timer_lib.rst revision 97f17497
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31.. _Timer_Library:
32
33Timer Library
34=============
35
36The Timer library provides a timer service to DPDK execution units to enable execution of callback functions asynchronously.
37Features of the library are:
38
39*   Timers can be periodic (multi-shot) or single (one-shot).
40
41*   Timers can be loaded from one core and executed on another. It has to be specified in the call to rte_timer_reset().
42
43*   Timers provide high precision (depends on the call frequency to rte_timer_manage() that checks timer expiration for the local core).
44
45*   If not required in the application, timers can be disabled at compilation time by not calling the rte_timer_manage() to increase performance.
46
47The timer library uses the rte_get_timer_cycles() function that uses the High Precision Event Timer (HPET)
48or the CPUs Time Stamp Counter (TSC) to provide a reliable time reference.
49
50This library provides an interface to add, delete and restart a timer. The API is based on BSD callout() with a few differences.
51Refer to the `callout manual <http://www.daemon-systems.org/man/callout.9.html>`_.
52
53Implementation Details
54----------------------
55
56Timers are tracked on a per-lcore basis,
57with all pending timers for a core being maintained in order of timer expiry in a skiplist data structure.
58The skiplist used has ten levels and each entry in the table appears in each level with probability ¼^level.
59This means that all entries are present in level 0, 1 in every 4 entries is present at level 1,
60one in every 16 at level 2 and so on up to level 9.
61This means that adding and removing entries from the timer list for a core can be done in log(n) time,
62up to 4^10 entries, that is, approximately 1,000,000 timers per lcore.
63
64A timer structure contains a special field called status,
65which is a union of a timer state (stopped, pending, running, config) and an owner (lcore id).
66Depending on the timer state, we know if a timer is present in a list or not:
67
68*   STOPPED: no owner, not in a list
69
70*   CONFIG: owned by a core, must not be modified by another core, maybe in a list or not, depending on previous state
71
72*   PENDING: owned by a core, present in a list
73
74*   RUNNING: owned by a core, must not be modified by another core, present in a list
75
76Resetting or stopping a timer while it is in a CONFIG or RUNNING state is not allowed.
77When modifying the state of a timer,
78a Compare And Swap instruction should be used to guarantee that the status (state+owner) is modified atomically.
79
80Inside the rte_timer_manage() function,
81the skiplist is used as a regular list by iterating along the level 0 list, which contains all timer entries,
82until an entry which has not yet expired has been encountered.
83To improve performance in the case where there are entries in the timer list but none of those timers have yet expired,
84the expiry time of the first list entry is maintained within the per-core timer list structure itself.
85On 64-bit platforms, this value can be checked without the need to take a lock on the overall structure.
86(Since expiry times are maintained as 64-bit values,
87a check on the value cannot be done on 32-bit platforms without using either a compare-and-swap (CAS) instruction or using a lock,
88so this additional check is skipped in favor of checking as normal once the lock has been taken.)
89On both 64-bit and 32-bit platforms,
90a call to rte_timer_manage() returns without taking a lock in the case where the timer list for the calling core is empty.
91
92Use Cases
93---------
94
95The timer library is used for periodic calls, such as garbage collectors, or some state machines (ARP, bridging, and so on).
96
97References
98----------
99
100*   `callout manual <http://www.daemon-systems.org/man/callout.9.html>`_
101    - The callout facility that provides timers with a mechanism to execute a function at a given time.
102
103*   `HPET <http://en.wikipedia.org/wiki/HPET>`_
104    - Information about the High Precision Event Timer (HPET).
105