Messages in this thread | | | Subject | Re: [PATCH 2/3] High-res-timers part 2 (x86 platform code) take 5.1 | From | (Eric W. Biederman) | Date | 10 Oct 2002 09:03:06 -0600 |
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george anzinger <george@mvista.com> writes:
> Linus Torvalds wrote: > > > > On Wed, 9 Oct 2002, george anzinger wrote: > > > > > > This patch, in conjunction with the "core" high-res-timers > > > patch implements high resolution timers on the i386 > > > platforms. > > > > I really don't get the notion of partial ticks, and quite frankly, this > > isn't going into my tree until some major distribution kicks me in the > > head and explains to me why the hell we have partial ticks instead of just > > making the ticks shorter. > > > Well, the notion is to provide timers that have resolution > down into the micro seconds. Since this take a bit more > overhead, we just set up an interrupt on an as needed > basis. This is why we define both a high res and a low res > clock. Timers on the low res clock will always use the 1/HZ > tick to drive them and thus do not introduce any additional > overhead. If this is all that is needed the configure > option can be left off and only these timers will be > available. > > On the other hand, if a user requires better resolution, > s/he just turns on the high-res option and incures the > overhead only when it is used and then only at timer expire > time. Note that the only way to access a high-res timer is > via the POSIX clocks and timers API. They are not available > to select or any other system call. > > Making ticks shorter causes extra overhead ALL the time, > even when it is not needed. Higher resolution is not free > in any case, but it is much closer to free with this patch > than by increasing HZ (which, of course, can still be > done). Overhead wise and resolution wise, for timers, we > would be better off with a 1/HZ tick and the "on demand" > high-res interrupts this patch introduces.
??? The issue of ticks is separate from the issue of how often timer interrupts fire. Ticks just becomes the maximum resolution you can support/express.
If it makes sense to have two maximum tick resolutions. The normal application maximum tick rate and the special task maximum tick rate it is probably worth making this only available as a capability or an rlimit.
Eric
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