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Re: [PATCH v4 1/3] x86/time: latch to-be-written TSC value early in rendezvous loop
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Tue, 20 Apr 2021 17:44:53 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Tue, 20 Apr 2021 15:45:10 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Thu, Apr 01, 2021 at 11:54:05AM +0200, Jan Beulich wrote:
> To reduce latency on time_calibration_tsc_rendezvous()'s last loop
> iteration, read the value to be written on the last iteration at the end
> of the loop body (i.e. in particular at the end of the second to last
> iteration).
>
> On my single-socket 18-core Skylake system this reduces the average loop
> exit time on CPU0 (from the TSC write on the last iteration to until
> after the main loop) from around 32k cycles to around 29k (albeit the
> values measured on separate runs vary quite significantly).
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Thanks, Roger.
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