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Top Forums UNIX for Advanced & Expert Users Ubuntu 14.04 - how to less ram usage Post 303029018 by Don Cragun on Saturday 19th of January 2019 06:40:24 PM
Old 01-19-2019
Note that some systems actively try to use otherwise unused memory to speed up running processes. For example, if a process starts reading from a large file, the system might read the next few blocks (or megabytes) of that file into a system buffer so the data will be in memory without having to wait for the disk to rotate if that process reads the next block.

If other processes start up need the memory that was used for read-ahead buffers they can be reallocated to the next process without affecting how fast the new process can be loaded.

Different versions of BSD, Linux, and UNIX systems use different algorithms to try to make efficient use of memory. And, different releases of each of those systems may change the algorithms as the vendors think they have learned something from past experiences.

In other words, there can be lots of "free" memory may be used to hold data that may soon be useful to someone even though it can be re-allocated immediately if someone asks for it to be put to a specific use.
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PS(1)							      General Commands Manual							     PS(1)

NAME
ps - process status SYNOPSIS
ps [ aklx ] [ namelist ] DESCRIPTION
Ps prints certain indicia about active processes. The a option asks for information about all processes with terminals (ordinarily only one's own processes are displayed); x asks even about processes with no terminal; l asks for a long listing. The short listing contains the process ID, tty letter, the cumulative execution time of the process and an approximation to the command line. The long listing is columnar and contains F Flags associated with the process. 01: in core; 02: system process; 04: locked in core (e.g. for physical I/O); 10: being swapped; 20: being traced by another process. S The state of the process. 0: nonexistent; S: sleeping; W: waiting; R: running; I: intermediate; Z: terminated; T: stopped. UID The user ID of the process owner. PID The process ID of the process; as in certain cults it is possible to kill a process if you know its true name. PPID The process ID of the parent process. CPU Processor utilization for scheduling. PRI The priority of the process; high numbers mean low priority. NICE Used in priority computation. ADDR The core address of the process if resident, otherwise the disk address. SZ The size in blocks of the core image of the process. WCHAN The event for which the process is waiting or sleeping; if blank, the process is running. TTY The controlling tty for the process. TIME The cumulative execution time for the process. The command and its arguments. A process that has exited and has a parent, but has not yet been waited for by the parent is marked <defunct>. Ps makes an educated guess as to the file name and arguments given when the process was created by examining core memory or the swap area. The method is inherently somewhat unreliable and in any event a process is entitled to destroy this information, so the names cannot be counted on too much. If the k option is specified, the file /usr/sys/core is used in place of /dev/mem. This is used for postmortem system debugging. If a second argument is given, it is taken to be the file containing the system's namelist. FILES
/unix system namelist /dev/mem core memory /usr/sys/core alternate core file /dev searched to find swap device and tty names SEE ALSO
kill(1) BUGS
Things can change while ps is running; the picture it gives is only a close approximation to reality. Some data printed for defunct processes is irrelevant PDP11 PS(1)
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