This series is a summary of Oracle’s JVM troubleshooting course which gives an overview on JVM memory management, Hotspot VM’s garbage collection options, various memory errors and how to troubleshoot them. In this post (part 1), we will have a look at how JVM manages memory and its different garbage collectors. You can find Part 2 here JVM Memory Management Overview The JVM provides automatic memory management to free the programmer from manually managing memory. New objects are allocated on heap memory. A root set consists of pointers to external memory, static variables, threads, JNI references and internal JVM structures. Objects directly reachable from the root set must be kept in heap. Objects reachable from any of the reachable objects must also be in heap. This group of objects are the only ones which can be used by a program. The unreachable objects (garbage) are removed using a process called garbage collection (GC). Reachable objects are compacted i.e moved to contiguous space in heap. This is important as otherwise, the heap will become fragmented. ...

Snake case and Camel case are conventions of naming variables, functions and classes. Most teams and projects prescribe a particular case in their style guides. Examples of camel case: MyClass MyClassFactory MyClassFactoryBuilder MyClassFactoryBuilderImpl myInstance myInstance2 abc patternMatcher Examples of snake case: add matrix_add diagonal_matrix_add pseudo_inverse If we want to convert back and forth between these cases, we must look for the points of interest - the word boundaries. Camel case boundaries have the first letter capitalized while the snake case word boundaries have an _. Snake case to camel case Here is a regular expression for finding out the _ and the first letter in the next word: (.*?)_([a-zA-Z]) This regex has 2 parts: (.*?) finds everything upto the _. The ‘.’ means any character. ‘*’ stands for match 0 or more instances ‘?’ stands for non-greedy match. We must use ‘?’ in the pattern because the regex engine will try to match as much as possible by default. So, if we use just (.*), the whole word will be consumed and nothing will be left for the rest of the pattern. ‘()’ stand for a group. A group is a way of saving a part of the match for later. Together, they mean that find all characters upto the first ‘_’ and capture them in a group. ([a-zA-Z]) finds the first alphabet after the _. We need this to convert to upper case for Camel case. ...

Background Today, I resumed a rust project of mine after a long time. In order to check my last working code, I ran cargo run. But it refused to run with error message: ...."-Wl,-Bdynamic" "-l" "dl" "-l" "rt" "-l" "pthread" "-l" "pthread" "-l" "gcc_s" "-l" "c" "-l" "m" "-l" "rt" "-l" "pthread" "-l" "util" "-l" "util" = note: /usr/bin/ld: cannot find Scrt1.o: No such file or directory collect2: error: ld returned 1 exit status Linker Issues! ld is used to link libraries. So, the error was not in the code but in the linking phase. I googled the error and the solution was to have build-essential package installed. But the package was already installed on my machine (it is one of the first packages I install on any development machine). Some more googling revealed that cargo uses the system cc linker to link to the C runtime. Running which cc gave me $HOME/anaconda3/bin/cc. This cc is part of my Anaconda root environment. (Anaconda is a package manager for scientific computing packages. It is a convenient way for installing multiple versions of packages in different environments). ...

Many times, smoothly running processes stop working mysteriously. You open the logs and see what is happening, only to find that even the logs have stopped updating. But the process itself is running. You SSH to the server and type cd TAB. Bash weeps “Unable to create temporary file”. The machine is out of disk space… Here is a checklist to make disk space debugging easier, using standard Linux utilities so you can get started without having to install anything extra: df -h command gives you an overview in a readable format about the number of disks mounted and their total and available capacities. To get an idea of which folders/directories are eating up the maximum space, try out du -ch / | sort -h | tail -n 30. This gives you the 30 most space consuming directories. If you already know which directories generate maximum disk output e.g logs and temp files, you can replace the ‘/’ with your directory (DIR) and run the command as du -ch DIR | sort -h | tail -n 30 Now that we have identified the directories with maximum space consumed, we may need to delete some files and get our process going again. The rm command is your friend here. You can delete old logs and temporary files to free up space. Many times, the culprit is a single large file which is already in use by a program e.g catalina.out by Apache Tomcat. If you want to free up space without shutting down the process, the truncate command will help you out. Example: truncate -s0 BIG_LOG.log. This will truncate the file to 0 bytes and still allow the other process to use it without issues (standard Unix permissions apply) Sometimes, you delete files and still, the space does not seem to be recovered. This can be because some process is still holding on to the file descriptor of the deleted file. Once these processes are stopped, the space will be recovered. The lsof command will help you out here. It stands for list open files. You can find out which processes are using deleted files as follows: lsof | grep deleted | grep OLD_FILENAME. The lsof command gives you the process name and the process id so you can run kill on the process. If you do not know the name of the deleted file, you can still run lsof | grep deleted and see the output to check for any familiar file / process. Finally, keep in mind that disk space is one of the metrics you should monitor on your server. This checklist must be used in a pinch. If you find yourself constantly having disk space issues, the solution is to set up periodic deletion/rotation of old log files, alerts when the disk space reaches a particular threshold or to increase the disk size if your processes require a lot of disk space e.g Kafka, MySQL and other databases. ...

Markdown Basics You can write regular markdown here and Jekyll will automatically convert it to a nice webpage. I strongly encourage you to take 5 minutes to learn how to write in markdown - it’ll teach you how to transform regular text into bold/italics/headings/tables/etc. Here is some bold text Here is a secondary heading Here’s a useless table: Number Next number Previous number Five Six Four Ten Eleven Nine Seven Eight Six Two Three One How about a yummy crepe? Code and Syntax highlighting The following are two code samples using syntax highlighting. ...

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