From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCH] credit: Change default timeslice to 5ms
Date: Wed, 5 Mar 2014 17:18:20 +0000
Message-ID: <53175C5C.4060505@citrix.com>
References: <1394036976-484-1-git-send-email-george.dunlap@eu.citrix.com>
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To: George Dunlap <george.dunlap@eu.citrix.com>
Cc: Marcus Granado <Marcus.Granado@eu.citrix.com>, Keir Fraser <keir@xen.org>, Dario Faggioli <dario.faggioli@citrix.com>, Tim Deegan <tim@xen.org>, xen-devel@lists.xen.org, Jan Beulich <jbeulich@suse.com>
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On 05/03/14 16:29, George Dunlap wrote:
> The 30ms timeslice was chosen nearly a decade ago now, with cpu
> "burning" workloads in mind.  In the mean time, processors have gotten
> faster and VMEXITs have gotten faster.  A timeslice of 30ms has a
> major cost when running latency-sensitive workloads like network or
> audio streaming: getting caught behind just one or two other VMs can
> introduce a processing delay of up to 60ms, and the "round-robin"
> nature of the credit scheduler means this delay may be introduced
> every time the VM yields for periods of time.
> 
> The XenServer performance team at Citrix have done extensive testing
> with various timeslices, including 30ms, 10ms, 5ms, and 2ms.  None of
> the workloads exhibited any performance degradation with a 5ms
> timeslice.
[...]
> --- a/xen/common/sched_credit.c
> +++ b/xen/common/sched_credit.c
> @@ -29,9 +29,9 @@
>   * Basic constants
>   */
>  #define CSCHED_DEFAULT_WEIGHT       256
> -#define CSCHED_TICKS_PER_TSLICE     3
> -/* Default timeslice: 30ms */
> -#define CSCHED_DEFAULT_TSLICE_MS    30
> +#define CSCHED_TICKS_PER_TSLICE     1

The TICKS_PER_TSLICE change doubles the tick rate.  Is this intentional?
 It's not mentioned in the commit message.

> +/* Default timeslice: 5ms */
> +#define CSCHED_DEFAULT_TSLICE_MS    5
>  #define CSCHED_CREDITS_PER_MSEC     10

David