Re: Caches in embedded systems

On 3 May 2006 18:04:17 -0700, "shrey" <shreyas76@xxxxxxxxx> wrote:

I know caches are avoided in real time applications

Unless the cache is very badly implemented, the worst case timing
occurs when the cache is disabled.

Thus, it is sufficient to verify that all high priority tasks with
some definite deadlines are executed within the deadlines even when
the cache is disabled.

A system usually also contains tasks that are not time critical (such
as user interfaces or calculating weekly statistics), which are
executed, when _no_ high priority task is using the processor. Thus,
the low priority tasks can progress quite slowly, if there is much
high priority activity.

Enabling the cache reduces _on_average_ the execution time of the high
priority tasks, freeing up some additional time for the low priority
tasks, which will progress more rapidly and ultimately allows the null
task to run at times when there is no useful work to be done.

The net effect of enabling the cache is that the high priority tasks
still complete within the deadlines, but the visible result is that
the low priority background tasks executes faster.

Enabling the cache will also reduce the number of main memory
accesses, thus reducing the memory and memory driver power
consumption. This is usually true even if the low priority tasks can
now perform more work, since the cache hit rate is usually quite high.

Paul

.

Relevant Pages

  • Re: Scheduler: Process priority fed back to parent?
    ... Mac OS X has a special cache ... on disk of things that get loaded on boot. ... >>initial priority is a guess, and isn't set until the priority info has ... This prefetch activity could be turned on/off ...
    (Linux-Kernel)
  • Re: Scheduler: Process priority fed back to parent?
    ... There would have to be some sort of cache. ... > The kernel already does disk access to load a process... ... > would be an artificial one which always exists for this, or the priority ...
    (Linux-Kernel)
  • Re: [PATCH v3 17/17] (Avoid overload)
    ... to try to pick another runqueue if the currently running task ... Preempting a normal task to run a RT task to keep ... priority task preempt a lower priority RT task, ... hot cache with a completely cold one. ...
    (Linux-Kernel)
  • [PATCH v4 16/20] Avoid overload
    ... to try to pick another runqueue if the currently running task ... Preempting a normal task to run a RT task to keep ... priority task preempt a lower priority RT task, ... hot cache with a completely cold one. ...
    (Linux-Kernel)
  • [PATCH 16/23] Subject: SCHED - Avoid overload
    ... to try to pick another runqueue if the currently running task ... Preempting a normal task to run a RT task to keep ... priority task preempt a lower priority RT task, ... hot cache with a completely cold one. ...
    (Linux-Kernel)