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Subject: Re: [PATCH] x86/xen: Add "xen_timer_slop" command line option
To:     luca abeni <luca.abeni@santannapisa.it>,
        Dario Faggioli <dfaggioli@suse.com>
Cc:     thibodux@gmail.com, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, oleksandr_andrushchenko@epam.com,
        tglx@linutronix.de, jgross@suse.com, ryan.thibodeaux@starlab.io
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From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <69e40698-f7ae-11c3-e4b7-dda4f1fadcf6@oracle.com>
Date:   Mon, 25 Mar 2019 09:43:20 -0400
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On 3/25/19 8:05 AM, luca abeni wrote:
> Hi all,
>
> On Sat, 23 Mar 2019 11:41:51 +0100
> luca abeni <luca.abeni@santannapisa.it> wrote:
> [...]
>>>> Is there any data that shows effects of using this new parameter?
>>>>     
>>> Yes, I've done some research and experiments on this. I did it
>>> together with a friend, which I'm Cc-ing, as I'm not sure we're
>>> ready/capable to share the results, yet (Luca?).  
>> I think we can easily share the experimental data (cyclictest output
>> and plots).
>>
>> Moreover, we can share the scripts and tools for running the
>> experiments (so, everyone can easily reproduce the numbers by simply
>> typing "make" and waiting for some time :)
>>
>>
>> I'll try to package the results and the scripts/tools this evening,
>> and I'll send them.
> Sorry for the delay. I put some quick results here:
> http://retis.santannapisa.it/luca/XenTimers/
> (there also is a link to the scripts to be used for reproducing the
> results). The latencies have been measured by running cyclictest in the
> guest (see the scripts for details).
>
> The picture shows the latencies measured with an unpatched guest kernel
> and with a guest kernel having TIMER_SLOP set to 1000 (arbitrary small
> value :).
> All the experiments have been performed booting the hypervisor with a
> small timer_slop (the hypervisor's one) value. So, they show that
> decreasing the hypervisor's timer_slop is not enough to measure low
> latencies with cyclictest.


I have a couple of questions:
* Does it make sense to make this a tunable for other clockevent devices
as well?
* This patch adjusts min value. Could max value (ever) need a similar
adjustment?

-boris