Re: sysbench+mysql(oltp, readonly) 30% regression with 2.6.26-rc1

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

On Wed, 2008-05-07 at 18:26 +0200, Peter Zijlstra wrote: 
> On Wed, 2008-05-07 at 12:55 +0800, Zhang, Yanmin wrote:
> > Comparing with kernel 2.6.25, sysbench+mysql(oltp, readonly) has many
> > regression with 2.6.26-rc1.
> > 
> > 1) 8-core stoakley: 28%;
> > 2) 16-core tigerton: 20%;
> > 3) Itanium Montvale: 50%.
> > 
> > Bisect located below patch.
> > 
> > 8f1bc385cfbab474db6c27b5af1e439614f3025c is first bad commit
> > commit 8f1bc385cfbab474db6c27b5af1e439614f3025c
> > Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Date:   Sat Apr 19 19:45:00 2008 +0200
> > 
> >     sched: fair: weight calculations
> >     
> >     In order to level the hierarchy, we need to calculate load based on the
> >     root view. That is, each task's load is in the same unit.
> > 
> > 
> > 
> > After I manually reverted the patch against 2.6.26-rc1 while fixing a couple of
> > conflictions/errors, sysbench oltp regression became less than 3% on 8-core
> > stoakley.
> 
> Does this patch help?
With the patch, oltp testing result is about 50% worse than the one of pure
2.6.26-rc1.

cpu idle with 16 threads( a parameter of sysbench):
1) 2.6.25: 1%
2) 2.6.26-rc1: 33%
3) 2.6.26-rc1+new_patch: 70%.

> 
> ---
> From: Mike Galbraith <efault@gmx.de>
> Subject: sched: fix weight calculations
> 
> The conversion between virtual and real time is as follows:
> 
>   dvt = rw/w * dt <=> dt = w/rw * dvt
> 
> Since we want the fair sleeper granularity to be in real time, we actually
> need to do:
> 
>   dvt = - rw/w * l
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  kernel/sched_fair.c |   11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6-2/kernel/sched_fair.c
> ===================================================================
> --- linux-2.6-2.orig/kernel/sched_fair.c
> +++ linux-2.6-2/kernel/sched_fair.c
> @@ -662,10 +662,15 @@ place_entity(struct cfs_rq *cfs_rq, stru
>  	if (!initial) {
>  		/* sleeps upto a single latency don't count. */
>  		if (sched_feat(NEW_FAIR_SLEEPERS)) {
> +			unsigned long thresh = sysctl_sched_latency;
> +
> +			/*
> +			 * convert the sleeper threshold into virtual time
> +			 */
>  			if (sched_feat(NORMALIZED_SLEEPER))
> -				vruntime -= calc_delta_weight(sysctl_sched_latency, se);
> -			else
> -				vruntime -= sysctl_sched_latency;
> +				thresh = calc_delta_fair(thresh, se);
> +
> +			vruntime -= thresh;
>  		}
>  
>  		/* ensure we never gain time by being placed backwards. */
> 
>