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Re: [Xen-devel] [PATCH] x86/domain_page: implement pure per-vCPU mapping infrastructure
- To: Hongyan Xia <hongyxia@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 4 Feb 2020 14:17:17 +0000
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- Cc: jgrall@xxxxxxxxxx, Wei Liu <wl@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Tue, 04 Feb 2020 14:17:36 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 03/02/2020 18:36, Hongyan Xia wrote:
> Rewrite the mapcache to be purely per-vCPU instead of partly per-vCPU
> and partly per-domain.
>
> This patch is needed to address performance issues when we start relying
> on the mapcache, e.g., when we do not have a direct map. Currently, the
> per-domain lock on the mapcache is a bottleneck for multicore, causing
> performance degradation and even functional regressions.
Do you mean that this patch causes a regression, or that removing the
directmap causes a regression?
The rest of the commit message is very confusing to follow.
> This patch
> makes the mapping structure per-vCPU and completely lockless.
>
> Functional regression:
>
> When a domain is run on more than 64 cores, FreeBSD 10 panicks frequently
> due to occasional simultaneous set_singleshot_timer hypercalls from too
> many cores. Some cores will be blocked waiting on map_domain_page,
> eventually failing to set a timer in the future. FreeBSD cannot handle
> this and panicks. This was fixed in later FreeBSD releases by handling
> -ETIME, but still the degradation in timer performance is a big issue.
>
> Performance regression:
>
> Many benchmarks see a performance drop when having a large core count.
> I have done a Geekbench on a 32-vCPU guest.
>
> perf drop old new
> single 0.04% 0.18%
> multi 2.43% 0.08%
>
> Removing the per-domain lock in the mapcache brings the multi-core
> performance almost identical to using the direct map for mappings.
>
> There should be room for futher optimisations, but this already
> improves over the old mapcache by a lot.
>
> Note that entries in the maphash will occupy inuse slots. With 16 slots
> per vCPU and a maphash capacity of 8, we only have another 8 available,
> which is not enough for nested page table walks. We need to increase the
> number of slots in config.h.
I'm afraid that I don't follow what you're trying to say here. The
number of slots should either be fine, or we've got a pre-exiting bug.
~Andrew
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