$ mean current shell PID. It is a special shell variable.

Wireshark - What to do after you found a suspicious packet?

noreply@blogger.com (Thompson Ng) — Fri, 03 Mar 2023 04:47:00 +0000

Now given that you found a suspicious packet from Expert Information and want to investigate further, what should you do next?

For example, I would like to investigate the Connection Reset packet. I double click on it and it brings me to the specific packet

What next? Of course, you could inspect the packet detail, including IP, port and data. But, what you usually want to know is the trail of the packet.

Wireshark allow you to follow the packet. You can right click the packet, Follow -> TCP stream (this is a tcp packet) to get the trail.

The result is the list of packets (time ordered) to let you know what had happen along the way

Worth mentioning, Wireshark provide a Flow Graph under Statistics menu. It shows a graphical view of connections between host. It is rather useful to understand packets sequences.

Wireshark - Where to start? Take a look at Expert Information

noreply@blogger.com (Thompson Ng) — Wed, 15 Feb 2023 06:24:00 +0000

Often, when you are looking at the tcpdump, you will question yourself... What are you looking at? Where should I start?

In Wireshark, there is a Expert Information at the Analyze menu

It will tell you analysis information that is detected by Wireshark. There could be Error, Warning, Note, Chat message and these are based on Wireshark tagging base on packet characteristic. See Wireshark TCP Analysis for more details. It will also tell you the count of each type of message.

From the above example, the first thing that catch my eyes are

Connection reset message
Retransmission anb Duplicate Ack message
Any not capture related message

If you expand each category, Wireshark will show you the packets summary that is related to the message. You can further drill into the packet by double clicking the packet row.

Tomcat - Live monitoring with JavaMelody

noreply@blogger.com (Thompson Ng) — Wed, 04 Jan 2023 07:03:00 +0000

Live Monitoring of Tomcat Resource

I used JavaMelody to provide live monitoring of Tomcat. It is as simple as dropping the following jar file

javamelody-core-1.88.0.jar
jrobin-1.5.9.jar

into the WEB-INF directory. And run http://host:port/context_path/monitoring

They provide HTTP statistic which is very useful

and many others which I think it is useful

See https://github.com/javamelody/javamelody/wiki/Screenshots#charts

SQL - Select ANY operator with using sub query

noreply@blogger.com (Thompson Ng) — Wed, 16 Nov 2022 06:56:00 +0000

Most of the online guide provide Select ANY operator with sub query, for example

SELECT ProductName FROM Products WHERE ProductID = ANY(SELECT ProductID FROM OrderDetails WHERE Quantity = 10);

It is possible not to use sub query, at least for PostgreSQL, and the working syntax is

SELECT ProductName FROM Products WHERE ProductName = ANY('{Samsung}');

SQL - Duplicate table structure with existing table

noreply@blogger.com (Thompson Ng) — Mon, 07 Nov 2022 03:46:00 +0000

Some simply SQL which could duplicate table with table structure quickly from existing table.

Duplicate with full data

create table A_dup as (select * from A);

Duplicate with no data

create table A_dup as (select * from A) with no data;

Chrome - Replaying Network Request

noreply@blogger.com (Thompson Ng) — Tue, 11 Oct 2022 02:10:00 +0000

From debugging point of view, some time, you will want do stress test your web application by replaying network request. Chrome Developer Tools allows you to do so.

The steps are:

1) Open Chrome Developer Tools, then navigate to Network Tab

2) Then perform the UI action which you want to replay the network request.

3) Look at the Network tab and it will capture the network request

4) Then, right click on the request, select Copy → Copy as fetch

5) Paste the copied fetch request to Console tab and press Enter. It will replay the request

6) You could write fancy javascript at the Console, for example, loop the fetch request