Messages in this thread | | | Date | Thu, 4 Sep 2008 06:25:42 +0200 | From | Willy Tarreau <> | Subject | Re: [PATCH] Fix TSC calibration issues |
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On Wed, Sep 03, 2008 at 08:59:05PM -0700, Linus Torvalds wrote: > > > On Wed, 3 Sep 2008, Arjan van de Ven wrote: > > > > but at some point, even doing things in parallel/asynchronous isn't > > helping, "parallel shit is still shit" :) > > Well, the thing is, you can't call ti "shit" when the fact is that we > don't have any other options than to wait. > > The only frequency we can trust on 99% of all machines is the PIT, and > it's a very uncomfortable programming model due to all the history (it is > one of the few truly 8-bit things left in a modern PC). The other options > are just not reliably there, or are known to not have a stable frequency. > > So how would you suggest we do it? Lowering the wait to 5ms (times 5, so > it's really 25ms, although we can probably stop early if the first > iterations are very consistent) will work, but it _will_ reduce precision. > And it's still real time. > > But we simply don't have alternatives. That 'shit' is originally from the > company you work for, btw, and while it was good for its time, the > replacement (HPET) was horribly misdesigned by the same company, and is > deficient in many ways (not the least of which is the idiotic enumeration: > another ACPI braindamage), and it often isn't even exposed. > > As a result, the PIT remains to this day the most reliable source of a > reference timer. That includes even on really modern machines (ie the one > I have from Intel that contains hardware not even released yet!).
15 years ago when I only knew DOS, I used the PIT a lot for precise delay calculations. I can attest that it can be a very precise timer for delays when you run busy loops. You even need very few ticks because you detect the falling edge with a high accuracy. Basically, I would do this :
pit1 = readpit(); while (readpit() == pit1); t1 = rdtsc(); // precise beginning of tick 0 while (readpit() != pit1 - 5000); t2 = rdtsc(); // precise beginning of tick 5000
(t2 - t1) will be exactly 5000 PIT ticks long, or 4.1904767 ms.
Additional sanity checks are needed of course, such as rollover detection, and a max loop counter in case we boot on a machine with a broken PIT.
If someone wants to test this, I'd be interested in the number of ticks required to get a good accuracy, I bet that even with a few hundred ones it's already precise by a few ppm (about the precision of the input clock in fact).
Willy
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