How to find information about a given process?

NotionCommotion · 04-20-2017, 07:08 AM

For instance, what is "writeback"? Maybe it has always been there, but I don't recognize it. How do I learn what it is? Below is my interpretation of ps's output. Thanks

User: The user who started the process
PID: the process ID
%CPU and %MEM: The resources being used.
VSZ: Virtual memory being used?
RSS: Resident set size, non-swapped virtual memory?
TTY: The terminal accessing?
STAT: The status? For this process, it is in interruptible sleep (waiting for an event to complete) and is high-priority (not nice to other users)?
START: When it was started? Date only and not time?
TIME: The amount of time it has run?
COMMAND: The name of the process?

Code:

michael@raspberrypi3:~ $ ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
...
root        26  0.0  0.0      0     0 ?        S<   Apr13   0:00 [writeback]
...
michael   5330  0.0  0.2   4816  2164 pts/0    R+   11:53   0:00 ps aux
michael@raspberrypi3:~ $ whereis writeback
writeback:
michael@raspberrypi3:~ $ whereis [writeback]
[writeback]:
michael@raspberrypi3:~ $

Tanner9 · 04-20-2017, 07:14 AM

man writeback

NotionCommotion · 04-20-2017, 07:21 AM

Quote:

Originally Posted by Tanner9

man writeback

Code:

michael@raspberrypi3:~ $ man writeback
No manual entry for writeback
michael@raspberrypi3:~ $

In addition to what writeback is, what do the brackets around it mean?

Tanner9 · 04-20-2017, 07:26 AM

Quote:

Originally Posted by NotionCommotion

Code:

michael@raspberrypi3:~ $ man writeback
No manual entry for writeback
michael@raspberrypi3:~ $

In addition to what writeback is, what do the brackets around it mean?

It's not in his post.

NotionCommotion · 04-20-2017, 03:18 PM

Quote:

Originally Posted by Tanner9

It's not in his post.

Meaning???

linux4evr5581 · 04-20-2017, 03:37 PM

The /proc directory is a virtual file system that the kernel uses to communicate with you (and to programs like top) about the the state of a process/program. In that directory you'll find information about all running processes. The processes are the numbers displayed and to view info available on it type ls /proc/number... Also look into file descriptors (fd) as they are given to a process based on what that process wants to do to a file.

Habitual · 04-20-2017, 04:25 PM

Quote:

Originally Posted by NotionCommotion

Code:

michael@raspberrypi3:~ $ ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
...
root        26  0.0  0.0      0     0 ?        S<   Apr13   0:00 [writeback]

Code:

sudo lsof -p 26

and process the results. The pid may change, but lsof -p <pid> is gold.

ss utility/program does same but my.fu there is weak.

Just sayin'

wrt:

Quote:

Originally Posted by NotionCommotion

what do the brackets around it mean?

Believe that would be be indicative of a kernel process or thread. AFAIK, best guess.

hydrurga · 04-20-2017, 04:50 PM

Square brackets indicate that the process's arguments cannot be located.

https://unix.stackexchange.com/quest...processes-mean

AwesomeMachine · 04-20-2017, 08:26 PM

Writeback is a form of data cacheing. It has to do with Linux cacheing memory to a disk and back again. Writeback cacheing is faster but less reliable than writethrough, because it notifies the system that data has been written, before that data is committed to disk. When the system is less busy, it writes the cache commits to disk.

Cacheing is fundamental to computing. Disk is the slowest part of the machine. Memory is faster than disk. And CPU is faster than memory. Cacheing is the art of guessing which data will be needed and keeping it in the faster media.

So, if the CPU can fetch data from memory it's quicker than fetching it from disk. If the cacheing algorithm works, it is correct more often than not.

The CPU also has cacheing in which the CPU guesses what data it might need and keeps it in the CPU cache rather than fetching it out of slower memory. Without cacheing, computers wouldn't work well.

Writeback is the kernel cacheing process.

Shadow_7 · 04-20-2017, 10:53 PM

The [writeback] brackets means that it's part of the linux kernel. (or very oddly named).

$ egrep -r -i writeback /lib/modules/$(uname -r)/* 2>&1 | grep -i .ko

Seems to be in a lot of things. I'd guess nfs. Although I'm not sure how to test that theory without stopping the usage and unloading the module and checking if it's still in the ps output. Which you can't do if it's not a module and compiled into the kernel as implied by the [] around the process name.

$ lsmod

Should be in both lists whatever it is. Except when not a module.

$ cat /boot/config | grep -i \=y$

If it's not a module it'll be in that list (quite a long one probably).