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Re: [PATCH v4] x86/x2apic: introduce a mixed physical/cluster mode
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 6 Nov 2023 16:06:58 +0000
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- Cc: Henry Wang <Henry.Wang@xxxxxxx>, Community Manager <community.manager@xxxxxxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Mon, 06 Nov 2023 16:07:05 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 06/11/2023 2:27 pm, Roger Pau Monne wrote:
> The current implementation of x2APIC requires to either use Cluster Logical or
> Physical mode for all interrupts. However the selection of Physical vs
> Logical
> is not done at APIC setup, an APIC can be addressed both in Physical or
> Logical
> destination modes concurrently.
>
> Introduce a new x2APIC mode called Mixed, which uses Logical Cluster mode for
> IPIs, and Physical mode for external interrupts, thus attempting to use the
> best method for each interrupt type.
>
> Using Physical mode for external interrupts allows more vectors to be used,
> and
> interrupt balancing to be more accurate.
>
> Using Logical Cluster mode for IPIs allows fewer accesses to the ICR register
> when sending those, as multiple CPUs can be targeted with a single ICR
> register
> write.
>
> A simple test calling flush_tlb_all() 10000 times on a tight loop on AMD EPYC
> 9754 with 512 CPUs gives the following figures in nano seconds:
>
> x mixed
> + phys
> * cluster
> N Min Max Median Avg Stddev
> x 25 3.5131328e+08 3.5716441e+08 3.5410987e+08 3.5432659e+08 1566737.4
> + 12 1.231082e+09 1.238824e+09 1.2370528e+09 1.2357981e+09 2853892.9
> Difference at 95.0% confidence
> 8.81472e+08 +/- 1.46849e+06
> 248.774% +/- 0.96566%
> (Student's t, pooled s = 2.05985e+06)
> * 11 3.5099276e+08 3.5561459e+08 3.5461234e+08 3.5415668e+08 1415071.9
> No difference proven at 95.0% confidence
>
> So Mixed has no difference when compared to Cluster mode, and Physical mode is
> 248% slower when compared to either Mixed or Cluster modes with a 95%
> confidence.
>
> Note that Xen uses Cluster mode by default, and hence is already using the
> fastest way for IPI delivery at the cost of reducing the amount of vectors
> available system-wide.
>
> Make the newly introduced mode the default one.
>
> Note the printing of the APIC addressing mode done in connect_bsp_APIC() has
> been removed, as with the newly introduced mixed mode this would require more
> fine grained printing, or else would be incorrect. The addressing mode can
> already be derived from the APIC driver in use, which is printed by different
> helpers.
>
> Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
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