Messages in this thread | | | Subject | Re: [RFC] sched: Limit idle_balance() when it is being used too frequently | From | Jason Low <> | Date | Wed, 17 Jul 2013 01:11:41 -0700 |
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On Wed, 2013-07-17 at 09:25 +0200, Peter Zijlstra wrote: > On Tue, Jul 16, 2013 at 03:48:01PM -0700, Jason Low wrote: > > On Tue, 2013-07-16 at 22:20 +0200, Peter Zijlstra wrote: > > > On Tue, Jul 16, 2013 at 12:21:03PM -0700, Jason Low wrote: > > > > When running benchmarks on an 8 socket 80 core machine with a 3.10 kernel, > > > > there can be a lot of contention in idle_balance() and related functions. > > > > On many AIM7 workloads in which CPUs go idle very often and idle balance > > > > gets called a lot, it is actually lowering performance. > > > > > > > > Since idle balance often helps performance (when it is not overused), I > > > > looked into trying to avoid attempting idle balance only when it is > > > > occurring too frequently. > > > > > > > > This RFC patch attempts to keep track of the approximate "average" time between > > > > idle balance attempts per CPU. Each time the idle_balance() function is > > > > invoked, it will compute the duration since the last idle_balance() for > > > > the current CPU. The avg time between idle balance attempts is then updated > > > > using a very similar method as how rq->avg_idle is computed. > > > > > > > > Once the average time between idle balance attempts drops below a certain > > > > value (which in this patch is sysctl_sched_idle_balance_limit), idle_balance > > > > for that CPU will be skipped. The average time between idle balances will > > > > continue to be updated, even if it ends up getting skipped. The > > > > initial/maximum average is set a lot higher though to make sure that the > > > > avg doesn't fall below the threshold until the sample size is large and to > > > > prevent the avg from being overestimated. > > > > > > One of the things I've been talking about for a while now is how I'd > > > like to use the idle guestimator used for cpuidle for newidle balance. > > > > > > Basically based on the estimated idle time limit how far/wide you'll > > > search for tasks to run. > > > > > > You can remove the sysctl and auto-tune by measuring how long it takes > > > on avg to do a newidle balance. > > > > Hi Peter, > > > > When you say how long it takes on avg to do a newidle balance, are you > > referring to the avg time it takes for each call to CPU_NEWLY_IDLE > > load_balance() to complete, or the avg time it takes for newidle balance > > attempts within a domain to eventually successfully pull/move a task(s)? > > Both :-), being as the completion time would be roughly equivalent for the > top domain and the entire call. > > So I suppose I was somewhat unclear :-) I initially started out with a > simpler model, where you measure the avg time of the entire > idle_balance() call and measure the avg idle time and compare the two. > > Then I progressed to the more complex model where you measure the > completion time of each domain in the for_each_domain() iteration of > idle_balance() and compare that against the estimated idle time, bailing > out of the domain iteration when the avg completion time exceeds the > expected idle time.
Hi Peter,
For the more complex model, are you suggesting that each completion time is the time it takes to complete 1 iteration of the for_each_domain() loop?
Based on some of the data I collected, a single iteration of the for_each_domain() loop is almost always significantly lower than the approximate CPU idle time, even in workloads where idle_balance is lowering performance. The bigger issue is that it takes so many of these attempts before idle_balance actually "worked" and pulls a tasks.
I initially was thinking about each "completion time" of an idle balance as the sum total of the times of all iterations to complete until a task is successfully pulled within each domain.
Jason
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