FreeBSD/Linux Kernel Cross Reference
sys/Documentation/hrtimers.txt

     
     hrtimers - subsystem for high-resolution kernel timers
     ----------------------------------------------------
     
     This patch introduces a new subsystem for high-resolution kernel timers.
     
     One might ask the question: we already have a timer subsystem
     (kernel/timers.c), why do we need two timer subsystems? After a lot of
     back and forth trying to integrate high-resolution and high-precision
    features into the existing timer framework, and after testing various
    such high-resolution timer implementations in practice, we came to the
    conclusion that the timer wheel code is fundamentally not suitable for
    such an approach. We initially didn't believe this ('there must be a way
    to solve this'), and spent a considerable effort trying to integrate
    things into the timer wheel, but we failed. In hindsight, there are
    several reasons why such integration is hard/impossible:
    
    - the forced handling of low-resolution and high-resolution timers in
      the same way leads to a lot of compromises, macro magic and #ifdef
      mess. The timers.c code is very "tightly coded" around jiffies and
      32-bitness assumptions, and has been honed and micro-optimized for a
      relatively narrow use case (jiffies in a relatively narrow HZ range)
      for many years - and thus even small extensions to it easily break
      the wheel concept, leading to even worse compromises. The timer wheel
      code is very good and tight code, there's zero problems with it in its
      current usage - but it is simply not suitable to be extended for
      high-res timers.
    
    - the unpredictable [O(N)] overhead of cascading leads to delays which
      necessitate a more complex handling of high resolution timers, which
      in turn decreases robustness. Such a design still led to rather large
      timing inaccuracies. Cascading is a fundamental property of the timer
      wheel concept, it cannot be 'designed out' without unevitably
      degrading other portions of the timers.c code in an unacceptable way.
    
    - the implementation of the current posix-timer subsystem on top of
      the timer wheel has already introduced a quite complex handling of
      the required readjusting of absolute CLOCK_REALTIME timers at
      settimeofday or NTP time - further underlying our experience by
      example: that the timer wheel data structure is too rigid for high-res
      timers.
    
    - the timer wheel code is most optimal for use cases which can be
      identified as "timeouts". Such timeouts are usually set up to cover
      error conditions in various I/O paths, such as networking and block
      I/O. The vast majority of those timers never expire and are rarely
      recascaded because the expected correct event arrives in time so they
      can be removed from the timer wheel before any further processing of
      them becomes necessary. Thus the users of these timeouts can accept
      the granularity and precision tradeoffs of the timer wheel, and
      largely expect the timer subsystem to have near-zero overhead.
      Accurate timing for them is not a core purpose - in fact most of the
      timeout values used are ad-hoc. For them it is at most a necessary
      evil to guarantee the processing of actual timeout completions
      (because most of the timeouts are deleted before completion), which
      should thus be as cheap and unintrusive as possible.
    
    The primary users of precision timers are user-space applications that
    utilize nanosleep, posix-timers and itimer interfaces. Also, in-kernel
    users like drivers and subsystems which require precise timed events
    (e.g. multimedia) can benefit from the availability of a separate
    high-resolution timer subsystem as well.
    
    While this subsystem does not offer high-resolution clock sources just
    yet, the hrtimer subsystem can be easily extended with high-resolution
    clock capabilities, and patches for that exist and are maturing quickly.
    The increasing demand for realtime and multimedia applications along
    with other potential users for precise timers gives another reason to
    separate the "timeout" and "precise timer" subsystems.
    
    Another potential benefit is that such a separation allows even more
    special-purpose optimization of the existing timer wheel for the low
    resolution and low precision use cases - once the precision-sensitive
    APIs are separated from the timer wheel and are migrated over to
    hrtimers. E.g. we could decrease the frequency of the timeout subsystem
    from 250 Hz to 100 HZ (or even smaller).
    
    hrtimer subsystem implementation details
    ----------------------------------------
    
    the basic design considerations were:
    
    - simplicity
    
    - data structure not bound to jiffies or any other granularity. All the
      kernel logic works at 64-bit nanoseconds resolution - no compromises.
    
    - simplification of existing, timing related kernel code
    
    another basic requirement was the immediate enqueueing and ordering of
    timers at activation time. After looking at several possible solutions
    such as radix trees and hashes, we chose the red black tree as the basic
    data structure. Rbtrees are available as a library in the kernel and are
    used in various performance-critical areas of e.g. memory management and
    file systems. The rbtree is solely used for time sorted ordering, while
    a separate list is used to give the expiry code fast access to the
    queued timers, without having to walk the rbtree.
    
    (This separate list is also useful for later when we'll introduce
   high-resolution clocks, where we need separate pending and expired
   queues while keeping the time-order intact.)
   
   Time-ordered enqueueing is not purely for the purposes of
   high-resolution clocks though, it also simplifies the handling of
   absolute timers based on a low-resolution CLOCK_REALTIME. The existing
   implementation needed to keep an extra list of all armed absolute
   CLOCK_REALTIME timers along with complex locking. In case of
   settimeofday and NTP, all the timers (!) had to be dequeued, the
   time-changing code had to fix them up one by one, and all of them had to
   be enqueued again. The time-ordered enqueueing and the storage of the
   expiry time in absolute time units removes all this complex and poorly
   scaling code from the posix-timer implementation - the clock can simply
   be set without having to touch the rbtree. This also makes the handling
   of posix-timers simpler in general.
   
   The locking and per-CPU behavior of hrtimers was mostly taken from the
   existing timer wheel code, as it is mature and well suited. Sharing code
   was not really a win, due to the different data structures. Also, the
   hrtimer functions now have clearer behavior and clearer names - such as
   hrtimer_try_to_cancel() and hrtimer_cancel() [which are roughly
   equivalent to del_timer() and del_timer_sync()] - so there's no direct
   1:1 mapping between them on the algorithmical level, and thus no real
   potential for code sharing either.
   
   Basic data types: every time value, absolute or relative, is in a
   special nanosecond-resolution type: ktime_t. The kernel-internal
   representation of ktime_t values and operations is implemented via
   macros and inline functions, and can be switched between a "hybrid
   union" type and a plain "scalar" 64bit nanoseconds representation (at
   compile time). The hybrid union type optimizes time conversions on 32bit
   CPUs. This build-time-selectable ktime_t storage format was implemented
   to avoid the performance impact of 64-bit multiplications and divisions
   on 32bit CPUs. Such operations are frequently necessary to convert
   between the storage formats provided by kernel and userspace interfaces
   and the internal time format. (See include/linux/ktime.h for further
   details.)
   
   hrtimers - rounding of timer values
   -----------------------------------
   
   the hrtimer code will round timer events to lower-resolution clocks
   because it has to. Otherwise it will do no artificial rounding at all.
   
   one question is, what resolution value should be returned to the user by
   the clock_getres() interface. This will return whatever real resolution
   a given clock has - be it low-res, high-res, or artificially-low-res.
   
   hrtimers - testing and verification
   ----------------------------------
   
   We used the high-resolution clock subsystem ontop of hrtimers to verify
   the hrtimer implementation details in praxis, and we also ran the posix
   timer tests in order to ensure specification compliance. We also ran
   tests on low-resolution clocks.
   
   The hrtimer patch converts the following kernel functionality to use
   hrtimers:
   
    - nanosleep
    - itimers
    - posix-timers
   
   The conversion of nanosleep and posix-timers enabled the unification of
   nanosleep and clock_nanosleep.
   
   The code was successfully compiled for the following platforms:
   
    i386, x86_64, ARM, PPC, PPC64, IA64
   
   The code was run-tested on the following platforms:
   
    i386(UP/SMP), x86_64(UP/SMP), ARM, PPC
   
   hrtimers were also integrated into the -rt tree, along with a
   hrtimers-based high-resolution clock implementation, so the hrtimers
   code got a healthy amount of testing and use in practice.
   
           Thomas Gleixner, Ingo Molnar

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