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<== Date ==> | <== Thread ==> |
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Subject: | RE: epicsTimer and rounding |
From: | "Hill, Jeff" <[email protected]> |
To: | "Hill, Jeff" <[email protected]>, Eric Norum <[email protected]>, Ralph Lange <[email protected]> |
Cc: | EPICS Core Talk <[email protected]>, Dirk Zimoch <[email protected]> |
Date: | Fri, 8 Jun 2012 15:18:37 +0000 |
Ø
Furthermore, I feel that no one would be (should be) calling
epicsThreadSleep if they Ø
wanted no amount of slumber. So of course on certain, but not all, OS if one requests a sleep of zero then this yields to threads
of same priority. I have also never used that feature in portable code BTW.
Nevertheless, we might decide for
epicsThreadSleep (double seconds). seconds <= 0.0 implies an OS specific sleep of zero ticks. 0.0 > seconds <= 1.5 ticks implies an OS specific sleep of one tick. 1.5 > seconds <= 2.5 ticks implies an OS specific sleep of two ticks. . . . Whatever is decided, we certainly need to document this, and keep
all of the OS specific implementations consistent. Jeff From: Hill,
Jeff Ø
Next question; If we decide that a sleep of less than one tick should be a
Ø
sleep of one tick, then perhaps Ralph’s issue is arguably caused by an issue
Ø
in the posix implementation of epicsThreadSleep instead of in epicsTimer.
Ø
Fixing only posix timer (and any other inconsistent implementations) would
Ø
presumably be an optimal solution where Ralph’s issue is solved and also
Ø
periodic timers can remain more statistically accurate (on average)? Note
Ø
that the epicsTimer class _is_ used to schedule the scan threads, ca beacons, etc. BTW, it occurred to me even yesterday (and probably before) that a good idea would
be to subtract the one half of a tick from the delay, to the next timer expiration, returned
from epicsTimer::process instead of subtracting this one half tick amount from the expiration
time stored in all of the timer objects.
This would have two beneficial impacts. o The timer would never be allowed to expire early, as verified using epicsTime::getCureent(). o We can still make adjustments to this returned delay amount to get the wakeup to occur as
close as possible to the correct time on average.
I was thinking about this alternative even yesterday, but there is a hitch. What does one do if
waking up between zero and one half of a tick too early that doesn’t consume cpu or delay timer expiration by much worse - between 1 tick <= delay <= 2 ticks. So the bottom line is that, before sending my message today and yesterday, I considered the
above alternative and rejected it, due to concerns about wasted cpu.
PS: Note that if we don’t allow a timer to expire as much as one half tick too early then we
will also be obliged to accept that a timers will expire on average one half of a tick too late. Nevertheless, I am happy to implement that alterantive if this what everyone wants. PPS: I still think that we should make all of the epicsThreadSleep OS specific implementations
consistent WRT sleeps requested in seconds less than one
half of a tick. Furthermore, I feel that no one would be (should be) calling epicsThreadSleep if they wanted no amount of slumber. PPPS: Ralph’s issue might also be resolved by deciding _not_ to subtract out the one half tick
from the timer expiration time if the requested timer delay is less than one tick. This additional
overhead would be necessary only if the epicsThreadSleep is allowed to interpret sleeps requested
less than one half of a tick as no sleep. Jeff From:
Hill, Jeff
Ø
The posix implementation of epicsThreadSleep calls nanosleep
Ø
asking for a 0 delay, and gets a 0 delay. This is certainly inconsistent with the windows implementation which (intends) to round
any floating point seconds sleep requested of less than one tick to a tick. Rounding sleeps
of between 0 and .5 seconds to no sleep definitely seems wrong to me. We should probably
clarify what behavior should be implemented by each OS specific stub in the application
developers guide, and then make all of them consistent? Ø
I've never encountered a 'sleep/pause/nap/delay' function that could return early. Next question; If we decide that a sleep of less than one tick should be a sleep of one tick, then
perhaps Ralph’s issue is arguably caused by an issue in the posix implementation of epicsThreadSleep instead of in epicsTimer. Fixing only posix timer (and any other inconsistent
implementations) would presumably be an optimal solution where Ralph’s issue is solved and
also periodic timers can remain more statistically accurate (on average)? Note that the epicsTimer
class _is_ used to schedule the scan threads, ca beacons, etc. I will certainly go with whatever the consensus ends up being. Jeff From:
[email protected] [mailto:[email protected]]
On Behalf Of Eric Norum On Jun 8, 2012, at 6:35 AM, Ralph Lange wrote:
I agree with Ralph 100% I've never encountered a 'sleep/pause/nap/delay' function that could return early. -- |