Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt

Ingo Molnar
2005-07-14T08:39:04Z

* Linus Torvalds <torvalds@osdl.org> wrote:

> I suspect that it is impractical to reprogram the PIT on a very fine
> granularity.

yes - reprogramming the PIT can take up to 10 usecs even on recent PCs.
(in fact the cost is pretty much system-independent due to PIO.) On
modern, PIT-less timesources (e.g. HPET) it can be faster.

> Btw, if somebody really gets excited about all this, let me say (once
> again) what I think might be an acceptable situation.
>
> First off, I'm _not_ a believer in "sub-HZ ticks". Quite the reverse.
> I think we should have HZ be some high value, but we would _slow_down_
> the tick when not needed, and count by 2's, 3's or even 10's when
> there's not a lot going on.

i think that would be an acceptable solution for high-precision timers,
as long as two other problems are solved too:

- there are real-time applications (robotic environments: fast rotating
tools, media and mobile/phone applications, etc.) that want 10
usecs precision. If such users increased HZ to 100,000 or even
1000,000, the current timer implementation would start to creek: e.g.
jiffies on 32-bit systems would wrap around in 11 hours or 1.1 hours.
(To solve this cleanly, pretty much the only solution seems to be to
increase the timeout to a 64 bit value. A non-issue for 64-bit
systems, that's why i think we could eventually look at this
possibility, once all the other problems are hashed out.)

- at very high HZ values the clustering of e.g. network timers is lost,
creating an artificially high number of timer interrupts. So likely
we'd still need some way to 'blur' timeouts and to round e.g. network
timers to the next 1 msec or 500 usecs boundary, to cluster up timers
for bulk processing. But in any case, such a solution does not sound
nearly as messy as the sub-jiffies method.

if the 'high precision' uses are not addressed [*] i fear the whole HRT
game starts again: embedded folks trying to standardize on Linux for
everything [**] will want HRT timers and will do addons and sub-jiffy
approaches [***], and will push for inclusion. I think we could as well
solve this whole problem area by making ridiculously high HZ values
practical too!

Ingo

[*] there's also a third problem: timer prioritization. It's not
necessarily a problem the upstream kernel should care about, but
it's a problem for things that try to offer hard-real-time, like
PREEMPT_RT: HRT timers need to be prioritizable. If e.g. the system
is soaked handling network timers, it should still be possible for
that single mega-important HRT timer to run and wake up the
mega-high-priority RT task that will preempt all network activity
within 10 usecs worst-case. The sub-jiffies approach does this
prioritization in a natural way, because there HRT timers are
separate, so the prioritization of them is easy. With the grand
unified 'big HZ' scheme the HRT folks would have to implement a
mechanism to split off highprio timers from the stream of normal
timers. ]

[**] having high precision is also a perception and uniformity of
platform issue: most embedded developers will find 500 usecs
precision good enough for most uses, but it does not 'sound' good
enough, and there's no easy way out either. So _if_ there's the
occasional need for higher precision they'll have no easy solution
for Linux, and this prevents them from standardizing on Linux for
_everything_.

[***]
a side-thought about sub-jiffies: the biggest conceptual problem
the sub-jiffy method has is the sorting needed when timers move from
the jiffy bucket into the HRT list which doesnt scale - but this is
really a HRT-timers-internal problem. It could be further improved
by e.g. dividing the last jiffy up into say 10 usec buckets - with a
1msec jiffy clock that's 100 buckets, and having a bitmap to see
which bucket is active. Then the 'get the next timer' act becomes a
matter of searching the bitmap for the next bit set - at pretty much
constant overhead. Even a 1 usec precision would mean only 1000
buckets for the last jiffy, with 1000 bits (128 bytes) to search,
still quite ok. But, in any case, it would be nice to avoid the
"conceptual dualness" of the HRT patch.