Posts tagged “Operating Systems”
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8 posts
write() succeeded. Is the data on disk? Short writes, late errors and fsync on Linux
A successful write() does not mean the data is on disk. Handling short writes, Linux 4.13+ writeback error reporting, what fsync and fdatasync cover, and when the directory needs fsync too.
Reading Linux Transparent Huge Pages settings: enabled, per-size controls, madvise and khugepaged
THP has a top-level enabled file and per-size hugepages-<size>kB/enabled files. It covers inherit, MADV_HUGEPAGE/MADV_NOHUGEPAGE and khugepaged, and how to check real use in /proc and sysfs.
Linux memory overcommit: overcommit_memory 0, 1, 2 and reading CommitLimit
What the three vm.overcommit_memory modes do, how to read CommitLimit and Committed_AS in /proc/meminfo, and what to check before switching to mode 2, based on the kernel documentation.
Is a load average of 4.0 high? What Linux load average counts and what 1, 5 and 15 minutes mean
Linux load average isn't CPU usage. It's a decaying average of runnable (R) and uninterruptible (D) tasks. How the kernel computes it, how to read it against CPU count, and how PSI differs.
real, user and sys in time output: wall-clock time vs CPU time
What real, user and sys in time output count according to POSIX, why user + sys can exceed real, and how the shell's time differs from /usr/bin/time on Linux.
Linux free shows almost no free memory? Read available first
A small free column does not mean Linux is out of memory. How free derives buff/cache, available and used from /proc/meminfo, and why routinely dropping caches can hurt performance.
What chmod 755 and 644 mean: permission digits, rwx, and x on directories
What chmod 755 means, based on POSIX: how octal digits map to rwx, which permission class applies, directory permissions, umask, and recursive chmod.
Deleted a file but the disk space didn't come back? unlink and open files
Deleting a large file with rm doesn't always free space in df. The POSIX rules on link counts and open files, why du and df disagree, and how to find the cause and get space back on Linux.
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