From ac2a61ab289111df0c752d22f4875623110a201e Mon Sep 17 00:00:00 2001 From: наб Date: Sun, 17 Mar 2024 16:37:52 +0100 Subject: proc_pid_io.5: Dewafflify MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This page copies verbatim the contents of Documentation/filesystems/proc.rst, added wholesale in commit f9c99463b0cd05603d125c915e2886d55a686b82 ("[PATCH] Documentation for io-accounting / reporting via procfs") in 2007. As such, it mirrors the sensibilities of the time ‒ writing "successful read returns" as "data pulled from storage. actually just the data the process gave to read(). this also means from non-regular files! whether the data was pulled from storage doesn't matter actually (obligatory cache mention)" for the modern reader this is just a lot of waffling (note also that processes give no data to read()!) ‒ and sensibilities of the sheepish implementer in kernel documentation ‒ "an attempt" for a well-defined kernel behaviour, mentioning the "current implementation", consistent mentions of specific kernel-internal caching mechanisms, "the big inaccuracy here". Re-write to be more useful and less misleading as documentation; the syscall enumeration is accurate for kernel v6.8, but the sysc? stats are also bumped by kernel_{read,write}(), which is sometimes used by too many syscalls in too many scenarios to usefully enumerate. Signed-off-by: Ahelenia Ziemiańska Signed-off-by: Alejandro Colomar --- man5/proc_pid_io.5 | 68 +++++++++++++++++++++++++----------------------------- 1 file changed, 31 insertions(+), 37 deletions(-) diff --git a/man5/proc_pid_io.5 b/man5/proc_pid_io.5 index dc75a91de..531de8c0d 100644 --- a/man5/proc_pid_io.5 +++ b/man5/proc_pid_io.5 @@ -33,63 +33,57 @@ The fields are as follows: .TP .IR rchar ": characters read" The number of bytes -which this task and its waited-for children -have caused to be read from storage. -This is simply the sum of bytes which this process passed to +returned by successful .BR read (2) and similar system calls. -It includes things such as terminal I/O and -is unaffected by whether or not actual -physical disk I/O was required (the read might have been satisfied from -pagecache). .TP .IR wchar ": characters written" The number of bytes -which this task and its waited-for children -have caused, or shall cause to be written to disk. -Similar caveats apply here as with -.IR rchar . +returned by successful +.BR write (2) +and similar system calls. .TP .IR syscr ": read syscalls" -Attempt to count the number of read I/O operations\[em]that is, -system calls such as +The number of "file read" system calls\[em]those from the .BR read (2) +family, +.BR sendfile (2), +.BR copy_file_range (2), and -.BR pread (2). +.BR ioctl (2) +.BR BTRFS_IOC_ENCODED_READ [ _32 ] +(including when invoked by the kernel as part of other syscalls). .TP .IR syscw ": write syscalls" -Attempt to count the number of write I/O operations\[em]that is, -system calls such as +The number of "file write" system calls\[em]those from the .BR write (2) +family, +.BR sendfile (2), +.BR copy_file_range (2), and -.BR pwrite (2). +.BR ioctl (2) +.BR BTRFS_IOC_ENCODED_WRITE [ _32 ] +(including when invoked by the kernel as part of other syscalls). .TP .IR read_bytes ": bytes read" -Attempt to count the number of bytes -which this process and its waited-for children -really did cause to be fetched from the storage layer. +The number of bytes really fetched from the storage layer. This is accurate for block-backed filesystems. .TP .IR write_bytes ": bytes written" -Attempt to count the number of bytes -which this process and its waited-for children -caused to be sent to the storage layer. +The number of bytes really sent to the storage layer. .TP .IR cancelled_write_bytes : -The big inaccuracy here is truncate. -If a process writes 1 MB to a file and then deletes the file, -it will in fact perform no writeout. -But it will have been accounted as having caused 1 MB of write. -In other words: -this field represents the number of bytes -which this process and its waited-for children -caused to not happen, by truncating pagecache. -A task can cause "negative" I/O too. -If this task truncates some dirty pagecache, -some I/O which another task has been accounted for -(in its -.IR write_bytes ) -will not be happening. +The above statistics fail to account for truncation: +if a process writes 1 MB to a regular file and then removes it, +said 1 MB will not be written, but +.I will +have nevertheless been accounted as a 1 MB write. +This field represents the number of bytes "saved" from I/O writeback. +This can yield to having done negative I/O +if caches dirtied by another process are truncated. +.I cancelled_write_bytes +applies to I/O already accounted-for in +.IR write_bytes . .RE .IP .IR Note : -- cgit v1.2.3