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- Subject: Updating callout_reset
- From: silby at silby.com (Mike Silbersack)
- Date: Sun Jan 25 21:29:49 2004
Based on the work I've been doing with the rtc driver and some issues in
the tcp stack with high hz values, I'd like to propose a change in the
callout_reset interface. Currently, the timeout value is specified in
ticks. This creates two problems:
#1 - At high hz rates, the maximum timeout possible is reduced; we have
at least one example of this causing problems in the kernel at present
(16-bit sbtimeout values getting reduced from 327 to 32 seconds), there
could be others we haven't caught. Many of these are likely to result
from integer math errors introduced by authors who hadn't considered
larger hz values occuring.
#2 - Using ticks reduces the potential accuracy of wakeups with our
default hz setting. For example, if one wishes to sleep for 5ms when
hz=100, the only choices is to request one tick, or 10ms. However, if 5ms
could be specified, the callout subsystem would be able to schedule more
precise wakeups in places where the next timer interval was between 5 and
Case #1 also provides a second justification for a change; any piece of
code which requests a timeout must be aware of the system hz; moving to a
standard time format would create more straightforward code.
In order to ensure that modules and the like do not break, I would like to
propose that we leave callout_reset defined as is and create a new
function, callout_set (or some other clever name) which takes a struct
*timespec in place of ticks.
Optimizations as suggested in example #2 above would not be implemented
right away, but would be possible due to the better interface.
Mike "Silby" Silbersack
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