Linux Process States

Question

In Linux  what happens to the state of a process when it needs to read blocks from a disk  Is it blocked  If so  how is another process chosen to execute

User · Accepted Answer

While waiting for read   or write   to from a file descriptor return  the process will be put in a special kind of sleep  known as  D  or  Disk Sleep   This is special  because the process can not be killed or interrupted while in such a state  A process waiting for a return from ioctl   would also be put to sleep in this manner   An exception to this is when a file  such as a terminal or other character device  is opened in O NONBLOCK mode  passed when its assumed that a device  such as a modem  will need time to initialize  However  you indicated block devices in your question  Also  I have never tried an ioctl   that is likely to block on a fd opened in non blocking mode  at least not knowingly    How another process is chosen depends entirely on the scheduler you are using  as well as what other processes might have done to modify their weights within that scheduler   Some user space programs under certain circumstances have been known to remain in this state forever  until rebooted  These are typically grouped in with other  zombies   but the term would not be correct as they are not technically defunct

User · Answer

Yes  the task gets blocked in the read   system call  Another task which is ready runs  or if no other tasks are ready  the idle task  for that CPU  runs   A normal  blocking disc read causes the task to enter the  D  state  as others have noted   Such tasks contribute to the load average  even though they re not consuming the CPU   Some other types of IO  especially ttys and network  do not behave quite the same - the process ends up in  S  state and can be interrupted and doesn t count against the load average

User · Answer

A process performing I O will be put in D state  uninterruptable sleep   which frees the CPU until there is a hardware interrupt which tells the CPU to return to executing the program  See man ps for the other process states   Depending on your kernel  there is a process scheduler  which keeps track of a runqueue of processes ready to execute  It  along with a scheduling algorithm  tells the kernel which process to assign to which CPU  There are kernel processes and user processes to consider  Each process is allocated a time-slice  which is a chunk of CPU time it is allowed to use  Once the process uses all of its time-slice  it is marked as expired and  given lower priority in the scheduling algorithm   In the 2 6 kernel  there is a O 1  time complexity scheduler  so no matter how many processes you have up running  it will assign CPUs in constant time  It is more complicated though  since 2 6 introduced preemption and CPU load balancing is not an easy algorithm  In any case  it   s efficient and CPUs will not remain idle while you wait for the I O

User · Answer

When a process needs to fetch data from a disk  it effectively stops running on the CPU to let other processes run because the operation might take a long time to complete     at least 5ms seek time for a disk is common  and 5ms is 10 million CPU cycles  an eternity from the point of view of the program   From the programmer point of view  also said  in userspace    this is called a blocking system call  If you call write 2   which is a thin libc wrapper around the system call of the same name   your process does not exactly stop at that boundary  it continues  in the kernel  running the system call code  Most of the time it goes all the way up to a specific disk controller driver  filename   filesystem VFS   block device   device driver   where a command to fetch a block on disk is submitted to the proper hardware  which is a very fast operation most of the time   THEN the process is put in sleep state  in kernel space  blocking is called sleeping  ndash  nothing is ever  blocked  from the kernel point of view   It will be awakened once the hardware has finally fetched the proper data  then the process will be marked as runnable and will be scheduled  Eventually  the scheduler will run the process   Finally  in userspace  the blocking system call returns with proper status and data  and the program flow goes on   It is possible to invoke most I O system calls in non-blocking mode  see O NONBLOCK in open 2  and fcntl 2    In this case  the system calls return immediately and only report submitting the disk operation  The programmer will have to explicitly check at a later time whether the operation completed  successfully or not  and fetch its result  e g   with select 2    This is called asynchronous or event-based programming   Most answers here mentioning the D state  which is called TASK UNINTERRUPTIBLE in the Linux state names  are incorrect  The D state is a special sleep mode which is only triggered in a kernel space code path  when that code path can t be interrupted  because it would be too complex to program   with the expectation that it would block only for a very short time  I believe that most  D states  are actually invisible  they are very short lived and can t be observed by sampling tools such as  top    You can encounter unkillable processes in the D state in a few situations  NFS is famous for that  and I ve encountered it many times  I think there s a semantic clash between some VFS code paths  which assume to always reach local disks and fast error detection  on SATA  an error timeout would be around a few 100 ms   and NFS  which actually fetches data from the network which is more resilient and has slow recovery  a TCP timeout of 300 seconds is common   Read this article for the cool solution introduced in Linux 2 6 25 with the TASK KILLABLE state  Before this era there was a hack where you could actually send signals to NFS process clients by sending a SIGKILL to the kernel thread rpciod  but forget about that ugly trick  hellip

User · Answer

Yes  tasks waiting for IO are blocked  and other tasks get executed  Selecting the next task is done by the Linux scheduler

User · Answer

As already explained by others  processes in  D  state  uninterruptible sleep  are responsible for the hang of ps process  To me it has happened many times with RedHat 6 x and automounted NFS home directories   To list processes in D state you can use the following commands   cd  proc for i in  0-9   do echo -n   i    cat  i status  grep  State done grep D   To know the current directory of the process and  may be  the mounted NFS disk that has issues you can use a command similar to the following example  replace 31134 with the sleeping process number      ls -l  proc 31134 cwd lrwxrwxrwx 1 pippo users 0 Aug  2 16 25  proc 31134 cwd - gt   auto pippo   I found that giving the umount command with the -f  force  switch  to the related mounted nfs file system  was able to wake-up the sleeping process   umount -f  auto pippo   the file system wasn t unmounted  because it was busy  but the related process did wake-up and I was able to solve the issue without rebooting

User · Answer

Assuming your process is a single thread  and that you re using blocking I O  your process will block waiting for the I O to complete  The kernel will pick another process to run in the meantime based on niceness  priority  last run time  etc  If there are no other runnable processes  the kernel won t run any  instead  it ll tell the hardware the machine is idle  which will result in lower power consumption    Processes that are waiting for I O to complete typically show up in state D in  e g   ps and top

User · Answer

Generally the process will block   If the read operation is on a file descriptor marked as non-blocking or if the process is using asynchronous IO it won t block   Also if the process has other threads that aren t blocked they can continue running   The decision as to which process runs next is up to the scheduler in the kernel

[linux] Linux Process States

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