Quation about HZ kernel option

[Oliver Fromme]

Craig Boston wrote:
 > Oliver Fromme wrote:
 > > In that case, I would recommend not to override the
 > > default at all (which is 1000).
 > 
 > ISTM that it would be better to use kern.hz=100 in this case.

I haven't seen a benchmark yet which would support that.

 > My reasoning is that a web server shouldn't be terribly sensitive to
 > latency, so it's better to have longer quantums to get more work done
 > without context switching overhead.

With HZ=1000 any modern CPU still performs millions of
instructions per scheduling quantum.  The context switch
overhead for the case that a process exceeds its time
slice is negligible.

 > If you're not using polling, you'll
 > be getting interrupts for network traffic anyway.

Right.  Probably many more of them than the scheduler
switches between processes.

 > With polling on however, a high HZ value makes sense.

Yes, I've seen servers running at HZ=5000 and even more.

 > > Basically, the kernel cannot handle time slices smaller
 > > than 1/HZ seconds, for any purpose.
 > 
 > It should still be able to schedule a new process for the remainder of
 > the slice should the current one block or yield though, right?

Right.  And in that case a context switch happens anyway,
so reducing the HZ value doesn't buy you anything, except
reducing latency for those processes that can do their
job quickly (e.g. serving static inline images).

I still believe that it's best to not modify the default
value of HZ=1000 on such a server.

Best regards
   Oliver

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