On Tue, 6 Mar 2018, Richard Cochran wrote:
> On Tue, Mar 06, 2018 at 06:53:29PM -0800, Eric Dumazet wrote:
> > This is adding 32+1 bits to sk_buff, and possibly holes in this very
> > very hot (and already too fat) structure.
> >
> > Do we really need 32 bits for a clockid_t ?
>
> Probably we can live with fewer bits.
>
> For clock IDs with a positive sign, the max possible clock value is 16.
>
> For clock IDs with a negative sign, IIRC, three bits are for the type
> code (we have also posix timers packed like this) and the are for the
> file descriptor. So maybe we could use 16 bits, allowing 12 bits or
> so for encoding the FD.
>
> The downside would be that this forces the application to make sure
> and open the dynamic posix clock early enough before the FD count gets
> too high.
Errm. No. There is no way to support fd based clocks or one of the CPU
time/process time based clocks for this.
CLOCK_REALTIME and CLOCK_MONOTONIC are probably the only interesting
ones. BOOTTIME is hopefully soon irrelevant as we make MONOTONIC and
BOOTTIME the same unless this causes unexpectedly a major issues. I don't
think that CLOCK_TAI makes sense in that context, but I might be wrong.
The rest of the CLOCK_* space cannot be used at all.
So you need at max 2 bits for this, but I think 1 is good enough.
Thanks,
tglx
