https://frn.sh/Recent content in Fernando Simoes - Homepage onHugoen-USCopyright © Fernando Simões.Fri, 03 Apr 2026 00:00:00 +0000https://frn.sh/smaps/Fri, 03 Apr 2026 00:00:00 +0000https://frn.sh/smaps/A few weeks ago I profiled a Node.js server with recurring memory spikes. I found out dirty pages, allocator behavior and memory that never came back. To build a cleaner mental model, I stripped the problem down to something smaller: python3 -m http.server. root@debian:/# ps aux | grep http.server | grep -v grep root 479226 0.0 0.1 32104 19324 pts/1 T 16:10 0:00 python3 -m http.server 19 MiB of resident memory.https://frn.sh/pmem/Sat, 21 Mar 2026 00:00:00 +0000https://frn.sh/pmem/I restarted all 60+ pods of a Node.js websocket app earlier today. Every single pod sitting at ~330 MiB of memory. Except one, which was double the rest - at 640 MiB. This is a statefulset. When I built the cluster, I estimated each pod’s footprint: ~198 MiB base, plus ~25 MiB per websocket. With 30 websockets per pod, that’s roughly 900 MiB. I was wrong about the per-websocket cost - it’s lower than 25 MiB in practice.https://frn.sh/iops/Sun, 08 Feb 2026 00:00:00 +0000https://frn.sh/iops/We had a Postgres incident this week. Heroku timeouts, multiple queries running for 30+ minutes, and the IOPS pinned at the provisioned limit. I knew I needed a better index, but I wanted to understand what “reading from disk” actually means first. How many layers of caching sit between a SELECT and the storage device? Three. First: shared buffers, Postgres’ own cache living in the process memory. If the page is there, we need no system call - just a memory read.https://frn.sh/tforks/Thu, 11 Dec 2025 00:00:00 +0000https://frn.sh/tforks/First week at a new job. A colleague was showing me around our Grafana dashboards, just routine monitoring of the baremetal machines. One caught my eye: a machine with 32GB RAM and a top-of-the-line processor was hitting 90% CPU. A few containers running, no alerts, and nobody had reported anything. I found a process with cmd bash startup.sh that had been running for 28 minutes. I straced it for a few minutes:https://frn.sh/sigterm/Sun, 08 Jun 2025 00:00:00 +0000https://frn.sh/sigterm/Load average hit 12 on a 2 vCPU machine during a production incident. My first thought was that CPU must be the bottleneck - 12 is 6x the core count. But it wasn’t. Linux load average counts three things: processes running on a CPU, processes waiting in the run queue, and processes in uninterruptible sleep - D state. From the kernel source: The global load average is an exponentially decaying average of nr_running + nr_uninterruptible.https://frn.sh/til/osync/Sun, 18 May 2025 00:00:00 +0000https://frn.sh/til/osync/write(2) doesn’t actually write to disk imediatelly. Instead, it writes to a page cache and the OS periodically handles writes to disk. Using O_SYNC, though, write(2) returns only when it fully wrote the data to a data block. Linux exposes the actual timeframe for periodic writings: ➜ ~ cat /proc/sys/vm/dirty_writeback_centisecs 500https://frn.sh/tty/Sat, 26 Apr 2025 00:00:00 +0000https://frn.sh/tty/My birthday is next month, so I bought myself something I’d wanted for a while: a terminal keyboard. Last year I read this article about the history of the terminal and it fascinated me. I found a DEC-style Televideo terminal keyboard from the 70s or 80s (I can’t be sure of the exact date) and took advantage of the opportunity to learn a few things. These old terminal keyboards were sold alongside a “computer terminal,” which was essentially a dumb display and input device.https://frn.sh/til/source-and-export/Sun, 05 Jan 2025 00:00:00 +0000https://frn.sh/til/source-and-export/Non-interactive shells don’t load initialization files, so bash -c ‘declare -f’ doesn’t output anything. But we can source it: bash -c ‘source ~/.bashrc; hello’. Or even: bash -c ‘hello() { echo “hi”; }; declare -f’. It’s all about memory share in shell modes: source changes only affect current shell memory. export marks variables to be passed to child processes. Subtile difference that can save us a lof of debugging time.https://frn.sh/shell-source/Wed, 22 May 2024 00:00:00 +0000https://frn.sh/shell-source/Understanding how the shell manages environment variables and session state requires understanding builtins, process groups, and memory space. This post traces how these pieces connect. Types, builtins There is a classification in the shell called builtin. Some commands are builtin: ➜ ~ type cd cd is a shell builtin ➜ ~ type echo echo is a shell builtin But some are not: ➜ ~ type mkdir mkdir is /usr/bin/mkdir When a shell runs a command, it does one of two things: execute something available in its own memory space (a builtin), or execute something external (via execve).https://frn.sh/about/Mon, 01 Jan 0001 00:00:00 +0000https://frn.sh/about/Hello! I’m Fernando Simões. I enjoy sharing debugging stories and other computer-kernel-database-runtime-performance explorations. Also, structured procrastination. You can reach me at frn@frn.sh or the only social account I have.