Jesse Barnes wrote:
On Thursday, May 12, 2005 2:16 pm, George Anzinger wrote:
The tickless system differs from VST in that it is designed to only
"tick" when there is something in the time list to do and it does
this ALWAYS.
Right, that's what I gathered (and what I was getting at).
And this is exactly where the tickless system runs into overload.
Simply speaking, each task has with it certain limits and
requirements WRT time. It is almost always time sliced, but it may
also have execution limits and settimer execution time interrupts
that it wants.
Right...
These need to be set up for each task when it is
switched to and removed when the system switches away from it.
Why do they need to be switched when the task's slice is up?
No, that would not be too bad, but they need to be removed when the task is
switched away from. This happens FAR more often than a slice expiring. Most
tasks are switched away from because they block, not because their time is over.
Can't they
remain in the timer list?
We are timeing the tasks slice. It is only active when the task has the cpu...
I guess I'm imagining a global outstanding
timer list that's manipulated by add/remove_timer, settimer, etc..
When a timeout occurs the timer interrupt handler could mark tasks
runnable again, bump their priority, send SIGALRM, or whatever. Most
The timers that cause the problem are the ones that only run when the task is
active. These are the slice timer, the profile timer (ITIMER_PROF), the
execution limit timer and the settime timer that is relative to execution time
(ITIMER_VIRTUAL).
Again, we can colapse all these to one, but still it needs to be setup when the
task is switched to and removed when it is switched away.
timers are deleted before they expire anyway, right? If that's true
you definitely don't want to save and restore them on every context
switch...
One of these timers is almost ALWAYS needed. And yes, mostly they do not
expire. That is usually good as an expire cost more than a removal (provided
you do not need to reprogram the timer as a result).
Timers that run on system time (ITIMER_REAL) stay in the list even when the task
is inactive.
In
the test I did, I reduced all these timers to one (I think I just
used the slice time, but this is not the thing to do if the user is
trying to do profiling. In any case, only one timer needs to be set
up, possibly some work needs to be done to find the min. of all the
execution time timers it has, but only that one needs to go in the
time list.).
Or at least only the nearest one has to be programmed as the next
interrupt, and before going back to sleep the kernel could check if any
timers had expired while the last one was running (aren't missing
jiffies accounted for this way too, but of course jiffies would go away
in this scheme)?
BUT it needs to happen each context switch time and
thus adds overhead to the switch time. In my testing, the overhead
for this became higher than the ticked system overhead for the same
services at a relatively low context switch rate.
Yeah, that does sound expensive.
From a systems
point of view, you just don't want to increase overhead when the load
goes up. This leads to fragile systems.
Now, the question still remains, if a cpu in an SMP system is
sleeping because of VST, who or how is it to be wakened to responded
to increases in the system load. If all CPUs are sleeping, there is
some event (i.e. interrupt) that does this. I think, in an SMP
system, any awake CPUs should, during their load balancing, notice
that there are sleeping CPUs and wake them as the load increases.
Sounds reasonable to me, should be as simple as a 'wake up and load
balance' IPI, right?
I think there is already an IPI to tell another cpu that it has work. That
should be enough. Need to check, however. From the VST point of view, any
interrupt wake the cpu from the VST sleep, so it need not even target the
scheduler..
--
George Anzinger [email protected]
High-res-timers: http://sourceforge.net/projects/high-res-timers/
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