On Fri, 8 Jan 2010 14:37:50 +0100 Rob van der Heij <[email protected]> wrote:
> On Fri, Jan 8, 2010 at 1:56 PM, Agblad Tore <[email protected]> wrote: > > > SLES also have the hz_timer file. > > It is zero here as well, SLES10 SP2, z10. > > and the HZ contains 100. > > Correct. The "hz_timer" is to enable the "tick timer" - the "0" > setting is the right one, since that gets us the "tickless timer" > which avoids overhead. > > When the other platforms changed from 10 ms to 1 ms granularity, s390 > did not dare because it would seriously impact installations that > still run with the "tick timer" on. With the tickless timer, it would > be helpful for granularity of the CPU metrics to have HZ raised to > 1000. But.. The increase of the number of timer ticks from 100 to 1000 does not change the output format of the different /proc entries. That would be an incompatible change that would break quite a few user space tools. > The HZ also determines the granularity of wake-up requests. Some > applications misbehave and ask to sleep for very short delays > (polling). Today (after the right kernel fixes) these get rounded up > to the next 10 ms. So even when some application is polling with 1 ms > (!) delay, the process will wake up "only" once every 10 ms. Waking up > the server every ms would be 10 times as much expensive. The 10 ms > granularity also helps to improve timer merging, again reducing the > overhead in a virtualized environment. That is true for the kernels used in the SLES10 / RHEL5 and older distributions. More recent kernel version support true high-resolution wake-up. The hrtimer system call allows to specify timeout with a granularity of nano-seconds. With the clockevents infrastructure that gets rounded to the capabilities of the currently active clock event device. For s390 this always is the clock comparator which has microsecond or better resolution. You loose the timer merging though.. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin. ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
