dear kernel, please let me do my work …

April 22, 2012 — Leave a comment

until now we had an introduction to processes, how they are managed, what signals are and what they are used for, how the linux kernel ( and oracle ) uses double linked list to quickly look up memory structures and how critical regions like shared memory can be protected. this post gives an introduction to timing and process scheduling.

as the cpu can execute only one process at a time but because maybe hundreds or thousands of processes want to do their work the kernel must provide a mechanism to decide which process to run next ( process switching ). this is the task of the scheduler. for being able to do what it does, the scheduler must be able to make decisions, and the decisions are based on time and priorities.

lots and lots of work behind the scenes is driven by time measurements. consider cronjobs, for example. without being able to measure time they would not work. in short the kernel must be able to keep the current time and to provide a mechanism to notify programs when a specific interval has elapsed.

on the one hand there is the real time clock ( accessible through the /dev/rtc interface ) which is a special chip that continues to tick even if the computer is powered off ( there is a small battery for this chip ). the real time clock is used by linux to derive the date and time.

on the other hand there are several other mechanisms which can be used for timing:

one of the time related activities the kernel must perform is to determine how long a process has been running. each process is given a time slot in which it may run, which is called a quanta. if the quantum expires and the process did not terminate a process switch may occur ( another process is selected for execution ). these processes are called expired. active processes are those which did not yet consume their quantum.
additionally each process has a priority assigned, which is used by the scheduler to decide how appropriate it is to let the process do its work on the cpu.

in general processes can be divided in three classes:

  • interactive: typical interactive processes are those which respond to keyboard and mouse inputs of an end user. as an user wants to see quick responses, for example when editing text, these processes must be woken up quickly
  • batch: batch processes do not interact with the user and often run in the background.
  • real-time: real-time processes have very strong scheduling requirements and should not be blocked by processes with lower priorities.

in general the scheduler will give more attention to interactive processes than to batch processes, although this must not always be true.

one way we can change the base priority of processes from the command line is by using the “nice” command:

nice -19 vi

if you check the process without the nice call:

ps -aux | grep vi
oracle 4185 0.5 0.0 5400 1504 pts/0 S+ 10:51 0:00 vi

… and compare it to when you call vi with a nice value:

ps -aux | grep vi
oracle 4194 1.6 0.0 5400 1496 pts/0 SN+ 10:52 0:00 vi

.. you will see that “S+” changes to “SN+” ( the “N” stands for “low-priority (nice to other users)”

processes in linux are preemptable, which means that higher priority processes may suspend lower priority processes when they enter the running state. another reason a process can be preempted is when its time quantum expires.

consider this example: a user is writing an email while copying music from a cd to her computer. the email client is considered an interactive program while the copy job is considered a batch program. each time the user presses a key on her keyboard an interrupt occurs and the scheduler selects the email program for execution. but because users tend to think when writing emails there is plenty of time ( regarding the cpu ) between the key presses to wake up the copy job and let it do its work.

the time a process is allowed to be on a cpu, the quantum, is derived from a so called “static priority” which can be in the range of 100 to 139 ( with 100 being the highest priority and 139 being the lowest ). the higher the priority the more time the process is granted ( which ranges from 800ms for the highest priority to 5ms for the lowest priority ). in addition to the static priority there is a “dynamic priority” for each process ( again ranging from 100 to 139 ). without going too much into detail again: the dynamic priority is the one the scheduler uses for its decisions. as the name suggest, this priority may change over time ( depending on the average sleep time of a process ). processes with longer sleep times usually get a bonus ( the priority will be increased ) while processes with lower sleep times will get a penalty ( the priority will be decreased ). the average sleep time is also used by the scheduler to decide if processes are interactive or batch.

recall the post about double linked lists. the most important data structure used by the scheduler is the runqueue, which in fact is another linked list. this list links together all the process descriptors of the processes which want to run ( there is one runqueue per cpu ). one process can be in one runqueue only, but processes may migrate to others runqueues if the load between the cpus becomes unbalanced.

what to keep in mind: as only one process can run on one cpu at a time the scheduler decides which process to run next and which processes to suspend in case higher priority processes enter the running state. in general interactive processes are favored over batch processes and real-time processes should not be blocked by lower priority processes.

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