linux-kernel - Re: [PATCH] sched: introduce configurable delay before entering idle

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Date:   Tue, 7 May 2019 20:44:45 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     kvm-devel <kvm@...r.kernel.org>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Bandan Das <bsd@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] sched: introduce configurable delay before entering idle

On Wed, May 08, 2019 at 12:15:19AM +0200, Peter Zijlstra wrote:
> On Tue, May 07, 2019 at 03:56:49PM -0300, Marcelo Tosatti wrote:
> > 
> > Certain workloads perform poorly on KVM compared to baremetal
> > due to baremetal's ability to perform mwait on NEED_RESCHED
> > bit of task flags (therefore skipping the IPI).
> > 
> > This patch introduces a configurable busy-wait delay before entering the
> > architecture delay routine, allowing wakeup IPIs to be skipped 
> > (if the IPI happens in that window).
> > 
> > The real-life workload which this patch improves performance
> > is SAP HANA (by 5-10%) (for which case setting idle_spin to 30 
> > is sufficient).
> > 
> > This patch improves the attached server.py and client.py example 
> > as follows:
> > 
> > Host:                           31.814230202231556
> > Guest:                          38.17718765199993       (83 %)
> > Guest, idle_spin=50us:          33.317709898000004      (95 %)
> > Guest, idle_spin=220us:         32.27826551499999       (98 %)
> > 
> > Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> 
> Thanks for the CC..
> 
> NAK, this is something that should live in a virt idle governor or
> something along those lines.

Ok, makes sense, will rework the patch!