Re: [PATCH] Consolidate multiple implementations of jiffies-msecs conversions.

From: Bill Rugolsky Jr.
Date: Thu Mar 25 2004 - 16:07:01 EST

On Thu, Mar 25, 2004 at 12:17:41PM -0800, Sridhar Samudrala wrote:
> The following patch to 2.6.5-rc2 consolidates 6 different implementations
> of msecs to jiffies and 3 different implementation of jiffies to msecs.
> All of them now use the generic msecs_to_jiffies() and jiffies_to_msecs()
> that are added to include/linux/time.h

I was inplementing precisely the same diff while cleaning up the
select/poll timeout logic, when I came across this lkml post by George

the relevant part of which is:

As to small drifts of ~170 PPM, they are caused by code (ps I would
that assumes that jiffies is exactly 1/HZ whereas it is NOT in the 2.6.*
kernel. The size of the jiffie that the kernel uses is returned by:

struct timespec tv;
clock_res(CLOCK_REALTIME, &tv);

I inferred from the above that a generic msec_to_jiffies()/jiffies_to_msec()
ought to use TICK_NSEC, as with the other routines in time.h. For 32-bit
platforms the scaled arithmetic is simple; one has to be more careful
when BITS_PER_LONG == 64.

After e-mailing George about it, he replied:

You might want to look at the code in time.h that does the jiffies to
timespec conversion. We did a lot of work to get that right for both 32 and
64 bit platforms. I don't think you really need more precision than we get
on the 32 bit platforms (if I recall correctly it is in the 100 PPB range,
yeah that is parts per BILLION).

Unless this conversion is done a lot, I would just start with the timespec
conversions and convert from / to using simple math. As it is all power of
10 stuff it should not have a precision problem.

He also supplied me with his test harness.

I haven't gotten around to doing this properly yet. It seems that the
only place where precision is actually important (due to the possibility
of very long timeouts) is in poll/epoll, so perhaps it is best
to just code up a special version for them, as the simple version
becomes a no-op everywhere else for the default HZ==1000.


Bill Rugolsky
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