Messages in this thread | | | Date | Thu, 6 Sep 2012 13:37:42 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 13/23] rcu: Control grace-period duration from sysfs |
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On Thu, Sep 06, 2012 at 08:28:21PM +0200, Peter Zijlstra wrote: > On Thu, 2012-09-06 at 10:53 -0700, Paul E. McKenney wrote: > > > > - how do I know if my workload wants a longer or shorter forced qs > > > period? > > > > Almost everyone can do just fine with the defaults. If you have more > > than about 1,000 CPUs, you might need a longer period. > > Because the cost of starting a grace period is on the same order (or > larger) in cost as this period?
Because the overhead of rcu_gp_fqs() can then be multiple jiffies, so it doesn't make sense to run it so often. If nothing else, the rcu_gp_kthread() will start chewing up appreciable CPU time.
> > Some embedded > > systems might need a shorter period -- the only specific example I know > > of is network diagnostic equipment running wireshark, which starts up > > slowly due to grace-period length. > > But but but 3 jiffies.. however is that too long?
Because wireshark startup runs through a great many grace periods when starting up, and those 3-jiffy time periods add up.
> > > Also, whatever made you want to provide this 'feature' in the first > > > place? > > > > Complaints from the two groups called out above. > > Does this really warrant a boot time knob for which even you cannot > quite explain what values to use when?
If people look at me funny when I explain, I just tell them to leave it alone.
One alternative at the low end would be to have a sysfs variable that converted normal grace periods to expedited grace periods. Would that be preferable?
Thanx, Paul
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