Comprendre les compteurs dans/proc/[pid]/io

Question

Je crée un plugin pour Munin pour surveiller les statistiques des processus nommés. L'une des sources d'information serait /proc/[pid]/io. Mais j'ai du mal à trouver quelle est la différence entre rchar/wchar et read_bytes/written_bytes.

Ils ne sont pas identiques car ils fournissent des valeurs différentes. Que représentent-ils?

Answer 1

Alors que le proc manpage est lamentablement derrière (et sont donc la plupart des pages de manuel/documentation sur quoi que ce soit non liées au développement de l'espace utilisateur emporte-pièce), ce genre de choses est heureusement documenté complètement dans le Linux kernel source sous Documentation/filesystems/proc.txt. Voici les bits pertinents:

rchar 
----- 

I/O counter: chars read 
The number of bytes which this task has caused to be read from storage. This 
is simply the sum of bytes which this process passed to read() and pread(). 
It includes things like tty IO and it is unaffected by whether or not actual 
physical disk IO was required (the read might have been satisfied from 
pagecache) 


wchar 
----- 

I/O counter: chars written 
The number of bytes which this task has caused, or shall cause to be written 
to disk. Similar caveats apply here as with rchar. 


read_bytes 
---------- 

I/O counter: bytes read 
Attempt to count the number of bytes which this process really did cause to 
be fetched from the storage layer. Done at the submit_bio() level, so it is 
accurate for block-backed filesystems. <please add status regarding NFS and 
CIFS at a later time> 


write_bytes 
----------- 

I/O counter: bytes written 
Attempt to count the number of bytes which this process caused to be sent to 
the storage layer. This is done at page-dirtying time.