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

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Message-ID: <20190507221519.GE2677@hirez.programming.kicks-ass.net>
Date:   Wed, 8 May 2019 00:15:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
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 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.