Am Mittwoch, 19. Dezember 2007 schrieb Robert Hancock:
> Karsten Wiese wrote:
> > Am Mittwoch, 19. Dezember 2007 schrieb Robert Hancock:
> >> That seems fishy. What is your value of HZ and what is the timeout value
> >> that was passed in the bad case?
> >
> > HZ set to 250, timeout to 4ms.
> > Time spent in poll() taken by clock_gettime(CLOCK_MONOTONIC, &time)
> > before and after poll()call: i.e 62us.
> > Time measured with hpet gave 166us once.
>
> msecs_to_jiffies (kernel/time.c) has this:
>
> #if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ)
> /*
> * HZ is equal to or smaller than 1000, and 1000 is a nice
> * round multiple of HZ, divide with the factor between them,
> * but round upwards:
> */
> return (m + (MSEC_PER_SEC / HZ) - 1) / (MSEC_PER_SEC / HZ);
>
> With HZ=250 and m=4 this gives 7/4 or only 1 jiffy, which is not more
> than 4ms, but if we are already at near the end of the current jiffy it
> could be much less than that (potentially almost no time at all).
>
> Maybe we could convert poll to use a hrtimer for this instead?
That wouldn't fix configs without hrtimers.
The linux manpage reflects current behaviour:
from man2/poll.2.gz
"The timeout argument specifies an upper limit on the time for which
poll() will block, in milliseconds."
To achieve "at least" timeouts we'd have to add a jiffy's ms to the
timeout in userspace.
I'd like to let sys_poll() behave according to posix's manpage:
from man3p/poll.3p.gz
"poll() shall wait at least timeout milliseconds"
Thats easier to specify in userspace. No need to know the running kernel's
HZ.
Above posix/linux difference also shows in select() manpages.
epoll_wait() (linux only) manpage also says "maximum time of timeout"
A more complete patch would tweek (p)poll (p)select and epoll_(p)wait.
There are possibly more syscalls affected that I'm not aware off.
Is there upstream interest?
Karsten
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